Targeting PYK2 mediates microenvironment-specific cell death in multiple myeloma.

Multiple myeloma (MM) remains an incurable malignancy due, in part, to the influence of the bone marrow microenvironment on survival and drug response. Identification of microenvironment-specific survival signaling determinants is critical for the rational design of therapy and elimination of MM. Previously, we have shown that collaborative signaling between ß1 integrin-mediated adhesion to fibronectin and interleukin-6 confers a more malignant phenotype via amplification of signal transducer and activator of transcription 3 (STAT3) activation. Further characterization of the events modulated under these conditions with quantitative phosphotyrosine profiling identified 193 differentially phosphorylated peptides. Seventy-seven phosphorylations were upregulated upon adhesion, including PYK2/FAK2, Paxillin, CASL and p130CAS consistent with focal adhesion (FA) formation. We hypothesized that the collaborative signaling between ß1 integrin and gp130 (IL-6 beta receptor, IL-6 signal transducer) was mediated by FA formation and proline-rich tyrosine kinase 2 (PYK2) activity. Both pharmacological and molecular targeting of PYK2 attenuated the amplification of STAT3 phosphorylation under co-stimulatory conditions. Co-culture of MM cells with patient bone marrow stromal cells (BMSC) showed similar ß1 integrin-specific enhancement of PYK2 and STAT3 signaling. Molecular and pharmacological targeting of PYK2 specifically induced cell death and reduced clonogenic growth in BMSC-adherent myeloma cell lines, aldehyde dehydrogenase-positive MM cancer stem cells and patient specimens. Finally, PYK2 inhibition similarly attenuated MM progression in vivo. These data identify a novel PYK2-mediated survival pathway in MM cells and MM cancer stem cells within the context of microenvironmental cues, providing preclinical support for the use of the clinical stage FAK/PYK2 inhibitors for treatment of MM, especially in a minimal residual disease setting.

Dopamine has no effect on thyrotropin-releasing hormone mobilization of calcium from intracellular stores in rat anterior pituitary cells.

To investigate whether dopaminergic reduction of PRL secretion is caused by an inhibition of release of Ca2+ from intracellular stores, we used a perifusion system to monitor simultaneously changes in intracellular Ca2+ concentrations ([Ca2+]c) and changes in PRL secretion from rat anterior pituitary cells, and from enriched populations of lactotrophs. We eliminated influx of extracellular Ca2+ by using medium with no added Ca2+ and 0.2 mM EGTA, conditions which abolished the increase in [Ca2+]c caused by 56 mM KCl. In this low Ca2+-containing medium, 100 nM TRH induced a burst of [Ca2+]c; the magnitude of the peak was the same whether the cells were perifused with low Ca2+-containing medium for 2 or 20 min before adding TRH, indicating the source of intracellular Ca2+ was stable under these conditions. After 2 min in this medium, TRH was still able to stimulate almost as much PRL release as in medium containing 1.8 mM Ca2+, and 1 microM dopamine inhibited this release, but did not affect the magnitude of the TRH-induced increase in [Ca2+]c. Preincubation for 5 min with dopamine did not affect the ability of a 30-sec incubation with TRH to stimulate the accumulation of inositol phosphate, inositol bisphosphate, and inositol trisphosphate, either in medium containing 1.8 mM Ca2+, or in low Ca2+-containing medium. Preincubation with dopamine for 5 min had no effect on TRH-induced mobilization of intracellular calcium. A source of Ca2+ is needed to refill internal Ca2+-stores discharged by TRH, and as dopamine lowered [Ca2+]c which might fill these stores, we tested to see if dopamine prevented.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ transients induced by thyrotropin-releasing hormone rapidly lose their ability to cause release of prolactin.

To investigate the relationship of changes in cytosolic free calcium concentrations [( Ca2+]c) caused by TRH to changes in PRL secretion, we simultaneously monitored PRL release and [Ca2+]c, using the fluorescent Ca2+ indicator indo-1, in freshly isolated perifused cells from rat anterior pituitary glands. We found that a 30-sec pulse of 100 nM TRH triggered a transient spike of [Ca2+]c, but prolonged PRL release for up to 30 min; continuous administration of TRH caused a sustained elevation in [Ca2+]c, but the same pattern and amount of PRL release as that caused by the pulse of TRH. PRL secretion was refractory to further pulses of TRH given at 10-min intervals for 40 min, but did respond to a second pulse of TRH given 40 min after the first pulse with no intervening pulses. Pulses of TRH given every 10 min still triggered spikes of [Ca2+]c of the same magnitude as the first pulse, indicating that the cause of the refractory state must occur at a post-receptor step that is after the mobilization of [Ca2+]c. A 30-sec pulse of a high concentration of KCl caused a transient spike of [Ca2+]c and transient, not prolonged, release. Additional pulses of KCl cause progressively less PRL release, although the magnitude of the spikes in [Ca2+]c did not change.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression and characterization of recombinant type 2 3 alpha-hydroxysteroid dehydrogenase (HSD) from human prostate: demonstration of bifunctional 3 alpha/17 beta-HSD activity and cellular distribution.

In androgen target tissues, 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD) may regulate occupancy of the androgen receptor (AR) by catalyzing the interconversion of 5alpha-dihydrotestosterone (5alpha-DHT) (a potent androgen) and 3alpha-androstanediol (a weak androgen). In this study, a 3alpha-HSD cDNA (1170 bp) was isolated from a human prostate cDNA library. The human prostatic 3alpha-HSD cDNA encodes a 323-amino acid protein with 69.9%, 84.1%, 99.4%, and 87.9% sequence identity to rat liver 3alpha-HSD and human type 1, type 2, and type 3 3alpha-HSDs, respectively, and is a member of the aldo-keto reductase superfamily. The close homology with human type 2 3alpha-HSD suggests that it is either identical to this enzyme or a structural allele. Surprisingly, when the recombinant protein was expressed and purified from Escherichia coli, the enzyme did not oxidize androsterone when measured spectrophotometrically, an activity previously assigned to recombinant type 2 3alpha-HSD using this assay. Complete kinetic characterization of the purified protein using spectrophotometric, fluorometric, and radiometric assays showed that the catalytic efficiency favored 3alpha-androstanediol oxidation over 5alpha-DHT reduction. Using [14C]-5alpha-DHT as substrate, TLC analysis confirmed that the reaction product was [14C]-3alpha-androstanediol. However, in the reverse reaction, [3H]-3alpha-androstanediol was oxidized first to [3H]-androsterone and then to [3H]-androstanedione, revealing that the expressed protein possessed both 3alpha- and 17beta-HSD activities. The 17beta-HSD activity accounted for the higher catalytic efficiency observed with 3alpha-androstanediol. These findings indicate that, in the prostate, type 2 3alpha-HSD does not interconvert 5alpha-DHT and 3alpha-androstanediol but inactivates 5alpha-DHT through its 3-ketosteroid reductase activity. Levels of 3alpha-HSD mRNA were measured in primary cultures of human prostatic cells and were higher in epithelial cells than stromal cells. In addition, elevated levels of 3alpha-HSD mRNA were observed in epithelial cells derived from benign prostatic hyperplasia and prostate carcinoma tissues. Expression of 3alpha-HSD was not prostate specific, since high levels of mRNA were also found in liver, small intestine, colon, lung, and kidney. This study is the first complete characterization of recombinant type 2 3alpha-HSD demonstrating dual activity and cellular distribution in the human prostate.

A peptide histidine isoleucine/peptide histidine methionine-like peptide in the rabbit retina: colocalization with vasoactive intestinal peptide, synaptic relationships and activation of adenylate cyclase activity.

Antisera against peptide histidine isoleucine and peptide histidine methionine were found to label a subpopulation of amacrine and displaced amacrine cells in the rabbit retina with processes ramifying in sublaminas 1, 3 and 5 of the inner plexiform layer. Preadsorption controls demonstrated that this immunoreactivity was specific for a peptide histidine isoleucine- or peptide histidine methionine-like (peptide histidine isoleucine/peptide histidine methionine-like) peptide, and was not caused by cross-reactivity of the peptide histidine isoleucine or peptide histidine methionine antibodies with vasoactive intestinal peptide vasoactive intestinal peptide. In double-label studies, vasoactive intestinal peptide and peptide histidine isoleucine/peptide histidine methionine-like immunoreactivity were colocalized in the same population of retinal neurons. Electron microscopic analysis revealed that the peptide histidine isoleucine/peptide histidine methionine-labelled cells interacted with processes of bipolar cells, amacrine cells and ganglion cells. Peptide histidine methionine and peptide histidine isoleucine were slightly less potent than vasoactive intestinal peptide in stimulating adenylate cyclase activity in the rabbit retina, while the related peptides secretin, glucagon, and the C-terminal vasoactive intestinal peptide fragment, vasoactive intestinal peptide (10-28), showed little or no stimulatory activity. Stimulation of adenylate cyclase by high concentrations of vasoactive intestinal peptide and peptide histidine methionine were non-additive. These results suggest that a peptide histidine isoleucine/peptide histidine methionine-like peptide may function as a neuroactive peptide in the mammalian retina, and that this peptide appears to be cosynthesized and colocalized with vasoactive intestinal peptide and to mimic the activity of vasoactive intestinal peptide through interaction with vasoactive intestinal peptide receptor-adenylate cyclase complexes.

Ca2+ channel agonists enhance thyrotropin-releasing hormone-induced inositol phosphates and prolactin secretion.

The dihydropyridine Ca2+ channel activator BAY K 8644 (1 microM) stimulated basal prolactin secretion from perifused primary cultures of anterior pituitary cells and potentiated the stimulation of prolactin secretion by 1 microM thyrotropin-releasing hormone (TRH) 5-fold over 30 min. This potentiation was mimicked by other dihydropyridine agonists CGP 28392 and (+)-SDZ 202-791 and by (-)-BAY K 8644 (1 microM), but not by (+)-BAY K 8644. The Ca2+ channel antagonist nimodipine, at a concentration sufficient to block BAY K 8644-stimulated 45Ca2+ uptake in GH4C1 anterior pituitary tumor cells, decreased basal prolactin secretion and blocked the enhancement of basal and TRH-stimulated secretion by BAY K 8644. These results suggest that dihydropyridine agonists potentiate TRH-induced secretion through interaction with known stereospecific sites on Ca2+ channels. In GH4C1 cells, BAY K 8644 alone did not affect inositol polyphosphate accumulation, but potentiated TRH-stimulated accumulation of inositol 1,3,4-trisphosphate and inositol 1,3,4,5-tetrakisphosphate. Accumulation of the Ca(2+)-mobilizing isomer inositol 1,4,5-trisphosphate was not potentiated, suggesting that potentiation of TRH-stimulated hormone secretion by BAY K 8644 does not result from synergistic stimulation of phospholipase C, but may correlate with enhanced inositol trisphosphate-3-kinase activity.

Regulation of phospholipid hydrolysis and second messenger formation by protein kinase C.

The binding of a variety of agonists to their receptors leads to the breakdown of membrane phospholipids and the formation of intracellular second messengers. Hydrolysis of inositol phospholipids by phospholipase C results in the formation of two second messengers, inositol-1,4,5-trisphosphate which mobilizes intracellular calcium and the neutral lipid diacylglycerol (DAG) which binds to and activates protein kinase C (PKC). PKC is actually a family of homologous serine/threonine protein kinases which play a central role in regulation of growth, differentiation and secretion reactions in a variety of cell types. In addition to these feedforward roles of PKC, it is thought to play an important feedback role, regulating early events in signal transduction. To explore these feedback functions we have examined the effect of PKC inhibitors on second messenger formation in thrombin-stimulated human platelets (a rapidly responding system) and the effect of PKC overexpression on second messenger formation and mitogenesis in rat fibroblasts (a system where sustained signaling occurs). Treatment of platelets with inhibitors of PKC potentiates DAG mass formation in response to thrombin while prior activation of PKC with phorbol esters blocks DAG mass formation, consistent with PKC playing a negative feedback role, inhibiting inositol phospholipid breakdown. DAG can also be formed by the sequential hydrolysis of phosphatidylcholine by phospholipase D and phosphatidic acid phosphohydrolase. This is a minor reaction in the rapidly responding platelet system, but may play a role in sustained signaling events. We have found that fibroblasts which overexpress the beta 1 isozyme of PKC display greatly enhanced DAG formation and phospholipase D activation in response to phorbol ester treatment. Upon stimulation of fibroblasts with thrombin, phospholipase D activation is also enhanced by PKC overexpression while formation of inositol phosphates is suppressed. These data suggest that PKC may act as a switch, terminating inositol phospholipid hydrolysis and activating the hydrolysis of phosphatidylcholine. Furthermore, we have observed a strong correlation between activation of phospholipase D and mitogenesis, suggesting an important role for this enzyme in long-term cellular responses to activation.

Noncompetitive inhibition of inositol monophosphatase by K-76 monocarboxylic acid.

K-76COONa, a fungal product that was previously isolated for its inhibition of complement activation, was found to inhibit myo-inositol monophosphatase activity. K-76COONa was slightly more potent than lithium, with a Ki of approximately 0.5 mM. Kinetic analyses with D-myo-inositol 1-phosphate as the substrate showed that myo-inositol monophosphatase inhibition by K-76COONa was noncompetitive relative to substrate but competitive with activation by magnesium. Higher concentrations of K-76COONa were necessary to inhibit myo-[3H]inositol 1,4-bisphosphate hydrolysis by inositol 1,4-bisphosphate/inositol 1,3,4-trisphosphate 1-phosphatase (IC50 = approximately 7.5 mM). K-76COONa may be useful for further investigation of the mechanism of myo-inositol monophosphatase and for determination of whether inhibition of this enzyme plays a role in the therapeutic effectiveness of lithium in treatment of affective disorders.

Interactions between vasoactive intestinal peptide and dopamine in the rabbit retina: stimulation of a common adenylate cyclase.

Although 3,4-dihydroxyphenylethylamine (dopamine, DA) and vasoactive intestinal peptide (VIP) have been reported to stimulate adenylate cyclase activity in the rabbit retina, possible interactions between VIP-sensitive and DA-sensitive adenylate cyclase systems have not been previously investigated. To elucidate the interactions between these two putative transmitter-stimulated cyclase systems, the effects of VIP, DA, and VIP + DA on the conversion of [alpha-32P]ATP to [32P]cyclic AMP in rabbit retinal homogenates were measured. VIP stimulated adenylate cyclase activity in a biphasic manner, suggesting that two classes of VIP receptors may be involved in the induction of cyclic AMP formation. DA was less potent than VIP, and stimulated cyclase activity with a monophasic dose-response curve. When assayed together, these stimulations were partially nonadditive, implying the existence of a common adenylate cyclase pool that may be stimulated by both putative neurotransmitters. The dopaminergic antagonist (+)-butaclamol completely blocked dopaminergic stimulation, but had no significant effect on VIP-induced stimulation, indicating that VIP interacts with specific VIP receptor sites, which are distinct from the dopaminergic receptor sites. Furthermore, the specific D-2 dopaminergic receptor agonist LY141865 demonstrated no inhibitory effect on adenylate cyclase activity, suggesting that the interaction between the VIP- and DA-sensitive adenylate cyclase systems does not result from a D-2 receptor-mediated cyclase inhibition in the rabbit retina. Finally, at maximally effective concentrations, DA and VIP were less potent than fluoride or forskolin in the stimulation of cyclic AMP formation, suggesting that adenylate cyclase pools that are not sensitive to DA and VIP may also be present in this retina.(ABSTRACT TRUNCATED AT 250 WORDS)

Scintillation proximity assay to measure binding of soluble fibronectin to antibody-captured alpha 5 beta 1 integrin.

A scintillation proximity assay (SPA) has been developed to measure binding of alpha 5 beta 1 integrin, a heterodimeric cell-surface adhesion receptor, to fibronectin. This assay utilizes an anti-beta 1 integrin monoclonal antibody to simultaneously capture alpha 5 beta 1 from a cellular lysate and couple the integrin to anti-mouse IgG antibody-coated SPA beads for detection of 125I-fibronectin binding. The assay does not require prior purification of alpha 5 beta 1 nor physical separation of bound and free 125I-fibronectin. Chinese hamster ovary cells that stably overexpress human alpha 5 integrin (CHO#7 cells) were used as a source of alpha 5 beta 1 fibronectin receptor. Using the anti-hamster beta 1 monoclonal antibody 7E2 to capture alpha 5 beta 1 from a CHO#7 cell lysate, this SPA assay allowed measurement of specific 125I-fibronectin binding as defined by displacement by the Arg-Gly-Asp containing peptide GRGDSP or the anti-human alpha 5 antibody P1D6. IC50 values for displacement of 125I-fibronectin binding by GRGDSP and the novel cyclic peptides cRGDGF, cRGEGF, and cRRETAWA were 2.6, 0.045, 3.2, and 37 microM, respectively. Specific 125I-fibronectin binding to alpha 5 beta 1 from C8161 human melanoma cells was also measured using anti-human beta 1 antibodies. This method should be generally useful to measure cell-free ligand binding to receptors that are difficult to purify.

Deoxyribonucleic acid binding studies on several new anthracycline antitumor antibiotics. Sequence preference and structure--activity relationships of marcellomycin and its analogues as compared to adriamycin.

The deoxyribonucleic acid (DNA) binding characteristics of adriamycin and several new anthracycline glycosides, including marcellomycin, aclacinomycin, rudolfomycin, musettamycin, and pyrromycin, have been studied. The fluorescence spectra were determined for all six anthracyclines, and the fluorescence quenching effects caused by interactions with the natural DNAs poly(dAdT)--poly(dAdT) and poly(dGdC) were characterized. Binding parameters were determined by Scatchard analyses of results obtained by spectrofluorometric titrations of anthracyclines with DNA. Consistent with earlier structure--activity relationship studies of nucleic acid synthesis inhibitory effects, the results demonstrate a correlation between the length of the glycosidic side chain and DNA binding affinity. In addition, the sugar residue 2-deoxyfucose appears to confer greater DNA binding ability than do the sugars rednosamine and cinerulose when present in the terminal position of the glycosidic side chain, also in agreement with earlier studies. The sequence preference of anthracycline--DNA interaction has been examined by using DNAs of varying GC content, including the naturally occurring calf thymus DNA (43% GC), Clostridium perfringens DNA (28% GC), and Micrococcus luteus DNA (72% GC) and the synthetic double-stranded copolymers poly(dGdC)--poly(dGdC) and poly(dAdT)--POLY(DAdT). The results demonstrate that although adriamycin shows an absolute requirement for GC sequences for DNA binding, marcellomycin and its analogues showed no such sequence requirement. Furthermore, an AT preference for DNA binding was demonstrated with marcellomycin and its analogues.

TRH and BAY K 8644 synergistically stimulate prolactin release but not 45Ca2+ uptake.

Thyrotropin-releasing hormone (TRH) (1 microM) and the Ca2+-channel agonist BAY K 8644 (1 microM) each induced transient increases in prolactin secretion from primary cultures of rat anterior pituitary cells in perifusion. When BAY K 8644 was added after a TRH-induced secretory peak, the additional effect of BAY K 8644 on prolactin release was approximately twofold greater over a 30-min period than the effect of BAY K 8644 on previously untreated cells. TRH and BAY K 8644 were also synergistic when added in the opposite order or simultaneously. Substitution of other agents for BAY K 8644 revealed that only high K+ (40 mM) was at least additive with TRH in stimulating prolactin secretion; treatment with TRH inhibited, rather than facilitated, subsequent stimulation of prolactin secretion by angiotensin II (100 nM) or the ionophore A23187 (20 microM). The cooperative effect was not specific for TRH because BAY K 8644 also acted synergistically with angiotensin II or 40 mM K+. In GH4C1 cells, in which TRH and BAY K 8644 were also synergistic in releasing prolactin, measurements with the fluorescent indicator indo-1 showed that TRH and BAY K 8644 could each elevate cytosolic Ca2+ above the level stimulated by the other. Unexpectedly, TRH was found to inhibit BAY K 8644-stimulated 45Ca2+ uptake in both GH4C1 and primary cultured cells. These results indicate that BAY K 8644 and TRH synergistically stimulate prolactin secretion by a mechanism other than a cooperative effect on the activity of dihydropyridine-sensitive Ca2+ channels.

Bombesin stimulates inositol polyphosphate production in GH4C1 pituitary tumor cells: comparison with TRH.

The hormones bombesin and thyrotropin-releasing hormone (TRH) stimulated formation of inositol- monophosphate, bisphosphate, trisphosphate and tetrakisphosphate with parallel time courses in GH4C1 cells, while a more polar inositol polyphosphate peak, consisting of inositol-pentakisphosphate and perhaps also inositol-hexakisphosphate, was unaffected by either hormone. Although bombesin and TRH had similar potencies in stimulating inositol trisphosphate production (Km = 30 nM and 40 nM, respectively), TRH was significantly more efficacious than bombesin. Maximal stimulation of inositol-1,4,5-trisphosphate formation by TRH was not further increased by addition of a maximally effective dose of bombesin, suggesting that the two hormones act through stimulation of a common pool of phospholipase C, and this enzyme pool can be fully stimulated by TRH, alone.

Ability of repetitive Ca2+ spikes to stimulate prolactin release is frequency dependent.

Measurements of concentrations of cytosolic free Ca2+ ([Ca2+]c) in individual cells has frequently demonstrated periodic transients in [Ca2+]c rather than sustained elevated levels. To determine in anterior pituitary cells if such short and repetitive [Ca2+]c transients stimulated prolactin release, we used a perifusion system with cells loaded with the fluorescent Ca2+-indicator, indo-1. A one second pulse of 100 mM KCl caused an increase in [Ca2+]c with a half peak width of about 18 seconds and an almost coincident increase in prolactin secretion. Subsequent pulses of KCl each caused increases in [Ca2+]c and prolactin release that were the same as the first, up to a pulse frequency of one every two minutes. Increasing the frequency to 1 pulse every minute or 1 pulse every 30 seconds, however, resulted in a serial decline in the amount of prolactin released by each pulse even though each pulse caused a similar peak Ca2+ response. These findings demonstrate that cells become adapted to transient increases in [Ca2+]c of the same magnitude so that they no longer release prolactin if the increases in [Ca2+]c occur frequently enough. Cells may use frequency-encoded Ca2+ signals to stimulate release of prolactin at low frequency to prevent the adaptation that occurs at higher frequencies.

Multiple receptors coupled to adenylate cyclase regulate Na-H exchange independent of cAMP.

We have previously determined that beta-adrenergic and somatostatin receptors stimulate and inhibit, respectively, Na-H exchange independent of changes in cAMP accumulation (Barber, D.L., McGuire, M.E., and Ganz, M.B. (1989) J. Biol. Chem. 264, 21038-21042). The present study extends our work on the beta-adrenergic receptor (beta AR) by investigating receptor activation of Na-H exchange in multiple cell types that either endogenously express the beta AR or that have been transfected with cDNA of the hamster lung beta 2AR or the turkey erythrocyte beta AR. Exchanger activity was determined by monitoring intracellular pH in cell populations loaded with the pH-sensitive dye BCECF (2,7-biscarboxyethyl-5(6)-carboxyfluorescein). In addition to the action of the beta AR, activation of prostaglandin E1 and parathyroid hormone receptors induced an intracellular alkalinization by stimulating a Na(+)-dependent amiloride-sensitive Na-H exchange. In contrast, activation of D2-dopaminergic receptors induced an intracellular acidification by inhibiting Na-H exchange. beta-Adrenergic, prostaglandin E1, and parathyroid hormone receptors activated Na-H exchange independent of changes in intracellular cAMP accumulation and independent of a cholera toxin-sensitive stimulatory GTP regulatory protein. D2-dopaminergic receptors inhibited exchanger activity independent of a pertussis toxin-sensitive inhibitory GTP regulatory protein. We suggest that these receptors are functionally coupled to adenylate cyclase and Na-H exchange through divergent signaling mechanisms.

Endothelin (ETA) receptor number and calcium signalling are up-regulated by protein kinase C-beta 1 overexpression.

To evaluate the role of protein kinase C (PKC) in regulation of cellular responsiveness to mitogens, we used rat 6 (R6) fibroblasts that stably overexpress the beta 1 isoenzyme of protein kinase C (PKC-beta 1). The potent vasoconstrictor and mitogen endothelin-1 (ET-1; 100 nM) was substantially more effective in stimulating InsP3 accumulation in PKC-beta 1-overexpressing fibroblasts (PKC3 cells) than in control fibroblasts lacking the PKC-beta 1 cDNA insert. PKC3 cells were found to express a 7-fold greater number of endothelin receptors than did control cells, whereas both cell lines showed equivalent Kd values. These receptors were of the ETA subtype, as defined by a 1000-fold greater affinity for ET-1 than for ET-3. Changes in intracellular free Ca2+ levels ([Ca2+]i) in response to ET-1 measured with the fluorescent Ca2+ indicator fura-2 showed that ET-1 was more potent and efficacious in stimulating [Ca2+]i in PKC3 cells than in control fibroblasts. The ET-1-induced Ca2+ rise was completely blocked by the selective ETA antagonist BQ123, but only slightly diminished by extracellular application of 2 mM EGTA. In contrast with the effects of PKC-beta 1 overexpression on responsiveness to ET-1, alpha-thrombin, which was previously found to have a weaker effect on InsP3 accumulation in PKC-beta 1-overexpressing cells, was also a less effective stimulator of [Ca2+]i in PKC3 cells than in control cells. These results demonstrate that, although the Ca2+ response to alpha-thrombin is diminished by PKC-beta 1 overexpression, ETA receptor number and cellular responsiveness to ET-1 are increased in PKC-beta 1-overexpressing cells.

Overexpression of protein kinase C beta 1 enhances phospholipase D activity and diacylglycerol formation in phorbol ester-stimulated rat fibroblasts.

We are using a Rat-6 fibroblast cell line that stably overexpresses the beta 1 isozyme of protein kinase C (PKC) to study regulation of phospholipid hydrolysis by PKC. Stimulation of control (R6-C1) or overexpressing (R6-PKC3) cells with phorbol ester results in an increase in diacylglycerol (DAG) mass with no increase in inositol phosphates, indicating that DAG is not formed by inositol phospholipid breakdown. A more dramatic DAG increase occurs in R6-PKC3 cells (4.0-fold over basal) compared to R6-C1 cells (1.5-fold over basal). To further define the source of DAG, phosphatidylcholine (PC) pools were labeled with [3H]myristic acid or with [3H]- or [32P]alkyllyso-PC and formation of labeled phosphatidylethanol, an unambiguous marker of phospholipase D activation, was monitored. Phorbol ester-stimulated phosphatidylethanol formation is 5-fold greater in the R6-PKC3 cell line. Formation of radiolabeled phosphatidic acid (PA) is also enhanced by PKC overexpression. In cells double-labeled with [3H]- and [32P]-alkyl-lysoPC, the 3H/32P ratio of PA and PC are identical 15 min after stimulation, suggesting that a phospholipase D mechanism predominates. In support of this, the PA phosphohydrolase inhibitor propranolol decreased phorbol 12-myristate 13-acetate-stimulated DAG formation by 72%. Increases in DAG and phosphatidylethanol were inhibited by the PKC inhibitors K252a and staurosporine. These results indicate that phospholipase D is regulated by the action of PKC. Enhanced phospholipase D activity may contribute to the growth abnormalities seen in PKC-overexpressing cells.

Thapsigargin, but not caffeine, blocks the ability of thyrotropin-releasing hormone to release Ca2+ from an intracellular store in GH4C1 pituitary cells.

Thapsigargin stimulates an increase of cytosolic free Ca2+ concentration [( Ca2+]c) in, and 45Ca2+ efflux from, a clone of GH4C1 pituitary cells. This increase in [Ca2+]c was followed by a lower sustained elevation of [Ca2+]c, which required the presence of extracellular Ca2+, and was not inhibited by a Ca2(+)-channel blocker, nimodipine. Thapsigargin had no effect on inositol phosphate generation. We used thyrotropin-releasing hormone (TRH) to mobilize Ca2+ from an InsP3-sensitive store. Pretreatment with thapsigargin blocked the ability of TRH to cause a transient increase in both [Ca2+]c and 45Ca2+ efflux. The block of TRH-induced Ca2+ mobilization was not caused by a block at the receptor level, because TRH stimulation of InsP3 was not affected by thapsigargin. Rundown of the TRH-releasable store by Ca2(+)-induced Ca2+ release does not appear to account for the action of thapsigargin on the TRH-induced spike in [Ca2+]c, because BAY K 8644, which causes a sustained rise in [Ca2+]c, did not block Ca2+ release caused by TRH. In addition, caffeine, which releases Ca2+ from intracellular stores in other cell types, caused an increase in [Ca2+]c in GH4C1 cells, but had no effect on a subsequent spike in [Ca2+]c induced by TRH or thapsigargin. TRH caused a substantial decrease in the amount of intracellular Ca2+ released by thapsigargin. We conclude that in GH4C1 cells thapsigargin actively discharges an InsP3-releasable pool of Ca2+ and that this mechanism alone causes the block of the TRH-induced increase in [Ca2+]c.

Differential regulation of phosphoinositide and phosphatidylcholine hydrolysis by protein kinase C-beta 1 overexpression. Effects on stimulation by alpha-thrombin, guanosine 5'-O-(thiotriphosphate), and calcium.

Rat 6 fibroblasts that stably overexpress cDNA for the beta 1 isozyme of protein kinase C (PKC3 cells) were used to determine the effect of protein kinase C (PKC) overexpression on hormonal stimulation of phospholipid hydrolysis. In control Rat 6 cells, inositol trisphosphate levels (InsP3) were increased 9-fold in 15 s in response to 10 nM alpha-thrombin, compared with only a 2-fold increase in PKC3 cells. PKC overexpression also inhibited thrombin-stimulated production of 1,2-diacylglycerol, the other product of phosphatidylinositol 4,5-bisphosphate hydrolysis, by 73% at 15 s. In permeabilized cells, PKC overexpression greatly reduced guanosine thiotriphosphate-stimulated InsP3 accumulation, but did not affect InsP3 stimulation by increased free calcium concentration. These data suggest that desensitization of thrombin-stimulated phosphoinositide-phospholipase C is enhanced by PKC-beta 1 overexpression and may involve modulation of G-protein/phospholipase C coupling. In contrast, thrombin was 4.5-fold more effective in stimulation of phosphatidylcholine-phospholipase D activity in PKC3 cells than in control cells, as determined by phosphatidylethanol formation. In permeabilized cells, guanosine thiotriphosphate also stimulated phospholipase D activity more effectively in PKC3 cells than in control cells, suggesting that upregulation of phospholipase D activity by PKC overexpression occurs distal to the thrombin receptor. These results suggest that PKC may act as a switch to up-regulate phosphatidylcholine-phospholipase D and down-regulate phosphoinositide-phospholipase C stimulations.