Regulation of GSH in alphaA-expressing human lens epithelial cell lines and in alphaA knockout mouse lenses.

PURPOSE: To study the mechanism of regulation of GSH in HLE-B3 cells expressing alphaA-crystallin (alphaA) and in alphaA knockout mouse lenses. METHODS: GSH levels and maximal rates of GSH synthesis were measured in immortalized, alphaA-transfected HLE-B3 cells containing varying amounts of alphaA. The mRNA and protein for the rate-limiting enzyme for GSH synthesis, gamma-glutamylcysteine synthetase (GCS), were also determined in alphaA- and mock-transfected cells by Northern blot analysis and Western blot analysis of heavy (GCS-HS) and light (GCS-LS) subunits. The effect of absence of alphaA and alphaB on lens GSH concentrations was evaluated in whole lenses of alphaA knockout and alphaB knockout mice as a function of age. GCS-HS mRNA and protein were determined in young, precataractous and cataractous alphaA knockout lenses. RESULTS: GSH levels were significantly higher in HLE-B3 cells expressing alphaA- compared with mock-transfected cells and were correlated positively with alphaA content. Mean rate of GSH synthesis was also higher in alphaA-expressing cells than in mock controls (0.84 vs. 0.61 nmol. min(-1) per mg protein, respectively). GCS-HS mRNA and GCS-LS mRNA were approximately twofold higher in alphaA-expressing cells, whereas the heavy and light GCS subunit proteins increased by 80% to 100%. In alphaA(-/-) mouse lenses, GSH level was not different from that of wild type up to 2 months from birth, after which it dropped to approximately 50% of controls. On the other hand, GCS-HS and GCS-LS proteins showed a significant decrease before cataract formation as early as 15 days after birth. GSH level in cataract-free alphaB(-/-) lenses was similar to that of wild type for up to 14 months. CONCLUSIONS: Expression of alphaA caused an increase in cellular GSH, in part, because of an increase in mRNA and protein of both GCS subunits. GSH levels decreased with increasing age in cataractous alphaA(-/-) lenses but not in the noncataractous alphaB(-/-) lenses. It is suggested that neonatal precataractous lenses (with normal GSH and decreased GCS) may maintain their GSH level by other compensatory mechanisms such as increased GSH transport.

Lens epithelial cells derived from alphaB-crystallin knockout mice demonstrate hyperproliferation and genomic instability.

alphaB-crystallin is a member of the small heat shock protein family and can act as a molecular chaperone preventing the in vitro aggregation of other proteins denatured by heat or other stress conditions. Expression of alphaB-crystallin increases in cells exposed to stress and enhanced in tumors of neuroectodermal origin and in many neurodegenerative diseases. In the present study, we examined the properties of lens epithelial cells derived from mice in which the alphaB-crystallin gene had been knocked out. Primary rodent cells immortalize spontaneously in tissue culture with a frequency of 10(-5) to 10(-6). Primary lens epithelial cells derived from alphaB-crystallin-/- mice produced hyperproliferative clones at a frequency of 7.6 x 10(-2), four orders of magnitude greater than predicted by spontaneous immortalization (1). Hyperproliferative alphaB-crystallin-/- cells were shown to be truly immortal since they have been passaged for more than 100 population doublings without any diminution in growth potential. In striking contrast to the wild-type cells, which were diploid, the alphaB-crystallin-/- cultures had a high proportion of tetraploid and higher ploidy cells, indicating that the loss of alphaB-crystallin is associated with an increase in genomic instability. Further evidence of genomic instability of alphaB-crystallin-/- cells was observed when primary cultures were infected with Ad12-SV40 hybrid virus. In striking contrast to wild-type cells, alphaB-crystallin-/- cells expressing SV40 T antigen exhibited a widespread cytocidal response 2 to 3 days after attaining confluence, indicating that SV40 T antigen enhanced the intrinsic genomic instability of alphaB-crystallin-/- lens epithelial cells. These observations suggest that the widely distributed molecular chaperone alphaB-crystallin may play an important nuclear role in maintaining genomic integrity.

Ubiquitin-activating enzyme (E1) isoforms in lens epithelial cells: origin of translation, E2 specificity and cellular localization determined with novel site-specific antibodies.

Lens development and response to peroxide stress are associated with dramatic changes in protein ubiquitination, reflecting dynamic changes in activity of the ubiquitin-activating enzyme (E1). Two isoforms of E1 (E1A and E1B) have been identified in lens cells although only one E1 mRNA, containing three potential translational start sites, has been detected. Novel, site-specific antibodies to E1 were generated and the hypothesis that the two isoforms of E1 are translated from alternative initiation codons of a single mRNA was tested. Antibodies raised against E1A-N peptide (Met(1)to Cys(23)of E1A) reacted only with E1A by immunoblot and immunoprecipitation. Antibodies raised against E1B-N peptide (Met(1)to Glu(25)of E1B or Met(41)to Glu(65)of E1A) and E1AB-C peptide (His(1030)to Arg(1058)of E1A or His(990)to Arg(1018)of E1B) reacted with both E1A and E1B. These results indicate that (1) E1A and E1B contain the same C-terminal residues; (2) E1A contains the N terminal sequence of E1B; and (3) E1B does not contain the N terminal sequence of E1A. The two isoforms of lens E1 are therefore translated from a single mRNA. Specifically, E1A is translated from the first initiation codon, and E1B translated from the second initiation codon. E1A and E1B were affinity-purified, and their ability to 'charge' ubiquitin carrier proteins (E2s) with activated ubiquitin was compared in a cell-free system. E1A and E1B were indistinguishable with respect to charging different E2s. However, E1 immunolocalization studies with human lens epithelial cells indicate that E1A and E1B are preferentially localized to the nucleus and cytosol, respectively. This observation suggests that E1A and E1B ubiquitinate different proteins and serve different functions in intact cells.

Differential protective activity of alpha A- and alphaB-crystallin in lens epithelial cells.

alphaA- and alphaB-crystallins are molecular chaperones expressed at low levels in lens epithelial cells, and their expression increases dramatically during differentiation to lens fibers. However, the functions of alphaA- and alphaB-crystallins in lens epithelial cells have not been studied in detail. In this study, the relative ability of alphaA- and alphaB-crystallin, in protecting lens epithelial cells from apoptotic cell death was determined. The introduction of alphaA-crystallin in the transformed human lens epithelial (HLE) B-3 lens epithelial cell line (which expresses low endogenous levels of alphaB-crystallin) led to a nearly complete protection of cell death induced by staurosporine, Fas monoclonal antibody, or the cytokine tumor necrosis factor alpha. To further study the relative protective activities of alphaA- and alphaB-crystallins, we created a cell line derived from alphaA-/-alphaB-/- double knockout mouse lens epithelia by infecting primary cells with Ad12-SV40 hybrid virus. The transformed cell line alphaAalphaBKO1 derived from alphaA/alphaB double knockout cells was transfected with alphaA- or alphaB-crystallin cDNA contained in pCIneo mammalian expression vector. Cells expressing different amounts of either alphaA-crystallin or alphaB-crystallin were isolated. The ability of alphaA- or alphaB-crystallin to confer protection from apoptotic cell death was determined by annexin labeling and flow cytometry of staurosporine- or UVA- treated cells. The results indicate that the anti-apoptotic activity of alphaA-crystallin was two to three-fold higher than that of alphaB-crystallin. Our work suggests that comparing the in vitro annexin labeling of lens epithelial cells is an effective way to measure the protective activity of alphaA- and alphaB-crystallin. Since the expression of alphaA-crystallin is largely restricted to the lens, its greater protective effect against apoptosis suggests that it may play a significant role in protecting lens epithelial cells from stress.

Induction of heme oxygenase-1 modulates cis-aconitase activity in lens epithelial cells.

Heme oxygenase-1 is the heme catabolic enzyme induced in human dermal fibroblasts by environmental stress. We report an increase of heme oxygenase-1 message in lens epithelial cells after exposure to UVA radiation, followed by a 10-fold increase of protein expression. The size of message was larger than previously demonstrated for fibroblasts. The relationship between heme oxygenase-1 activation and iron metabolism was investigated by measurement of activities of both cytosolic and mitochondrial cis-aconitase enzymes. A 2-fold increase in mitochondrial cis-aconitase activity in UVA-exposed cells coincided with the time of maximal heme oxygenase-1 expression. We propose that modulation of cis-aconitase activity at the translational level by an increase of cellular iron is an important consequence of heme oxygenase-1 activation. This might be a novel aspect of the protective role of heme oxygenase-1 in modulating the response of cells challenged with oxidative stress.

DNA repair and survival in human lens epithelial cells with extended lifespan.

PURPOSE: Ultraviolet-B radiation (290-320 nm) produces cataracts in animals and has been associated with human cataract formation in several epidemiological studies. UVB radiation decreases the long-term cell survival and changes the pattern of protein synthesis in cultured lens epithelial cells. However, the relationship between DNA photoproduct formation and long term cell survival in human lens epithelial cells is not known. In the present work, we used human lens epithelial cells with extended lifespan (HLE B-3 cells) to examine the kinetics of DNA repair and cell survival after UVB exposure. METHODS: Cyclobutane pyrimidine dimers and pyrimidine-pyrimidone (6-4) photoproducts were analyzed by radioimmunoassay. Long-term survival of the cells was determined by measuring their ability form colonies when plated at low density. RESULTS: HLE B-3 cells were repair competent after UVB (302 nm) exposure. Excision repair of the (6-4) photoproduct was more efficient than that of the cyclobutane dimer. Ninety five percent of the (6-4) photoproducts were repaired 24 h after 400 J/m2 UVB exposure, whereas 50% of the cyclobutane dimers were repaired during this time. When cells were split for the clonogenic assay immediately after irradiation, only 10% of the cells formed colonies following 7 days of culture in the serum-containing medium. When cells were split for the clonogenic assay after a 48 hour incubation in serum-containing medium, the colony-forming ability of the irradiated cells increased to 60% following culture in a serum-containing medium. CONCLUSIONS: These results indicate a close correlation between the repair of cyclobutane dimers and the increase in the long-term survival of the cells as measured by their colony-forming ability. The extended lifespan human lens epithelial cells HLE B-3 may be a useful model to investigate the mechanism and regulation of UVB-induced DNA repair in human lens cells.

Expression of prostaglandin receptors EP4 and FP in human lens epithelial cells.

PURPOSE: To examine the expression of prostaglandin (PG) receptors EP2, EP4, and FP in a human lens epithelial cell line (HLE-B3) at molecular and pharmacologic levels. METHODS: Reverse transcription-polymerase chain reactions (RT-PCR) were performed with total RNA preparations from HLE-B3 cells using sense and antisense primers for each of the three prostaglandin receptors. The PCR products were hybridized with specific 32P-labeled probes and, for further confirmation, digested with appropriate restriction enzymes. At the pharmacologic level, the expression of EP4 receptors was determined by measuring intracellular cyclic adenosine monophosphate (cAMP) formation in response to PGE2 (EP1, EP2, EP3, and EP4 agonist) and the EP4 receptor-selective antagonist AH23848. The expression of FP receptors in HLE-B3 cells was explored by measuring intracellular [Ca2+]i mobilization. RESULTS: RT-PCR generated DNA products of predicted sizes corresponding to the EP2, EP4, and FP receptors. Hybridization of the PCR products with specific 32P-labeled probes and restriction digestion of the PCR products further confirmed that they were generated from the respective EP2, EP4, and FP mRNAs. The EP receptor agonist PGE2 significantly increased the cAMP level in HLE-B3 cells. The formation of cAMP by PGE2 was concentration-dependently inhibited by the EP4 receptor-selective antagonist AH23848. Stimulation of HLE-B3 cells by the FP receptor agonist fluprostenol increased [Ca2+]i in a time-dependent manner. CONCLUSIONS: The results of the molecular biologic and pharmacologic experiments showed conclusively the presence of EP4 and FP receptor messenger RNAs and proteins, respectively, in HLE-B3 cells.

The molecular chaperone alphaA-crystallin enhances lens epithelial cell growth and resistance to UVA stress.

alphaA-Crystallin (alphaA) is a member of the small heat shock protein (sHSP) family and has the ability to prevent denatured proteins from aggregating in vitro. Lens epithelial cells express relatively low levels of alphaA, but in differentiated fiber cells, alphaA is the most abundant soluble protein. The lenses of alphaA-knock-out mice develop opacities at an early age, implying a critical role for alphaA in the maintenance of fiber cell transparency. However, the function of alpha-crystallin in the lens epithelium is unknown. To investigate the physiological function of alphaA in lens epithelial cells, we used the following two systems: alphaA knock-out (alphaA(-/-)) mouse lens epithelial cells and human lens epithelial cells that overexpress alphaA. The growth rate of alphaA(-/-) mouse lens epithelial cells was reduced by 50% compared with wild type cells. Cell cycle kinetics, measured by fluorescence-activated cell sorter analysis of propidium iodide-stained cells, indicated a relative deficiency of alphaA(-/-) cells in the G2/M phases. Exposure of mouse lens epithelial cells to physiological levels of UVA resulted in an increase in the number of apoptotic cells in the cultures. Four hours after irradiation the fraction of apoptotic cells in the alphaA(-/-) cultures was increased 40-fold over wild type. In cells lacking alphaA, UVA exposure modified F-actin, but actin was protected in cells expressing alphaA. Stably transfected cell lines overexpressing human alphaA were generated by transfecting extended life span human lens epithelial cells with the mammalian expression vector construct pCI-neoalphaA. Cells overexpressing alphaA were resistant to UVA stress, as determined by clonogenic survival. alphaA remained cytoplasmic after exposure to either UVA or thermal stress indicating that, unlike other sHSPs, the protective effect of alphaA was not associated with its relocalization to the nucleus. These results indicate that alphaA has important cellular functions in the lens over and above its well characterized role in refraction.

Glutathione transport in immortalized HLE cells and expression of transport in HLE cell poly(A)+ RNA-injected Xenopus laevis oocytes.

PURPOSE: To determine reduced glutathione (GSH) transport in cultured human lens epithelial cells (HLE-B3) and plasma membrane vesicles and to study the expression of GSH transport in Xenopus laevis oocytes injected with poly(A)+ RNA from HLE-B3 cells. METHODS: Confluent HLE-B3 cells pretreated with 10 mM DL-buthionine sulfoximine and 0.5 mM acivicin were used in GSH uptake studies. The uptake of 35S-GSH was performed for 30 minutes in either NaCl medium (Na+-containing) or choline chloride medium (Na+-free) at 37 degrees C and 4 degrees C. The molecular form of 35S uptake was determined by high-performance liquid chromatography. GSH uptake kinetics were studied in acivicin and buthionine sulfoximine-treated HLE-B3 cells in NaCl medium in the concentration range 0.01 microM to 50 mM. The transport of GSH and the effect of Na+ on uptake also were determined in mixed plasma membrane vesicles from HLE-B3 cells. In oocyte expression studies, HLE-B3 poly(A)+ RNA was injected into X. laevis oocytes and GSH uptake experiments were performed 3 days after injection. The uptake of 35S-GSH and GSH efflux rates were determined in HLE-B3 poly(A)+ RNA-injected oocytes. RESULTS: No significant difference was found in the uptake of 1 mM GSH+/-acivicin (17.7+/-4.3 versus 15.7+/-1.4 picomoles/min(-1) per 10(6) cells). However, GSH uptake was significantly lower in Na+-free medium compared with Na+-containing medium (10.3+/-0.7 versus 16.8+/-0.9 picomoles/min(-1) per 10(6) cells; P < 0.01). GSH uptake in NaCl medium was carrier mediated. GSH uptake showed partial sodium dependency from 5 microM to 5 mM GSH in mixed plasma membrane vesicles from HLE-B3 cells. Oocytes injected with HLE-B3 poly(A) RNA expressed uptake and efflux of GSH. Uptake showed partial Na+ dependency at various GSH concentrations. The efflux rates were approximately 30-fold higher than those in water-injected oocytes (0.48+/-0.03 versus 0.016+/-0.005 (nanomoles per hour(-1) per oocyte, respectively). The molecular form of uptake in cultured cells and in oocyte studies was predominantly as intact GSH. CONCLUSIONS: HLE-B3 cells and plasma membrane vesicles transported GSH by a carrier-mediated process. HLE-B3 poly(A)+ RNA injected X laevis oocytes expressed GSH transport. GSH uptake was partially Na+ dependent in all systems. HLE-B3 cells offer a useful model for characterizing GSH transport and for studying its regulatory role in the etiology of cataracts.

Expression of growth control and differentiation genes in human lens epithelial cells with extended life span.

PURPOSE: Peptide growth factors including hepatocyte growth factor (HGF), platelet-derived growth factor (PDGF), and epidermal growth factor (EGF) are mitogens for many cell types and may act as regulators of lens epithelial cell growth and differentiation. The present study was undertaken to investigate the expression of growth factor receptors and crystallin genes in the human lens epithelial cell line HLE B-3, created by infection with Adeno12-simian virus 40 (Ad12-SV40) hybrid virus. METHODS: Reverse transcriptase-polymerase chain reaction (RT-PCR) with gene-specific primers was used to detect transcripts, and Southern and western blot analyses were used for identification of gene products. Functional analysis of PDGF receptor was performed by measuring the effect of PDGF-BB on Ca2+ release, cell growth, and western blot analysis, by using an antiphosphotyrosine antibody. RESULTS: Human lens epithelial B-3 cells expressed the growth factor receptors HGF-R, EGF-R, and PDGF-Rbeta, but not PDGF-Ralpha, and also expressed the oncogenes H-ras and raf and the growth inhibitor transforming growth factor-beta1. Stimulation of PDGF-Rbeta with PDGF-BB in HLE B-3 cells increased phosphorylation of the receptor, was associated with an increase in intracellular Ca2+ levels, and produced a small increase in cell growth. In addition, HLE B-3 cells expressed transcripts for alphaA-, alphaB-, and betaB2-crystallins, and expressed the corresponding proteins. The transcripts for alphaA-crystallin decreased markedly at higher passages. CONCLUSIONS: The above findings suggest that the increased growth potential of human lens epithelial cells by Ad12-SV40 infection maintained certain lens-specific properties and response to PDGF.

Action spectrum for photocross-linking of human lens proteins.

The action spectrum for photocross-linking was measured for human lens beta gamma-crystallins from young adult noncataractous lenses at wavelengths of 297, 302, 313, 334 and 365 nm. The action spectrum had a maximal effectiveness at 297 nm that sharply decreased in effectiveness up to 313 nm, then remained flat until 334 nm and decreased markedly as wavelength increased to 365 nm. Radiation at 297 nm was 36 times more effective in producing cross-linking than 302 nm radiation. The 297 nm radiation was 220, 195 and 1300 times more effective than 313, 334 and 365 nm radiation, respectively. The action spectrum had a shape similar to the absorption spectrum of the lens proteins but the response was lower than expected from the absorption data, suggesting that some of the absorptions are not effective at cross-linking. Because most animal experimentation and epidemiological studies include broadband radiation, these studies would be useful in predicting the biological response of the lens to environmental UV stress.

Cloning, expression, and chaperone-like activity of human alphaA-crystallin.

One of the major protein components of the ocular lens, alpha-crystallin, is composed of alphaA and alphaB chain subunits that have structural homology to the family of mammalian small heat shock proteins. Like other small heat shock proteins, alpha-crystallin subunits associate to form large oligomeric aggregates that express chaperone-like activity, as defined by the ability to suppress nonspecific aggregation of proteins destabilized by treatment with a variety of denaturants including heat, UV irradiation, and chemical modification. It has been proposed that age-related loss of sequences at the C terminus of the alphaA chain subunit may be a factor in the pathogenesis of cataract due to diminished capacity of the truncated crystallin to protect against nonspecific aggregation of lens proteins. To evaluate the functional consequences of alpha-crystallin modification, two mutant forms of alphaA subunits were prepared by site-directed mutagenesis. Like wild type (WT), aggregates of approximately 540 kDa were formed from a tryptophan-free alphaA mutant (W9F). When added in stoichiometric amounts, both WT and W9F subunits completely suppressed the heat-induced aggregation of aldose reductase. In contrast, subunits encoded by a truncation mutant in which the C-terminal 17 residues were deleted (R157STOP), despite having spectroscopic properties similar to WT, formed much larger aggregates with a marked reduction in chaperone-like activity. Similar results were observed when the chaperone-like activity was assessed through inhibition of gamma-crystallin aggregation induced by singlet oxygen. These results demonstrate that the structurally conservative substitution of Phe for Trp-9 has a negligible effect on the functional interaction of alphaA subunits, and that deletion of C-terminal sequences from the alphaA subunit results in substantial loss of chaperone-like activity, despite overall preservation of secondary structure.

The role of prostaglandins E2 and F2 alpha in ultraviolet radiation-induced cortical cataracts in vivo.

PURPOSE: Previous work has shown that exposure of lens epithelial cells or rabbit eyes in vivo to ultraviolet B (UVB) radiation enhanced prostaglandin (PG)E2 synthesis. Such enhanced PGE2 synthesis was related to the increased DNA synthesis that followed UVB exposure. The current study examined the relationship between enhanced prostaglandin synthesis and UVB-induced cataract formation. METHODS: Seventy albino (New Zealand white) rabbit eyes were exposed to UVB radiation in vivo. Fluence of radiation at the cornea was 2.8 J/cm2, 5.6 J/cm2, or 11.2 J/cm2. Eyes were examined 24 hours after UVB exposure and for as long as 10 days by slit lamp biomicroscopy. Mass spectrometry was used to measure PGE2, PGF2 alpha, and 6-keto-PGF1 alpha content of the lens and iris-ciliary body using authentic standards. To determine the effect of inhibition of prostaglandin synthesis on UVB-induced cataract formation, animals were given indomethacin intraperitoneally. Other pharmacologic agents, such as PGE2, PGF2 alpha, and misoprostol, were applied topically to the eye. The effect of UVB on K+ pump was determined by incubating isolated lenses with [86Rb+]. RESULTS: Twenty-four hours after UVB exposure, PGE2 and PGF2 alpha concentrations in aqueous humor were increased by 100- and 30-fold, respectively. Lens PGE2 and PGF2 alpha increased by 6- and 4-fold, respectively, after UVB radiation exposure. Pretreatment of animals with indomethacin prevented the rise in lens and aqueous humor PGE2 and PGF2 alpha levels. Furthermore, indomethacin was partially protective against UVB cataract formation and lowered cataract severity from stage 3 to stage 1, but it did not prevent UVB-induced lens changes completely. Topical application of PGE2 before UVB exposure completely prevented cataract formation in the UVB-exposed eye. In contrast, topical administration of PGF2 alpha increased cataract severity. UVB-induced cataract formation preceded changes in [86Rb]+ uptake in lenses subsequently incubated in K(+)-free Tyrode's. CONCLUSIONS: Enhanced synthesis of cyclooxygenase products of arachidonic acid metabolism in the lens is associated with UVB-induced cataract formation in albino rabbit eyes, and inhibition of cyclooxygenase by indomethacin decreased the severity of cataracts. PGE2, the principal arachidonic acid metabolite, appears to have a protective role because pretreatment of the eye with topical PGE2 completely prevented UVB-induced cataract formation, whereas PGF2 alpha increased the severity of the cataract. The evidence presented for a role of PGF2 alpha in the development of cataract suggests that caution be exercised in the use of PGF2 alpha derivatives in the therapy of glaucoma.

Enhanced prostaglandin synthesis after ultraviolet-B exposure modulates DNA synthesis of lens epithelial cells and lowers intraocular pressure in vivo.

PURPOSE: To study the functional significance of prostaglandin synthesis after ultraviolet-B (UVB) exposure of cultured human lens epithelial cells and rabbit eyes in vivo. METHODS: Prostaglandin E2 (PGE2) was assayed using a radioimmunoassay (RIA) and mass spectroscopy. An immortalized human lens epithelial cell line (HLE-B3) was exposed to UV irradiation, and the synthesis of PGE2 was compared with the rabbit lens epithelial cell line N/N1003A. Intact human lenses were exposed to UVB in organ culture. [3H]Thymidine incorporation was measured in cultured lens epithelial cells by incubation with the radiolabel. The effects of isobutyl methyl xanthine (IBMX), an inhibitor of phosphodiesterase and of dibutyryl cyclic adenosine monophosphate (cAMP), an analog of cAMP, on PGE2 synthesis and DNA synthesis, were determined. Rabbit eyes were exposed to UVB radiation in vivo. Intraocular pressure was measured at specific times after exposure. Aqueous humor was remove from rabbit eyes, and its PGE2 content was measured by RIA. RESULTS: Cultured human lens epithelial cells (HLE), like rabbit lens epithelial cells (RLE), showed a dose-dependent increase in basal PGE2 synthesis 24 hours after UVB exposure. However, the amount of PGE2 synthesis was 2000-fold higher in the rabbit cells. Ultraviolet-B radiation enhanced the incorporation of [3H]thymidine in lens epithelial cells. Pretreatment of cells with indomethacin reduce PGE2 synthesis and [3H]thymidine incorporation. The human and rabbit cells responded in a similar manner to changes in DNA synthesis after UVB exposure. The addition of IBMX or dbcAMP to indomethacin-treated, UVB-exposed cells restored DNA synthesis toward the levels observed in the UVB-exposed cells. An increase in the concentration of cAMP was observed in lens epithelial cells exposed to exogenous PGE2. PGE2 synthesis in intact human lenses also increased twofold 24 hours after UVB exposure. Exposure of the rabbit eye in vivo to an optimal dose of UVB produced an increase in the PGE2 levels of the lens and the aqueous humor. Measurements of the intraocular pressure (IOP) of the animals showed a decrease in IOP by 2.21 +/- 0.66 and 6.45 +/- 0.79 mm Hg (mean +/- SEM, P = 0.004, t-test) at 6 and 24 hours after UVB exposure, respectively. The decrease in IOP was prevented by pretreatment with indomethacin. Exposure of the rabbit lens to UVB radiation in vivo enhanced [3H]thymidine incorporation twofold into the lens. Pretreatment of rabbits with indomethacin before exposure reduced this response. CONCLUSIONS: Results indicate that UVB exposure enhances PGE2 synthesis in HLE cultures as well as in rabbit lenses irradiated in vivo. This increased PGE2 synthesis is related to the increase in DNA synthesis observed after UVB treatment. The modulation of DNA synthesis in cultured lens epithelial cells after UVB exposure may be mediated by a cAMP-dependent mechanism.

Ultraviolet action spectra for photobiological effects in cultured human lens epithelial cells.

The action spectrum for cell killing by UV radiation in human lens epithelial (HLE) cells is not known. Here we report the action spectrum in the 297-365 nm region in cultured HLE cells with an extended lifespan (HLE B-3 cells) and define their usefulness as a model system for photobiological studies. Cells were irradiated with monochromatic radiation at 297, 302, 313, 325, 334 and 365 nm. Cell survival was determined using a clonogenic assay. Analysis of survival curves showed that radiation at 297 nm was six times more effective in cell killing than 302 nm radiation; 297 nm radiation was more than 260, 590, 1400 and 3000 times as effective in cell killing as 313, 325, 334 and 365 nm radiation, respectively. The action spectrum was similar in shape to that for other human epithelial cell lines and rabbit lens epithelial cells. The effect of UV radiation on crystallin synthesis was also determined at different wavelengths. To determine whether exposure to UV radiation affects the synthesis of beta-crystallin, cells were exposed to sublethal fluences of UV radiation at 302 and 313 nm, labeled with [35S]methionine and the newly synthesized beta-crystallin was analyzed by immunoprecipitation and western blotting using an antibody to beta-crystallin. The results show a decrease in crystallin synthesis in HLE cells irradiated at 302 and 313 nm at fluences causing low cytotoxicity. The effect of radiation on membrane perturbation was determined by measuring enhancement of synthesis of prostaglandin E2 (PGE2). Synthesis of PGE2 occurs at all UV wavelengths tested in the 297-365 nm region.(ABSTRACT TRUNCATED AT 250 WORDS)

Propagation and immortalization of human lens epithelial cells in culture.

PURPOSE: To establish primary and immortalized cell cultures of human lens epithelial cells for a model system investigating human lens epithelial physiology and cataract. METHODS: Human lens epithelial cells in culture were grown by isolating epithelium fragments from infant human lenses from patients who underwent treatment for retinopathy of prematurity and by allowing epithelial cells to grow from explants. To immortalize cells, the cultures were infected with an adenovirus 12-SV40 virus (Ad12-SV40). RESULTS: The primary cells from infant eyes proliferated for three passages before senescence was observed. However, the immortalized cells remained proliferative and retained the morphology of the primary cells. Immunohistochemical analysis demonstrated that these immortalized cells were SV40 large T antigen-positive and ceased to produce infectious virus after a few passages. Immortalized cells passaged to population doubling levels of 76 continued to form confluent cultures within 7 days of subculture. Analysis of proteins by SDS-PAGE and immunoblotting showed that immortalized cells produce a protein with molecular weight of about 25 kD, which reacted with an antibody to beta H-crystallin. CONCLUSIONS: This report constitutes the first successful immortalization of human lens epithelial cells. Currently, two cell lines have been created (B-3 and B-4) and passaged to population doubling levels of 76 and 52, respectively. These cells may provide an important human cell line specific to in vivo human lens epithelial cell physiology and would be of interest in establishing a human model to study lens cell differentiation and the etiology of cataract. These cells may also provide a constant and reproducible source of lens epithelial cells for eye-related toxicology studies and to assay inhibitory drugs for the prevention of cataracts and posterior capsular opacification observed after cataract extraction.

Expression of recombinant bovine gamma B-, gamma C- and gamma D-crystallins and correlation with native proteins.

Despite the use of bovine gamma-crystallins in numerous biophysical and chemical studies, characterization of these proteins at the molecular level is incomplete. Bovine lenses have at least six gamma-crystallin protein fractions currently assigned as gamma s/I, gamma A/IVb, gamma B/II, gamma C/IIIb, gamma D/IIIa and gamma E/IVa. A lack of primary sequence data for corresponding gamma-crystallin genes and proteins, however, has made these assignments tenuous. To clarify these assignments, we have over-expressed recombinant bovine gamma-crystallin proteins in Escherichia coli using complementary DNAs corresponding to gamma B-, gamma C-, and gamma D-crystallin genes. The recombinant crystallins were characterized by chromatographic and spectroscopic comparisons with native bovine crystallin fractions gamma II, gamma IIIa and gamma IIIb. The elution of recombinant gamma B and native gamma II proteins was identical on cation-exchange chromatography as expected; however, recombinant gamma C coeluted with gamma IIIa and recombinant gamma D co-eluted with gamma IIIb. Sequential Edman degradation through the first 29 residues of the native gamma IIIa and gamma IIIb polypeptides confirmed the colinearity of their sequences with those predicted from the gamma C- and gamma D-crystallin genes, respectively. Absorption and UV circular dichroism (CD) spectra of the recombinant proteins were almost identical to those of their native counterparts, indicating that the secondary and tertiary structures of the recombinant proteins were characteristic for gamma-crystallins. Based on these data the bovine gamma-crystallins proteins and genes are correlated as follows: II/gamma B, IIIa/gamma C and IIIb/gamma D. The assignment of gamma IIIb (previously characterized as having a low critical temperature for phase separation) with gamma D rather than gamma C proves an exception to the hypothesis that the gamma ABC-crystallin group is more resistant to phase separation than the gamma DEF group. These corrected assignments should provide a more substantial base for investigations of residues responsible for phase separation and other biophysical characteristics. Additionally, expression of recombinant gamma-crystallins with structures similar to native proteins may prove to be useful in probing specific structure-function relationships of the gamma-crystallins.

Action spectrum for cytotoxicity in the UVA- and UVB-wavelength region in cultured lens epithelial cells.

PURPOSE: This study was done to quantitate the biologic effects of different wavelengths of radiation in the UVA- and UVB-wavelength region on cultured rabbit lens epithelial cells. METHODS: An action spectrum for UV-induced cytotoxicity as measured by colony-forming ability was determined using six different monochromatic wavelengths from 297 to 405 nm in rabbit lens epithelial cell line N/N1003A. Cell survival was determined by clonogenic assay. Fluence rates were monitored with a calibrated radiometer. RESULTS: Survival curves show that cell killing was most efficient at 297 nm. After quantum correction, the efficiency of 297-nm radiation in cell killing was 7 times greater than was 302-nm radiation. Radiation at 297 nm was more than 170, 340, 560, and 2000 times as effective in cell killing as 313-, 325-, 334-, and 365-nm radiation, respectively. The action spectrum had a shape similar to the DNA absorption spectrum in the UVB region, suggesting that DNA may be one of the critical targets for damage to the cells. At wavelengths longer than 313 nm, the shape of the action spectrum deviated from the DNA absorption spectrum. CONCLUSIONS: Cytotoxicity of UV radiation in cultured lens epithelial cells varies greatly with wavelength within the UVA and UVB regions. Different mechanisms may predominate in the two wavelength regions. Cultured cells may provide a suitable system for investigating the mechanisms by which UV radiation damages lens epithelial cells and leads to cataract formation.

Modulation of lens epithelial cell proliferation by enhanced prostaglandin synthesis after UVB exposure.

PURPOSE: The goals of this investigation were to examine the synthesis of prostaglandins after UVB exposure of lens epithelial cells and to investigate their role in cell proliferation. METHODS: Cultured rabbit lens epithelial cells (cell line N/N1003A) were exposed to low levels of UV radiation. Prostaglandins were assayed by radioimmunoassay; products of arachidonic acid metabolism were analyzed by thin-layer chromatography and mass spectroscopy. Cell proliferation was measured by [3H]thymidine incorporation and proliferative autoradiography. RESULTS: Cultured lens epithelial cells exposed to UVB radiation showed a dose-dependent increase in basal prostaglandin synthesis measured 24 hours after UV exposure. At an optimal dose (250 J/m2) of UVB, prostaglandin E2 (PGE2) synthesis was enhanced tenfold. Product identity was confirmed using thin-layer chromatography and mass spectroscopy with authentic standards. Incubation of irradiated cells with exogenous arachidonic acid followed by extraction and thin-layer chromatography revealed that the cultures produced PGE2, prostaglandin I2 (measured as 6-keto-prostaglandin F1 alpha), prostaglandin F2 alpha, and hydroxyeicosatetraenoic acid. The synthesis of all of these products was enhanced threefold in cells exposed to 250-J/m2 UVB. Indomethacin pretreatment eliminated the synthesis of prostaglandins, further confirming their identity. To discover the relationship between PGE2 synthesis and irradiation-induced cell proliferation, [3H]thymidine incorporation into DNA was determined 24 or 48 hours after exposure. These experiments revealed a fivefold increase in incorporation induced by UVB exposure. UVB-enhanced incorporation of thymidine was eliminated by pretreatment of cultures with indomethacin to eliminate PG synthesis. However, when 100 nM PGE2 was added to the indomethacin-treated irradiated cultures, incorporation of the label was restored toward the level detected in the UVB-stimulated cells. Addition of other prostaglandins to the cultures was ineffective. CONCLUSIONS: The results indicate that the synthesis of PGE2 is enhanced by exposure of lens epithelial cells to UVB radiation. PGE2 seems to play a specific role in cell proliferation after UV exposure. This increase in PGE2 synthesis may be important in posterior subcapsular cataract formation in humans and in animals exposed to UVB radiation in vivo.

Effect of ultraviolet-B radiation on protein synthesis in cultured lens epithelial cells.

Exposure of cultured rabbit lens epithelial cells to repetitive doses of UV-B radiation delays their growth and alters the synthesis of specific proteins. Irradiated cells on the shoulder of the survival curve exhibited a dose-dependent decrease in growth when subcultured in serum-supplemented medium. UV-B irradiation did not affect the subsequent attachment efficiency of the cells. Control and UV-B irradiated cells were incubated with [35S]methionine and the pattern of protein synthesis in the cells was analyzed by SDS-PAGE and autoradiography. Analysis of the labeled proteins from cells exposed to UV-B radiation showed the induction of a 32 kD polypeptide and the loss of a 26 kD polypeptide compared with controls. Analysis of the proteins released by the UV-B irradiated cells into the culture medium revealed the 50% loss of a 37 kD radiolabeled protein compared with controls. The alteration of protein synthesis in lens epithelial cells by UV-B radiation may contribute to cataract formation.