Embryonic simple epithelial keratins 8 and 18: chromosomal location emphasizes difference from other keratin pairs.

The keratins 8 and 18 of simple epithelia differ from stratified epithelial keratins in tissue expression and regulation. To examine the specific properties of human keratin 8, we cloned and sequenced the cDNA from a placental mRNA expression library and defined the optimum state of such clones for expression in bacterial plasmid vectors. Using the polymerase chain reaction we identified and sequenced three introns and located the single active gene for keratin 8, out of a background of 9 to 24 pseudogenes, on chromosome 12. This chromosome contains several genes for type II keratins and also the gene for keratin 18, the type I keratin that is coexpressed with keratin 8. This location of both members of a keratin pair on a single chromosome is thus far unique among the keratin genes; it is consistent with the hypothesis that keratins 8 and 18 may be closer to an ancestral keratin gene than the keratins of more highly differentiated epithelia.     

The mechanical properties of simple epithelial keratins 8 and 18: discriminating between interfacial and bulk elasticities

The abundance and cytoplasmic organization of keratin filaments enables them to contribute to the maintenance of structural integrity in epithelial tissues. Co-polymers of the type II keratin 8 and type I keratin 18 form the major intermediate filament network in simple epithelia. We investigated the mechanical properties of K8-K18 filament suspensions using rheological assays in conjunction with light and electron microscopy. Suspensions of K8-K18 filaments behave like a viscoelastic solid under standard assembly conditions. Bulk elasticity is weakly dependent on deformation frequency but is very sensitive to the concentration (G' approximately C1.5) and size of individual keratin polymers, in agreement with recent models of semiflexible-polymer physics. K8-K18 filaments can self-organize to form a bundled network that exhibits gel-like mechanical properties. In all cases the mechanical properties of the suspensions correlate with the structural features of individual polymers, as seen under light and electron microscopy. Importantly, these bulk viscoelastic properties of K8-K18 filaments are revealed only when interfacial elastic effects are minimized by the application of phospholipids at the air-liquid interface. Suspensions of K5-K14 and vimentin filaments also exhibit interfacial elasticity, which distorts the interpretation of the viscoelastic moduli as determined by standard rheometry. The potential for modulation of mechanical properties through self-organization may be a general property of keratin polymers and contribute to their organization and function in vivo.     

Localization of the gene for human simple epithelial keratin 18 to chromosome 12 using polymerase chain reaction.

Many human genes encoding keratin intermediate filament proteins are clustered on chromosomes 17 (the type I genes) and 12 (the type II genes). Some have not yet been localized, notably the genes for the primary embryonic keratins 8 and 18, normally expressed in simple epithelia: this is because the numerous pseudogenes for these keratins have made it difficult to identify the true functional gene in each case. Through the use of human-specific primers from within introns of the published gene sequence for human type I keratin 18, human genomic DNA has been specifically amplified using the polymerase chain reaction. A single reaction product was obtained. DNA from a characterized series of mouse-human somatic cell hybrid lines was tested for the presence of sequences able to initiate the chain reaction from these primers, and the presence or absence of this genomic DNA PCR product allowed us to assign a gene for human keratin 18 to chromosome 12 unambiguously. This differs from the location of other human type I keratins on chromosome 17 and may indicate the early divergence of the genes for stratifying cell keratins from that of simple, or embryonic, keratin 18.     

The quest for the function of simple epithelial keratins

Simple epithelial keratins K8 and K18 are components of the intracellular cytoskeleton in the cells of the single-layered sheet tissues inside the body. As members of the intermediate filament family of proteins, their function has been a matter for debate since they were first discovered. Whilst there is an indisputable case for a structural cell-reinforcing function for keratins in the mutilayered squamous epithelia of external barrier tissues, some very different stress-protective features now seem to be emerging for the simple epithelial keratins. Even the emerging evidence of pathological mutations in K8/K18 looks very different from mutations in stratified epithelial keratins. K8/K18-like keratins were probably the first to evolve and, whilst stratified epithelial (keratinocyte) keratins have diversified into a large group of keratins highly specialised for providing mechanical stability, the simple epithelial keratins have retained early features that may protect the internal epithelia from a broader range of stresses, including osmotic stress and chemical toxicity.     

Sedimentation studies of epidermal keratins: keratin A and keratin B

Electrophoretically homogeneous keratin A and keratin B were studied in the ultracentrifuge. Both preparations revealed two fractions: one which sedimented rapidly and another which sedimented slowly. This indicated that both preparations are heterogeneous with respect to particle size.     

Posttranslational regulation of keratins: degradation of mouse and human keratins 18 and 8.

Human keratin 18 (K18) and keratin 8 (K8) and their mouse homologs, Endo B and Endo A, respectively, are expressed in adult mice primarily in a variety of simple epithelial cell types in which they are normally found in equal amounts within the intermediate filament cytoskeleton. Expression of K18 alone in mouse L cells or NIH 3T3 fibroblasts from either the gene or a cDNA expression vector results in K18 protein which is degraded relatively rapidly without the formation of filaments. A K8 cDNA containing all coding sequences was isolated and expressed in mouse fibroblasts either singly or in combination with K18. Immunoprecipitation of stably transfected L cells revealed that when K8 was expressed alone, it was degraded in a fashion similar to that seen previously for K18. However, expression of K8 in fibroblasts that also expressed K18 resulted in stabilization of both K18 and K8. Immunofluorescent staining revealed typical keratin filament organization in such cells. Thus, expression of a type I and a type II keratin was found to be both necessary and sufficient for formation of keratin filaments within fibroblasts. To determine whether a similar proteolytic system responsible for the degradation of K18 in fibroblasts also exists in simple epithelial cells which normally express a type I and a type II keratin, a mutant, truncated K18 protein missing the carboxy-terminal tail domain and a conserved region of the central, alpha-helical rod domain was expressed in mouse parietal endodermal cells. This resulted in destabilization of endogenous Endo A and Endo B and inhibition of the formation of typical keratin filament structures. Therefore, cells that normally express keratins contain a proteolytic system similar to that found in experimentally manipulated fibroblasts which degrades keratin proteins not found in their normal polymerized state.     

Complex formation and kinetics of filament assembly exhibited by the simple epithelial keratins K8 and K18

We have generated human recombinant keratins K8 and K18 and describe conditions to quantitatively follow their assembly into filaments. When renatured individually from 8M urea into a low ionic strength/high pH-buffer, K8 was present in a dimeric to tetrameric form as revealed by analytical ultracentrifugation. In contrast, K18 sedimented as a monomer. When mixed in 8 M urea and renatured together, K8 and K18 exhibited s-value profiles compatible with homogeneous tetrameric complexes. This finding was confirmed by sedimentation equilibrium centrifugation. Subsequently, these tetrameric starter units were subjected to assembly experiments at various protein concentrations. At low values such as 0.0025 g/l, unit-length filaments were abundantly present after 2s of assembly. During the following 5 min, filaments grew rapidly and by measuring the length of individual filaments we were able to generate time-dependent length profiles. These data revealed that keratins K8/K18 assemble several times faster than vimentin and desmin. In addition, we determined the persistence length l(p) of K8/K18 filaments to be in the range of 300 nm. Addition of 1 mM MgCl(2) increases l(p) to 480 nm indicating that magnesium ions affect the interaction of keratin subunits within the filament during assembly to some extent.     

Differential expression of keratin 19 in normal human epithelial tissues revealed by monospecific monoclonal antibodies

Summary Three monospecific monoclonal antibodies (BA16, BA17 and A53—B/A2) recognizing different epitopes of the human keratin 19 were used to determine tissue distribution of this 40 kDa keratin polypeptide. Immunohistochemical methods revealed four different staining patterns among normal human epithelial tissues: firstly, complete negativity of the epidermis, sebaceous glands, hepatocytes and other tissues; secondly, homogeneous positivity as seen for example in the gall bladder and urinary bladder epithelium, endometrium and many other epithelia; thirdly, a mosaic of positive and negative cells among mammary gland luminal cells, prostate epithelia and some other epithelia and fourthly, a more complex heterogeneous pattern found in non-keratinizing squamous epithelia and hair follicles with generally the basal layer being the most strongly or sometimes exclusively stained. The pattern seen in non-keratinizing squamous epithelia varied considerably according to the fixation method and the antibody used as well as among different donors and in different areas of the same organ. The other three staining patterns were on the other hand nearly identical with all three antibodies on both frozen sections and sections of methacarn-fixed paraffinembedded tissues. Our results provide evidence for differential expression of the human keratin 19 at the single cell level, an observation which could be exploited in the study of epithelial differentiation and pathology.     

A single human keratin 18 gene is expressed in diverse epithelial cells of transgenic mice

The expression of keratin 18 (K18) is restricted in humans primarily to a variety of single layered or simple epithelia. However, direct introduction of a cloned K18 gene into cultured, somatic cells by DNA transfection has been shown to result in the promiscuous expression of K18 even while the endogenous mouse form of K18 (Endo B) remains silent. To determine if the cloned K18 genomic DNA fragment contains sufficient information to be regulated appropriately when subjected to a normal developmental environment, and to determine if the cloned gene is expressed in diverse epithelia, the K18 gene, including 2.5 kb of 5' flanking sequence and 3.5 kb of 3' flanking sequence, has been introduced into the germ line of mice. Mice from all three resulting K18 transgenic lines express the gene in an appropriate tissue-specific pattern that includes hepatocytes, simple epithelia of the intestinal tract, ductal cells of several glands and epithelial cells of the thymus. No expression of K18 was found in muscle, heart, or in most of the brain even in mice carrying 18 copies of the K18 gene. In most tissues, the level of K18 RNA was directly proportional to copy number and was as efficiently expressed as the endogenous Endo B gene. The K18 protein was identified by both protein blotting methods and indirect immunofluorescence staining. No pathological consequences of overexpression of the K18 gene were observed. The cloned K18 gene appears to contain all cis-acting DNA sequences necessary for appropriate expression. In addition, diverse epithelial cell types are able to express this single human gene.     

14-3-3 proteins associate with phosphorylated simple epithelial keratins during cell cycle progression and act as a solubility cofactor

14-3-3 is a ubiquitous protein family that interacts with several signal transduction kinases. We show that 14-3-3 proteins associate with keratin intermediate filament polypeptides 8 and 18 (K8/18) that are expressed in simple-type epithelia. The association is stoichiometrically significant (> or = one 14-3-3 molecule/keratin tetramer), occurs preferentially with K18, and is phosphorylation- and cell cycle-dependent in that it occurs during S/G2/M phases of the cell cycle when keratins become hyperphosphorylated. Binding of phospho- K8/18 to 14-3-3 can be reconstituted in vitro using recombinant 14-3-3 or using total cellular cytosol. Phosphatase treatment results in dissociation of 14-3-3, and dephosphorylation of phospho-K8/18 prevents reconstitution of the binding. Three cellular keratin subpopulations were analyzed that showed parallel gradients of keratin phosphorylation and 14-3-3 binding. Incubation of 14-3-3 with keratins during or after in vitro filament assembly results in sequestering of additional soluble keratin, only in cases when the keratins were hyperphosphorylated. Our results demonstrate a stoichiometrically significant cell cycle- and phosphorylation-regulated binding of 14-3-3 proteins to K18 and in vitro evidence of a simple epithelial keratin sequestering role for 14-3-3 proteins.     

Mutation in keratin 18 induces mitochondrial fragmentation in liver-derived epithelial cells

Microtubules (MTs) and microfilaments (MFs) are known to modulate mitochondrial morphology, distribution and function. However, little is known evidence about the role of intermediate filaments (IFs) in modulating mitochondria except desmin. To investigate whether or not the IFs regulate mitochondrial morphology, distribution, and function, we manipulated the IFs of cultured epithelial cells to express a mutant keratin 18 (K18). In contrast to the filamentous expression of wild K18, mutant K18 induced aggregation of K8/18, showing no fine IF network in the cells. In mutant K18-transfected cells, the mitochondria were fragmented into small spheroids, although they were observed as mitochondrial fibers in un-transfected or wild K18-transfected cells. Fluorescence recovery after photobleaching of fluorescence-labeled mitochondria was markedly less in the mutant K18-transfected cells, although a significant recovery was confirmed in wild K18-transfected cells. These findings suggest that the IFs are important for the maintenance of normal mitochondrial structures.     

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.     

Transient expression of simple epithelial keratins by mesenchymal cells of regenerating newt limb

Structural proteins of the intermediate filament family are an early indicator of differentiation before organogenesis becomes apparent. Keratin intermediate filaments are characteristically expressed only by epithelial and not by mesenchymal cells. Here we show, using monoclonal antibodies, a transient expression of the keratin pair 8 and 18 in a population of mesenchymal cells in the regenerating newt limb, specifically in the undifferentiated progenitor cells (blastemal cells) which give rise to the new tissues. These keratins are also expressed in cultured limb cells that can differentiate into muscle. In contrast no reactivity with anti-keratin 8 and 18 antibodies was observed in the newt limb bud at an early stage of development, indicating a molecular difference between the developing and regenerating limb. The molecular weights of the newt proteins detected by these antibodies are very similar to those of human keratins 8 and 18, further supporting the immunocytochemical evidence that the newt homologs of these keratins are expressed in blastemal cells. This is the first demonstration of keratin expression in mesenchymal progenitor cells in an adult animal.     

Three epidermal and one simple epithelial type II keratin genes map to human chromosome 12.

We have localized the genes which encode the human type II epidermal keratins K5, K6a, and K6b and the simple epithelial keratin K7 (KRT5, KRT6A, KRT6B, and KRT7, respectively) to chromosome 12 using Southern blot analysis of somatic cell hybrids. In addition, we have sublocalized the genes for K6a and K7 to bands 12q12----q14 on the long arm of this chromosome by in situ hybridization of metaphase chromosomes.     

Expression and distribution of vimentin and keratin filaments in heterokaryons of human fibroblasts and amnion epithelial cells

The expression of intermediate filaments of the keratin- and the vimentin-type was studied in heterokaryons of human fibroblasts and amnion epithelial cells by immunofluorescence microscopy. Fibroblasts and their homokaryons showed a fibrillar, vimentin-specific fluorescence throughout the cytoplasm but were negative when stained for keratin. Amnion epithelial cells and their homokaryons, on the other hand, showed a keratin-specific fibrillar staining, and only some of them contained also detectable vimentin. When suspended epithelial cells were fused with adherent fibroblasts, keratin fibrils spread within 3 h into the fibroblasts, intermixing with the vimentin fibrils. 1-3 d after fusion, both vimentin and keratin filaments were expressed as typical fibrillar cytoplasmic arrays, and the distribution of keratin in heterokaryons resembled closely that of vimentin. A typical cell-to-cell arrangement of keratin fibrils, seen in cultures of amnion epithelial cells, could also be found between heterokaryons. Treatment of the cultures with vinblastine sulphate induced coiling of the vimentin filaments in both homo- and heterokaryons, whereas the keratin organization was only slightly affected. Our results show that both vimentin and keratin filaments are incorporated into the cytoskeleton of heterokaryons formed between fibroblasts and epithelial cells, and that they behave in the same way as in their parental cells. Both epithelial and fibroblastic characteristics thus appear to the coexpressed in such heterokaryons.     

Type II keratins are phosphorylated on a unique motif during stress and mitosis in tissues and cultured cells

Epithelial cell keratins make up the type I (K9-K20) and type II (K1-K8) intermediate filament proteins. In glandular epithelia, K8 becomes phosphorylated on S73 ((71)LLpSPL) in human cultured cells and tissues during stress, apoptosis, and mitosis. Of all known proteins, the context of the K8 S73 motif (LLS/TPL) is unique to type II keratins and is conserved in epidermal K5/K6, esophageal K4, and type II hair keratins, except that serine is replaced by threonine. Because knowledge regarding epidermal and esophageal keratin regulation is limited, we tested whether K4-K6 are phosphorylated on the LLTPL motif. K5 and K6 become phosphorylated in vitro on threonine by the stress-activated kinase p38. Site-specific anti-phosphokeratin antibodies to LLpTPL were generated, which demonstrated negligible basal K4-K6 phosphorylation. In contrast, treatment of primary keratinocytes and other cultured cells, and ex vivo skin and esophagus cultures, with serine/threonine phosphatase inhibitors causes a dramatic increase in K4-K6 LLpTPL phosphorylation. This phosphorylation is accompanied by keratin solubilization, filament reorganization, and collapse. K5/K6 LLTPL phosphorylation occurs in vivo during mitosis and apoptosis induced by UV light or anisomycin, and in human psoriatic skin and squamous cell carcinoma. In conclusion, type II keratins of proliferating epithelia undergo phosphorylation at a unique and conserved motif as part of physiological mitotic and stress-related signals.     

Apoptosis-induced cleavage of keratin 15 and keratin 17 in a human breast epithelial cell line

Keratin 15 (K15) and keratin 17 (K17) are intermediate filament (IF) type I proteins that are responsible for the mechanical integrity of epithelial cells. By analyzing the human breast epithelial cell line H184A1 before and after induction of apoptosis by high-resolution two-dimensional gel electrophoresis (2-DE) we identified the caspase-mediated cleavage of keratins 15 and 17. After induction of apoptosis three fragments of both K15 and K17 could be observed by 2 -DE. K15 and K17 proteolysis was observed during staurosporine-induced apoptosis and anoikis (anchorage-dependent apoptosis) as well and was shown to be caspase-dependent. By using mass spectrometry we could determine the caspase cleavage sites, one in K15 and two in K17. The sequence VEMD/A at the cleavage site located in the conserved linker region was found in K15 and K17. A further cleavage site was identified in the tail region of K17 with the recognition motif EVQD/G.