Identification of the major physiologic phosphorylation site of human keratin 18: potential kinases and a role in filament reorganization.

There is ample in vitro evidence that phosphorylation of intermediate filaments, including keratins, plays an important role in filament reorganization. In order to gain a better understanding of the function of intermediate filament phosphorylation, we sought to identify the major phosphorylation site of human keratin polypeptide 18 (K18) and study its role in filament assembly or reorganization. We generated a series of K18 ser-->ala mutations at potential phosphorylation sites, followed by expression in insect cells and comparison of the tryptic 32PO4-labeled patterns of the generated constructs. Using this approach, coupled with Edman degradation of the 32PO4-labeled tryptic peptides, and comparison with tryptic peptides analyzed after labeling normal human colonic tissues, we identified ser-52 as the major K18 physiologic phosphorylation site. Ser-52 in K18 is not glycosylated and matches consensus sequences for phosphorylation by CAM kinase, S6 kinase and protein kinase C, and all these kinases can phosphorylate K18 in vitro predominantly at that site. Expression of K18 ser-52-->ala mutant in mammalian cells showed minimal phosphorylation but no distinguishable difference in filament assembly when compared with wild-type K18. In contrast, the ser-52 mutation played a clear but nonexclusive role in filament reorganization, based on analysis of filament alterations in cells treated with okadaic acid or arrested at the G2/M stage of the cell cycle. Our results show that ser-52 is the major physiologic phosphorylation site of human K18 in interphase cells, and that its phosphorylation may play an in vivo role in filament reorganization.

Keratins turn over by ubiquitination in a phosphorylation-modulated fashion.

Keratin polypeptides 8 and 18 (K8/18) are intermediate filament (IF) proteins that are expressed in glandular epithelia. Although the mechanism of keratin turnover is poorly understood, caspase-mediated degradation of type I keratins occurs during apoptosis and the proteasome pathway has been indirectly implicated in keratin turnover based on colocalization of keratin-ubiquitin antibody staining. Here we show that K8 and K18 are ubiquitinated based on cotransfection of His-tagged ubiquitin and human K8 and/or K18 cDNAs, followed by purification of ubiquitinated proteins and immunoblotting with keratin antibodies. Transfection of K8 or K18 alone yields higher levels of keratin ubiquitination as compared with cotransfection of K8/18, likely due to stabilization of the keratin heteropolymer. Most of the ubiquitinated species partition with the noncytosolic keratin fraction. Proteasome inhibition stabilizes K8 and K18 turnover, and is associated with accumulation of phosphorylated keratins, which indicates that although keratins are stable they still turnover. Analysis of K8 and K18 ubiquitination and degradation showed that K8 phosphorylation contributes to its stabilization. Our results provide direct evidence for K8 and K18 ubiquitination, in a phosphorylation modulated fashion, as a mechanism for regulating their turnover and suggest that other IF proteins could undergo similar regulation. These and other data offer a model that links keratin ubiquitination and hyperphosphorylation that, in turn, are associated with Mallory body deposits in a variety of liver diseases.

Dynamics of human keratin 18 phosphorylation: polarized distribution of phosphorylated keratins in simple epithelial tissues.

Phosphorylation of keratin polypeptides 8 and 18 (K8/18) and other intermediate filament proteins results in their reorganization in vitro and in vivo. In order to study functional aspects of human K18 phosphorylation, we generated and purified a polyclonal antibody (termed 3055) that specifically recognizes a major phosphorylation site (ser52) of human K18 but not dephosphorylated K18 or a ser52-->ala K18 mutant. Pulse-chase experiments followed by immunoprecipitation and peptide mapping of in vivo 32PO4-labeled K8/18 indicated that the overall phosphorylation turnover rate is faster for K18 versus K8, and that ser52 of K18 is a highly dynamic phosphorylation site. Isoelectric focusing of 32PO4 labeled K18 followed by immunoblotting with 3055 showed that the major phosphorylated K18 species contain ser52 phosphorylation but that some K18 molecules exist that are preferentially phosphorylated on K18 sites other than ser52. Immunoblotting of total cell lysates obtained from cells at different stages of the cell cycle showed that ser52 phosphorylation increases three to fourfold during the S and G2/M phases of the cell cycle. Immunofluorescence staining of cells at different stages of mitosis, using 3055 or other antibodies that recognize the total keratin pool, resulted in preferential binding of the 3055 antibody to the reorganized keratin fraction. Staining of human tissues or tissues from transgenic mice that express human K18 showed that the phospho-ser52 K18 species are located preferentially in the basolateral and apical domains in the liver and pancreas, respectively, but no preferential localization was noted in other simple epithelial organs examined. Our results support a model whereby phosphorylated intermediate filaments are localized in specific cellular domains depending on the tissue type and site(s) of phosphorylation. In addition, ser52 of human K18 is a highly dynamic phosphorylation site that undergoes modulation during the S and G2/M phases of the cell cycle in association with filament reorganization.

Chronic hepatitis, hepatocyte fragility, and increased soluble phosphoglycokeratins in transgenic mice expressing a keratin 18 conserved arginine mutant.

The two major intermediate filament proteins in glandular epithelia are keratin polypeptides 8 and 18 (K8/18). To evaluate the function and potential disease association of K18, we examined the effects of mutating a highly conserved arginine (arg89) of K18. Expression of K18 arg89-->his/cys and its normal K8 partner in cultured cells resulted in punctate staining as compared with the typical filaments obtained after expression of wild-type K8/18. Generation of transgenic mice expressing human K18 arg89-->cys resulted in marked disruption of liver and pancreas keratin filament networks. The most prominent histologic abnormalities were liver inflammation and necrosis that appeared at a young age in association with hepatocyte fragility and serum transaminase elevation. These effects were caused by the mutation since transgenic mice expressing wild-type human K18 showed a normal phenotype. A relative increase in the phosphorylation and glycosylation of detergent solubilized K8/18 was also noted in vitro and in transgenic animals that express mutant K18. Our results indicate that the highly conserved arg plays an important role in glandular keratin organization and tissue fragility as already described for epidermal keratins. Phosphorylation and glycosylation alterations in the arg mutant keratins may account for some of the potential changes in the cellular function of these proteins. Mice expressing mutant K18 provide a novel animal model for human chronic hepatitis, and for studying the tissue specific function(s) of K8/18.

Mutation of a major keratin phosphorylation site predisposes to hepatotoxic injury in transgenic mice.

Simple epithelia express keratins 8 (K8) and 18 (K18) as their major intermediate filament (IF) proteins. One important physiologic function of K8/18 is to protect hepatocytes from drug-induced liver injury. Although the mechanism of this protection is unknown, marked K8/18 hyperphosphorylation occurs in association with a variety of cell stresses and during mitosis. This increase in keratin phosphorylation involves multiple sites including human K18 serine-(ser)52, which is a major K18 phosphorylation site. We studied the significance of keratin hyperphosphorylation and focused on K18 ser52 by generating transgenic mice that overexpress a human genomic K18 ser52--> ala mutant (S52A) and compared them with mice that overexpress, at similar levels, wild-type (WT) human K18. Abrogation of K18 ser52 phosphorylation did not affect filament organization after partial hepatectomy nor the ability of mouse livers to regenerate. However, exposure of S52A-expressing mice to the hepatotoxins, griseofulvin or microcystin, which are associated with K18 ser52 and other keratin phosphorylation changes, resulted in more dramatic hepatotoxicity as compared with WT K18-expressing mice. Our results demonstrate that K18 ser52 phosphorylation plays a physiologic role in protecting hepatocytes from stress-induced liver injury. Since hepatotoxins are associated with increased keratin phosphorylation at multiple sites, it is likely that unique sites aside from K18 ser52, and phosphorylation sites on other IF proteins, also participate in protection from cell stress.

Mutation of human keratin 18 in association with cryptogenic cirrhosis.

Mutations in 11 of the more than 20 keratin intermediate filaments cause several epidermal and oral associated diseases. No disease-associated mutations have been described in keratin 8 or 18 (K8/18) which are the major keratin pair in simple-type epithelia, as found in the liver, pancreas, and intestine. However, transgenic mice that express mutant keratin 18 develop chronic hepatitis, and have an increased susceptibility to drug-induced hepatotoxicity. Also, ectopic expression of epidermal K14 in mouse liver results in chronic hepatitis, and disruption of mouse K8 leads to embryo lethality with extensive liver hemorrhage. We tested if patients with liver disease of unknown cause may harbor mutations in K18. We describe a his127-->leu (H127L) K18 mutation in a patient with cryptogenic cirrhosis that is germline transmitted. The K18 H127L isolated from the liver explant, or after expression in bacteria, showed an altered migration on two-dimensional gel analysis as compared with normal human liver or bacterially expressed K18. Electron microscopy of in vitro assembled K18 H127L and wild type K8 showed an assembly defect as compared with normal K8/18 assembly. Our results suggest that mutations in K18 may be predispose to, or result in cryptogenic cirrhosis in humans.

Susceptibility to hepatotoxicity in transgenic mice that express a dominant-negative human keratin 18 mutant.

Keratins 8 and 18 (K8/18) are intermediate filament phosphoglycoproteins that are expressed preferentially in simple-type epithelia. We recently described transgenic mice that express point-mutant human K18 (Ku, N.-O., S. Michie, R.G. Oshima, and M.B. Omary. 1995. J. Cell Biol. 131:1303-1314) and develop chronic hepatitis and hepatocyte fragility in association with hepatocyte keratin filament disruption. Here we show that mutant K18 expressing transgenic mice are highly susceptible to hepatotoxicity after acute administration of acetaminophen (400 mg/Kg) or chronic ingestion of griseofulvin (1.25% wt/wt of diet). The predisposition to hepatotoxicity results directly from the keratin mutation since nontransgenic or transgenic mice that express normal human K18 are more resistant. Hepatotoxicity was manifested by a significant difference in lethality, liver histopathology, and biochemical serum testing. Keratin glycosylation decreased in all griseofulvin-fed mice, whereas keratin phosphorylation increased dramatically preferentially in mice expressing normal K18. The phosphorylation increase in normal K18 after griseofulvin feeding appears to involve sites that are different to those that increase after partial hepatectomy. Our results indicate that hepatocyte intermediate filament disruption renders mice highly susceptible to hepatotoxicity, and raises the possibility that K18 mutations may predispose to drug hepatotoxicity. The dramatic phosphorylation increase in nonmutant keratins could provide survival advantage to hepatocytes.

Characterization of distinct early assembly units of different intermediate filament proteins.

We have determined the mass-per-length (MPL) composition of distinct early assembly products of recombinant intermediate filament (IF) proteins from the four cytoplasmic sequence homology classes, and compared these values with those of the corresponding mature filaments. After two seconds under standard assembly conditions (i.e. 25 mM Tris-HCl (pH 7.5), 50 mM NaCl, 37 degrees C), vimentin, desmin and the neurofilament triplet protein NF-L aggregated into similar types of "unit-length filaments" (ULFs), whereas cytokeratins (CKs) 8/18 already yielded long IFs at this time point, so the ionic strength had to be reduced. The number of molecules per filament cross-section, as deduced from the MPL values, was lowest for CK8/18, i.e. 16 and 25 at two seconds compared to 16 and 21 at one hour. NF-L exhibited corresponding values of 26 and 30. Vimentin ULFs yielded a pronounced heterogeneity, with major peak values of 32 and 45 at two seconds and 30, 37 and 44 after one hour. Desmin formed filaments of distinctly higher mass with 47 molecules per cross-section, at two seconds and after one hour of assembly. This indicates that individual types of IF proteins generate filaments with distinctly different numbers of molecules per cross-section. Also, the observed significant reduction of apparent filament diameter of ULFs compared to the corresponding mature IFs is the result of a "conservative" radial compaction-type reorganization within the filament, as concluded from the fact that both the immature and mature filaments contain very similar numbers of subunits per cross-section. Moreover, the MPL composition of filaments is strikingly dependent on the assembly conditions employed. For example, vimentin fibers formed in 0.7 mM phosphate (pH 7.5), 2.5 mM MgCl2, yield a significantly increased number of molecules per cross-section (56 and 84) compared to assembly under standard conditions. Temperature also strongly influences assembly: above a certain threshold temperature "pathological" ULFs form that are arrested in this state, indicating that the system is forced into strong but unproductive interactions between subunits. Similar "dead-end" structures were obtained with vimentins mutated to introduce principal alterations in subdomains presumed to be of general structural importance, indicating that these sequence changes led to new modes of intermolecular interactions.

Keratin modifications and solubility properties in epithelial cells and in vitro.

The gains that have been made in characterizing some of the keratin posttranslational modifications have helped answer some questions regarding these modifications and have generated an information base for asking additional refined questions in future studies. Highlights of where we believe we currently stand with regard to keratin posttranslational modifications are as follows: 1. Keratin glycosylation, via O-GlcNAc, is a dynamic modification that has been conclusively identified in K13, K8, and K18. Three serine glycosylation sites in the head domain of K18 have been identified, and it is possible that all keratins are glycosylated. The function of this modification remains to be defined, but is likely to be different from phosphorylation, since the two modifications are generally segregated on different molecules and several examples exist whereby both modifications increase simultaneously. 2. Keratin phosphorylation occurs within the tail and/or head domains of all keratins that have been examined. Several serine phosphorylation sites and some of the relevant kinases have been characterized in K8, K6, and K18, and serine/threonine sites have been identified in K1. Functions of keratin phosphorylation that have significant experimental support include a role in filament solubility and reorganization and a role in regulating keratin binding with other cytoplasmic proteins. The significance of filament reorganization and increased solubility under a variety of physiologic conditions such as mitosis and cell stress are important areas of future and ongoing investigation. Other associations with keratin phosphorylation include protection against cell stress, cell signaling, apoptosis, and cell compartment-specific roles. At this stage, however, it is not known if these associations play direct or indirect roles. 3. Keratin transglutamination occurs in epidermal and simple epithelial keratins under physiologic and pathologic states, respectively. In the physiological context, the role of this modification is clear in terms of providing a compact protective structure, while in the pathologic context of liver disease the role remains ambiguous. 4. Proteolysis of K18 and K19 by caspases occurs during apoptosis, and generates stable keratin fragments that are highly enriched within the cytoskeletal compartment. Proteolysis of the type II keratins appears to be spared for reasons that remain to be defined. It is likely that this apoptosis-associated degradation involves all type I keratins. Keratin fragments are also noted in sera of patients in association with a variety of epithelial tumors. If a signal does exist for the apoptosis-associated fragmentation, aside from caspase activation, then it appears that the overall increase in keratin phosphorylation during apoptosis does not account for this signal. 5. Keratins undergo several other posttranslational modifications including disulfide bond formation (not found in K8/18 due to lack of cystienes) and acetylation of their N-terminal serines. Modification by lipids is also possible, but this modification requires further confirmation. 6. Keratin solublility is highly dynamic and varies profoundly depending on the keratin pair and the physiologic state of the cell. Within the keratin family, simple epithelial keratins are among the most soluble (approximately 5% of K8/18 is soluble at basal conditions). Phosphorylation plays an important role in modulating keratin solubility, and distinct differences occur in site-specific phosphorylation depending on the soluble versus cytoskeletal partitioning of the keratin. Keratin solubility (at least for K8/18) also appears to be regulated by 14-3-3 proteins via K18 Ser33 phosphorylation.

IL-1 and IL-6 mediate increased production and synthesis by hepatocytes of acute-phase reactant mouse serum amyloid P-component (SAP).

Primary mouse hepatocytes exposed to the inflammatory cytokines IL-1 and IL-6 in vitro displayed an increase in the production of the major acute-phase reactant, serum amyloid P-component (SAP). Antiserum to recombinant human IL-6 selectively neutralized the SAP-inducing activity secreted by human diploid fibroblasts. Purified mouse interferon-beta (IFN-beta), but not IFN-alpha, also induced SAP production. Addition of 0.05 ng/ml of recombinant mouse IL-1 alpha induced a 10-fold increase in SAP production, whereas recombinant human and recombinant mouse IL-6 displayed optimal SAP-inducing activity of four-fold and seven-fold at 10 ng/ml and 1 unit/ml/2 x 10(5) mouse hepatocytes, respectively. The SAP-inducing activity was neutralized by antibodies to each of the recombinant cytokines. The kinetics of the SAP response in vitro was similar for all of the cytokines. Addition of a mixture of IL-1 and IL-6 to the hepatocytes resulted in SAP production that was not synergistic, but additive, over a range of concentrations for each cytokine. The increase in SAP production mediated by the cytokines was in part the result of an increase in the level of SAP mRNA. Metabolic incorporation of [35S]methionine into mouse SAP occurred in response to both IL-1 and IL-6. Therefore, mouse SAP should be classified among the subset of acute-phase proteins that can be induced by the direct action of either IL-1 or IL-6 on hepatocytes.

The mouse C-reactive protein (CRP) gene is expressed in response to IL-1 but not IL-6.

C-Reactive protein (CRP) is a minor acute phase reactant (APR) in the mouse, whereas CRP is the prototypical and one of the major positive APRs in all other mammals. MoCRP gene expression was tissue specific for the liver and induced by culture supernatants of LPS-activated macrophages. MoCRP gene expression by isolated hepatocytes in culture increased c, 3-fold in response to interleukin (IL)-1, but not IL-6. IL-6 is the most potent inflammatory cytokine for the induction of human CRP and many other APRs. By contrast, gene expression of the major APR of the mouse, serum amyloid P-component (SAP), a structural homologue of CRP, increased in response to either IL-1 or IL-6 under the same conditions. The region containing two potentially IL-1 responsive C/EBP elements in the moCRP gene failed to respond to IL-1 when a pCAT construct containing the elements was transfected into Hep 3B2 hepatoma cells. Therefore, IL-1 may influence the expression of the moCRP gene at the post-transcriptional rather than at the transcriptional level. The findings suggest that moCRP may be a minor APR because of the limited response of the gene to inflammatory cytokine signals.

Cloning and tissue-specific expression of the gene for mouse C-reactive protein.

C-reactive protein is a serum acute-phase reactant that increases several thousand-fold in concentration during inflammation in most mammals. However, mouse C-reactive protein is considered to be a minor acute-phase reactant, since its blood level increases only from approx. 0.1 to 1-2 micrograms/ml. A mouse genomic clone of approximately 5 kb was obtained to determine the molecular basis for the regulation of the expression of mouse C-reactive protein. Several cis-acting elements in the 5' flanking region that potentially regulate transcription were identified: two glucocorticoid-responsive elements, two CCAAT-enhancer-binding protein C (C/EBP) consensus elements that are required for the interleukin-1 responsiveness of some acute-phase reactant genes, an interleukin-6-responsive element, two hepatocyte nuclear factor-1 (HNF-1) elements and a single heat-shock element. Transfection of the hepatoma cell line Hep 3B.2 with a pCAT expression vector containing the 5' flanking sequence from -1083 to -3 bp from the transcriptional start site, and truncations of this sequence, localized elements that control the tissue-specific expression of mouse C-reactive protein to the two HNF-1 elements and a C/EBP, interleukin-1-responsive element located between -220 and -153, and -90 and -50 bp from the transcriptional start site. A constitutive nuclear protein from mouse-liver hepatocytes specifically binds to the HNF-1 elements. These findings explain the tissue-specific expression of the gene, as well as its limited expression during the acute-phase response.

Effect of mutation and phosphorylation of type I keratins on their caspase-mediated degradation.

Type I keratins K18 and K19 undergo caspase-mediated degradation during apoptosis. Two known K18 caspase cleavage sites are aspartates in the consensus sequences VEVDA and DALDS, located within the rod domain and tail domain, respectively. Several K14 (another type I keratin) mutations within the caspase cleavage motif have been described in patients with epidermolysis bullosa simplex. Here we use extensive mutational analysis to show that K19 and K14 are caspase substrates and that the ability to undergo caspase-mediated digestion of K18, K19, or K14 is highly dependent on the location and nature of the mutation within the caspase cleavage motif. Caspase cleavage of K14 occurs at the aspartate of VEMDA, a consensus sequence found in type I keratins K12-17 with similar but not identical sequences in K18 and K19. For K18, apoptosis-induced cleavage occurs sequentially, first at (393)DALD and then at (234)VEVD. Hyperphosphorylation of K18 protects from caspase-3 in vitro digestion at (234)VEVD but not at (393)DALD. Hence, keratins K12-17 are likely caspase substrates during apoptosis. Keratin hyperphosphorylation, which occurs early in apoptosis, protects from caspase-mediated K18 digestion in a cleavage site-specific manner. Mutations in epidermolysis bullosa simplex patients could interfere with K14 degradation during apoptosis, depending on their location.

Phosphorylation of human keratin 8 in vivo at conserved head domain serine 23 and at epidermal growth factor-stimulated tail domain serine 431.

Dynamic phosphorylation is one mechanism that regulates the more than 20 keratin type I and II intermediate filament proteins in epithelial cells. The major type II keratin in "simple type" glandular epithelia is keratin 8 (K8). We used biochemical and mutational approaches to localize two major in vivo phosphorylation sites of human K8 to the head (Ser-23) and tail (Ser-431) domains. Since Ser-23 of K8 is highly conserved among all type II keratins, we also examined if the corresponding Ser-59 in stratified epithelial keratin 6e is phosphorylated. Mutation of K6e Ser-59 abolished its phosphorylation in 32PO4-labeled baby hamster kidney cell transfectants. With regard to K8 phosphorylation at Ser-431, it increases dramatically upon stimulation of cells with epidermal growth factor (EGF) or after mitotic arrest and is the major K8 phosphorylated residue after incubating K8 immunoprecipitates with mitogen-activated protein or cdc2 kinases. A monoclonal antibody that specifically recognizes phosphoserine 431-K8 manifests increased reactivity with K8 and recognizes reorganized K8/18 filaments after EGF stimulation. Our results suggest that in vivo serine phosphorylation of K8 and K6e within the conserved head domain motif is likely to reflect a conserved phosphorylation site of most if not all type II keratins. Furthermore, K8 Ser-431 phosphorylation occurs after EGF stimulation and during mitotic arrest and is likely to be mediated by mitogen-activated protein and cdc2 kinases, respectively.

Expression, glycosylation, and phosphorylation of human keratins 8 and 18 in insect cells.

The filament forming ability and post-translational modifications of the human intermediate filaments, keratin polypeptides 8 and 18 (K8/18), were studied in recombinant baculovirus-infected insect (Spodoptera frugiperda, Sf9) cells. No change in cell morphology was noted after high levels of K8/18 were expressed in Sf9 cells coinfected with recombinant virus-containing human K8 and K18. Immunofluorescence staining showed that K8/18 expressed in Sf9 cells formed somewhat what disorganized and rope-like filaments, in contrast with K8 or K18 expression alone, which did not result in filament formation. K8/18 expressed in Sf9 cells were glycosylated (O-linked N-acetylglucosamine) and phosphorylated, and each modification occurred on different molecules of K8 and K18, as previously found in human HT29 cells. The glycosylation and phosphorylation of K18 in human and insect cells were very similar as determined by tryptic peptide mapping and localization to the head and proximal rod domains. In contrast, differences were noted in the relative intensity of the tryptic phospho- and glycopeptides of K8 expressed in human and insect cells and in the ratio of K8 to K18 phosphorylation in human and insect cells. Our results show that although quantitative differences exist, the post-translational modification of K8/18 expressed in insect cells is quite similar to its mammalian counterpart, especially for K18. Baculovirus expressed K8/18 should prove useful for mapping phosphorylation and glycosylation sites and for studying factors involved in organized filament assembly in mammalian cells.

Identification and mutational analysis of the glycosylation sites of human keratin 18.

Keratin polypeptides 8 and 18 (K8/18) are intermediate filament phosphoglycoproteins that are expressed preferentially in glandular epithelia. We previously showed that K8/18 phosphorylation occurs on serine residues and that K8/18 glycosylation consists of O-linked single N-acetylglucosamines (O-GlcNAc) that are linked to Ser/Thr. Since the function of these modifications is unknown, we sought as a first step to identify the precise modification sites and asked if they play a role in keratin filament assembly. For this, we generated a panel of K18 Ser and Thr-->Ala mutants at potential glycosylation sites followed by expression in a baculovirus-insect cell system. We identified the major glycosylation sites of K18 by comparing the tryptic 3H-glycopeptide pattern of the panel of mutant and wild type K18 expressed in the insect cells with the glycopeptides of K18 in human colonic cells. The identified sites occur on three serines in the head domain of K18. The precise modified residues in human cells were verified using Edman degradation and confirmed further by the lack of glycosylation of a K18 construct that was mutated at the molecularly identified sites then transfected into NIH-3T3 cells. Partial or total K18 glycosylation mutants transfected into mammalian cells manifested nondistinguishable filament assembly to cells transfected with wild type K8/18. Our results show that K18 glycosylation sites share some features with other already identified O-GlcNAc sites and may together help predict glycosylation sites of other intermediate filament proteins.

Regulation of cytokine-induced human C-reactive protein production by transforming growth factor-beta.

Transforming growth factor-beta (TGF-beta) modified production of the major human acute phase reactant, C-reactive protein (CRP), induced by the inflammatory cytokines, IL-1 beta or IL-6. CRP mRNA accumulation in the hepatoma PLC/PRF/5 cell line was slightly more rapid, but of smaller magnitude in response to IL-1 beta (fourfold increase) than to IL-6 (10-fold increase); however, the amount of CRP protein accumulating in the culture medium was similar for both cytokines. TGF-beta at concentrations greater than or equal to 0.1 pg/ml inhibited the induced IL-1 or IL-6 CRP production; whereas concentrations less than 0.1 pg/ml slightly enhanced CRP synthesis. Addition of TGF-beta to the cultures up to 16 h after the PLC/PRF/5 cells were already exposed to IL-1 or IL-6 resulted in the cessation of CRP production. CRP mRNA accumulated in hepatoma cells treated with both TGF-beta and IL-6, although CRP protein synthesis was inhibited. A similar pattern of inhibition of CRP production by TGF-beta occurred when Hep 3B.2 cells were treated with a mixture of IL-1 and IL-6. Enhanced production of CRP was observed only when TGF-beta was added to the cells before the cytokine. This enhanced CRP response was sensitive to cycloheximide. TGF-beta added along with IL-6 inhibited the metabolic labeling of CRP with [35S]methionine; however, enhanced incorporation of [35S]methionine into CRP was observed when the cells were exposed to TGF-beta before IL-6 addition. Therefore, TGF-beta is potentially a potent regulator of CRP synthesis by hepatocytes at the post-transcriptional level.

Stress, apoptosis, and mitosis induce phosphorylation of human keratin 8 at Ser-73 in tissues and cultured cells.

Simple epithelia express keratins 8 (K8) and 18 (K18) as their major intermediate filament proteins. We previously showed that several types of cell stress such as heat and virus infection result in a distinct hyperphosphorylated form of K8 (termed HK8). To better characterize K8/18 phosphorylation, we generated monoclonal antibodies by immunizing mice with hyperphosphorylated keratins that were purified from colonic cultured human HT29 cells pretreated with okadaic acid. One antibody specifically recognized HK8, and the epitope was identified as 71LLpSPL which corresponds to K8 phosphorylation at Ser-73. Generation of HK8 occurs in mitotic HT29 cells, basal crypt mitotic cells in normal mouse intestine, and in regenerating mouse hepatocytes after partial hepatectomy. Prominent levels of HK8 were also generated in HT29 cells that were induced to undergo apoptosis using anisomycin or etoposide. In addition, mouse hepatotoxicity that is induced by chronic feeding with griseofulvin resulted in HK8 formation in the liver. Our results demonstrate that a "reverse immunological" approach, coupled with enhancing in vivo phosphorylation using phosphatase inhibitors, can result in the identification of physiologic phosphorylation states. As such, K8 Ser-73 phosphorylation generates a distinct HK8 species under a variety of in vivo conditions including mitosis, apoptosis, and cell stress. The low steady state levels of HK8 during mitosis, in contrast to stress and apoptosis, suggest that accumulation of HK8 may represent a physiologic stress marker for simple epithelia.

Apoptosis generates stable fragments of human type I keratins.

Type I and II keratins help maintain the structural integrity of epithelial cells. Since apoptosis involves progressive cell breakdown, we examined its effect on human keratin polypeptides 8, 18, and 19 (K8, K18, K19) that are expressed in simple-type epithelia as noncovalent type I (K18, K19) and type II (K8) heteropolymers. Apoptosis induces rapid hyperphosphorylation of most known K8/18 phosphorylation sites and delayed formation of K18 and K19 stable fragments. In contrast, K8 is resistant to proteolysis and remains associated with the K18 fragments. Transfection of phosphorylation/glycosylation-mutant K8 and K18 does not alter fragment formation. The protein domains of the keratin fragments were determined using epitope-defined antibodies, and microsequencing indicated that K18 cleavage occurs at a conserved caspase-specific aspartic acid. The fragments are found preferentially within the detergent-insoluble pool and can be generated, in a phosphorylation-independent manner, by incubating keratins with caspase-3 or with detergent lysates of apoptotic cells but not with lysates of nonapoptotic cells. Our results indicate that type I keratins are targets of apoptosis-activated caspases, which is likely a general feature of keratins in most if not all epithelial cells undergoing apoptosis. Keratin hyperphosphorylation occurs early but does not render the keratins better substrates of the downstream caspases.

Phosphorylation of human keratin 18 serine 33 regulates binding to 14-3-3 proteins.

Members of the 14-3-3 protein family bind the human intermediate filament protein keratin 18 (K18) in vivo, in a cell-cycle- and phosphorylation-dependent manner. We identified K18 Ser33 as an interphase phosphorylation site, which increases its phosphorylation during mitosis in cultured cells and regenerating liver, and as an in vitro cdc2 kinase phosphorylation site. Comparison of wild-type versus K18 Ser33-->Ala/Asp transfected cells showed that K18 Ser33 phosphorylation is essential for the association of K18 with 14-3-3 proteins, and plays a role in keratin organization and distribution. Mutation of another K18 major phosphorylation site (Ser52) or K18 glycosylation sites had no effect on the binding of K18 to 14-3-3 proteins. The K18 phospho-Ser33 motif is different from several 14-3-3-binding phosphomotifs already described. Antibodies that are specific to K18 phospho-Ser33 or phospho-Ser52 show that although Ser52 and Ser33 phosphorylated K18 molecules manifest partial colocalization, these phosphorylation events reside predominantly on distinct K18 molecules. Our results demonstrate a unique K18 phosphorylation site that is necessary but not sufficient for K18 binding to 14-3-3 proteins. This binding is likely to involve one or more mitotic events coupled to K18 Ser33 phosphorylation, and plays a role in keratin subcellular distribution. Physiological Ser52 or Ser33 phosphorylation on distinct K18 molecules suggests functional compartmentalization of these modifications.