Keratins modulate hepatic cell adhesion, size and G1/S transition

Keratins (Ks) are the intermediate filament (IF) proteins of epithelial cells. Hepatocyte IFs are made solely of keratins 8 and 18 (K8/K18), the hallmark of all simple epithelia. While K8/K18 are essential for maintaining structural integrity, there is accumulating evidence indicating that they also exert non-mechanical functions. We have reported recently that K8/K18-free hepatocytes from K8-null mice are more sensitive to Fas-mediated apoptosis, in line with an increased Fas density at the cell surface and an altered c-Flip regulation of the anti-apoptotic ERK1/2 signaling pathway. In the present study, we show that K8-null hepatocytes attach more rapidly but spread more slowly on a fibronectin substratum and undergo a more efficient G1/S transition than wild-type hepatocytes. Moreover, plectin, an IF associated protein, receptor for activated C kinase 1 (RACK1), a plectin partner, and vinculin, a key component of focal adhesions, distribute differently in spreading K8-null hepatocytes. Cell seeding leads to no differential activation of ERK1/2 in WT versus K8-null hepatocytes, whereas a stronger Akt activation is detected in K8-null hepatocytes. Insulin stimulation also leads to a differential Akt activation, implying altered Akt signaling capacity as a result of the K8/K18 loss. In addition, a delayed autophosphorylation of FAK, a target for integrin beta1 signaling, was obtained in seeding K8-null hepatocytes. These alterations in cell cycle-related events in hepatocytes in primary culture are also found in a K8-knockdown H4-II-E-C3 rat hepatoma cell line. Besides, K8/K18-free cells are smaller and exhibit a reduced rate of protein synthesis. In addition, a distinctive cyclin interplay is observed in these K8/K18-free hepatic cells, namely a more efficient cyclin A-dependent G1/S phase transition. Furthermore, K8 re-expression in these cells, following transfer of a human K8 cDNA, restores proper cell size, spreading and growth. Together, these results suggest new interrelated signaling roles of K8/18 with plectin/RACK1 in the modulation of cell attachment/spreading, size/protein synthesis and G1/S transition.     

Simple epithelium keratins 8 and 18 provide resistance to Fas-mediated apoptosis. The protection occurs through a receptor-targeting modulation

Keratins 8 and 18 belong to the keratin family of intermediate filament (IF) proteins and constitute a hallmark for all simple epithelia, including the liver. Hepatocyte IFs are made solely of keratins 8 and 18 (K8/K18). In these cells, the loss of one partner via a targeted null mutation in the germline results in hepatocytes lacking K8/K18 IFs, thus providing a model of choice for examining the function(s) of simple epithelium keratins. Here, we report that K8-null mouse hepatocytes in primary culture and in vivo are three- to fourfold more sensitive than wild-type (WT) mouse hepatocytes to Fas-mediated apoptosis after stimulation with Jo2, an agonistic antibody of Fas ligand. This increased sensitivity is associated with a higher and more rapid caspase-3 activation and DNA fragmentation. In contrast, no difference in apoptosis is observed between cultured K8-null and WT hepatocytes after addition of the Fas-related death-factors tumor necrosis factor (TNF) alpha or TNF-related apoptosis-inducing ligand. Analyses of the Fas distribution in K8-null and WT hepatocytes in culture and in situ demonstrate a more prominent targeting of the receptor to the surface membrane of K8-null hepatocytes. Moreover, altering Fas trafficking by disrupting microtubules with colchicine reduces by twofold the protection generated against Jo2-induced lethal action in K8-null versus WT hepatocytes. Together, the results strongly suggest that simple epithelium K8/K18 provide resistance to Fas-mediated apoptosis and that this protection occurs through a modulation of Fas targeting to the cell surface.     

Switch in Fas-activated death signaling pathway as result of keratin 8/18-intermediate filament loss

Fas-induced apoptosis is initiated through the recruitment of FADD and procaspase 8 to form the death-inducing signaling complex (DISC). In some cells (type I cells) the initiator caspase 8 directly activates effector caspases such as procaspase 3, whereas in others (type II cells) the death signal is amplified through mitochondria. In epithelial cells, Fas-induced hierarchic caspase activation is also linked with DEDD, a member of the DED family that binds to keratin (K) intermediate filaments (IFs). Hepatocytes are type II cells and their IFs are made exclusively of K8/K18. We have shown previously that K8-null mouse hepatocytes, lacking K8/K18 IFs, are more sensitive than their wild-type counterparts to Fas-induced apoptosis. Here, by examining the cell-death kinetics and death-signaling ordering, we found that K8-null hepatocytes exhibited prominent DISC formation, higher procaspase 8 activation and direct procaspase 3 activation as reported for type I cells; however they experienced a reduced Bid cleavage and a stronger procaspase 9 activation. In addition, the K8/K18 loss altered the DEDD ubiquitination status and nuclear/cytoplasmic distribution. Together, the results suggest that the K8/K18 loss induces a switch in Fas-induced death signaling, likely through a DEDD involvement. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Intermediate filament cytoskeleton of the liver in health and disease

Intermediate filaments (IFs) represent the largest cytoskeletal gene family comprising approximately 70 genes expressed in tissue specific manner. In addition to scaffolding function, they form complex signaling platforms and interact with various kinases, adaptor, and apoptotic proteins. IFs are established cytoprotectants and IF variants are associated with >30 human diseases. Furthermore, IF-containing inclusion bodies are characteristic features of several neurodegenerative, muscular, and other disorders. Acidic (type I) and basic keratins (type II) build obligatory type I and type II heteropolymers and are expressed in epithelial cells. Adult hepatocytes contain K8 and K18 as their only cytoplasmic IF pair, whereas cholangiocytes express K7 and K19 in addition. K8/K18-deficient animals exhibit a marked susceptibility to various toxic agents and Fas-induced apoptosis. In humans, K8/K18 variants predispose to development of end-stage liver disease and acute liver failure (ALF). K8/K18 variants also associate with development of liver fibrosis in patients with chronic hepatitis C. Mallory-Denk bodies (MDBs) are protein aggregates consisting of ubiquitinated K8/K18, chaperones and sequestosome1/p62 (p62) as their major constituents. MDBs are found in various liver diseases including alcoholic and non-alcoholic steatohepatitis and can be formed in mice by feeding hepatotoxic substances griseofulvin and 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC). MDBs also arise in cell culture after transfection with K8/K18, ubiquitin, and p62. Major factors that determine MDB formation in vivo are the type of stress (with oxidative stress as a major player), the extent of stress-induced protein misfolding and resulting chaperone, proteasome and autophagy overload, keratin 8 excess, transglutaminase activation with transamidation of keratin 8 and p62 upregulation.     

Keratin 8/18 regulation of cell stiffness-extracellular matrix interplay through modulation of Rho-mediated actin cytoskeleton dynamics

Cell mechanical activity generated from the interplay between the extracellular matrix (ECM) and the actin cytoskeleton is essential for the regulation of cell adhesion, spreading and migration during normal and cancer development. Keratins are the intermediate filament (IF) proteins of epithelial cells, expressed as pairs in a lineage/differentiation manner. Hepatic epithelial cell IFs are made solely of keratins 8/18 (K8/K18), hallmarks of all simple epithelia. Notably, our recent work on these epithelial cells has revealed a key regulatory function for K8/K18 IFs in adhesion/migration, through modulation of integrin interactions with ECM, actin adaptors and signaling molecules at focal adhesions. Here, using K8-knockdown rat H4 hepatoma cells and their K8/K18-containing counterparts seeded on fibronectin-coated substrata of different rigidities, we show that the K8/K18 IF-lacking cells lose their ability to spread and exhibit an altered actin fiber organization, upon seeding on a low-rigidity substratum. We also demonstrate a concomitant reduction in local cell stiffness at focal adhesions generated by fibronectin-coated microbeads attached to the dorsal cell surface. In addition, we find that this K8/K18 IF modulation of cell stiffness and actin fiber organization occurs through RhoA-ROCK signaling. Together, the results uncover a K8/K18 IF contribution to the cell stiffness-ECM rigidity interplay through a modulation of Rho-dependent actin organization and dynamics in simple epithelial cells.     

Keratin-protein kinase C interaction in reactive oxygen species-induced hepatic cell death through mitochondrial signaling

Keratins (Ks), the intermediate filament (IF) proteins of epithelia, constitute at least 20 cytoskeletal proteins subdivided into type I (K9-20) and type II (K1-K8) and expressed as type I/type II pairs in a cell differentiation manner. Hepatocyte IFs are made only of K8/K18, the hallmark of simple epithelial cells. We have shown previously that a K8/K18 loss leads to a modulation of apoptosis in Fas-stimulated mouse hepatocytes. Here we report that K8-knockout mouse hepatocytes and K8-knockdown H4-II-E-C3 (shK8b1) rat hepatoma cells were much more resistant than their K8/K18-containing counterparts, wild-type hepatocytes, and H4ev hepatoma cells, in response to excess H2O2 or tert-butyl hydroperoxide, a ROS generator. While excess H2O2 altered glutathione (GSH) and ROS levels in H4ev versus shK8b1 cells, the differential death response was largely GSH level independent. Assessment of key cell death features revealed that hepatic cells exposed to H2O2 die through a mitochondrial involvement. Similarly, administration of the GSH depletor L-buthionine-sulfoximine to generate mitochondrial ROS-sensitized H4-II-E-C3 cells but not shK8b1 cells to death. Treatment with protein kinase C (PKC) inhibitors yielded a resistance of H2O2-treated H4-II-E-C3 cells comparable to that of nontreated shK8b1 cells, which in turn were not affected by the treatment. In addition, this differential death response was associated with altered PKCdelta activation and surface-membrane/mitochondria distribution in H2O2-treated shK8b1 cells. Together, these results point to a key regulatory function for K8/K18 in ROS-induced mitochondria-mediated death through PKCdelta involvement in hepatic cells.     

Keratins modulate c-Flip/extracellular signal-regulated kinase 1 and 2 antiapoptotic signaling in simple epithelial cells

Among the large family of intermediate filament proteins, the keratin 8 and 18 (K8/K18) pair constitutes a hallmark for all simple epithelial cells, such as hepatocytes and mammary cells. Functional studies with different cell models have suggested that K8/K18 are involved in simple epithelial cell resistance to several forms of stress that may lead to cell death. We have reported recently that K8/K18-deprived hepatocytes from K8-null mice are more sensitive to Fas-mediated apoptosis. Here we show that upon Fas, tumor necrosis factor alpha receptor, or tumor necrosis factor alpha-related apoptosis-inducing ligand receptor stimulation, an inhibition of extracellular signal-regulated kinase 1 and 2 (ERK1/2) activation sensitizes wild-type but not K8-null mouse hepatocytes to apoptosis and that a much weaker ERK1/2 activation occurs in K8-null hepatocytes. In turn, this impaired ERK1/2 activation in K8-null hepatocytes is associated with a drastic reduction in c-Flip protein, an event that also holds in a K8-null mouse mammary cell line. c-Flip, along with Raf-1, is part of a K8/K18-immunoisolated complex from wild-type hepatocytes, and Fas stimulation leads to further c-Flip and Raf-1 recruitment in the complex. This points to a new regulatory role of simple epithelium keratins in the c-Flip/ERK1/2 antiapoptotic signaling pathway.     

Keratin 8/18 Modulation of Protein Kinase C-mediated Integrin-dependent Adhesion and Migration of Liver Epithelial Cells

Keratins are intermediate filament (IF) proteins of epithelial cells, expressed as pairs in a lineage/differentiation manner. Hepatocyte and hepatoma cell IFs are made solely of keratins 8/18 (K8/K18), the hallmark of all simple epithelia. Cell attachment/spreading (adhesion) and migration involve the formation of focal adhesions at sites of integrin interactions with extracellular matrix, actin adaptors such as talin and vinculin, and signaling molecules such as focal adhesion kinase (FAK) and member(s) of the protein kinase C (PKC) family. Here, we identify the novel PKCδ as mediator of the K8/K18 modulation of hepatoma cell adhesion and migration. We also demonstrate a K8/K18-dependent relationship between PKCδ and FAK activation through an integrin/FAK-positive feedback loop, in correlation with a reduced FAK time residency at focal adhesions. Notably, a K8/K18 loss results to a time course modulation of the receptor of activated C-kinase-1, β1-integrin, plectin, PKC, and c-Src complex formation. Although the K8/K18 modulation of hepatocyte adhesion also occurs through a PKC mediation, these differentiated epithelial cells exhibit minimal migrating ability, in link with marked differences in protein partner content and distribution. Together, these results uncover a key regulatory function for K8/K18 IFs in the PKC-mediated integrin/FAK-dependent adhesion and migration of simple epithelial cells.     

Directed Expression of a Chimeric Type II Keratin Partially Rescues Keratin 5-null Mice

The crucial role of structural support fulfilled by keratin intermediate filaments (IFs) in surface epithelia likely requires that they be organized into cross-linked networks. For IFs comprised of keratins 5 and 14 (K5 and K14), which occur in basal keratinocytes of the epidermis, formation of cross-linked bundles is, in part, self-driven through cis-acting determinants. Here, we targeted the expression of a bundling-competent KRT5/KRT8 chimeric cDNA (KRT8bc) or bundling-deficient wild type KRT8 as a control to the epidermal basal layer of Krt5-null mice to assess the functional importance of keratin IF self-organization in vivo. Such targeted expression of K8bc rescued Krt5-null mice with a 47% frequency, whereas K8 completely failed to do so. This outcome correlated with lower than expected levels of K8bc and especially K8 mRNA and protein in the epidermis of E18.5 replacement embryos. Ex vivo culture of embryonic skin keratinocytes confirmed the ability of K8bc to form IFs in the absence of K5. Additionally, electron microscopy analysis of E18.5 embryonic skin revealed that the striking defects observed in keratin IF bundling, cytoarchitecture, and mitochondria are partially restored by K8bc expression. As young adults, viable KRT8bc replacement mice develop alopecia and chronic skin lesions, indicating that the skin epithelia are not completely normal. These findings are consistent with a contribution of self-mediated organization of keratin IFs to structural support and cytoarchitecture in basal layer keratinocytes of the epidermis and underscore the importance of context-dependent regulation for keratin genes and proteins in vivo.     

Cytoskeleton-mediated death receptor and ligand concentration in lipid rafts forms apoptosis-promoting clusters in cancer chemotherapy

While investigating the mechanism of action of the novel antitumor drug Aplidin, we have discovered a potent and novel cell-killing mechanism that involves the formation of Fas/CD95-driven scaffolds in membrane raft clusters housing death receptors and apoptosis-related molecules. Fas, tumor necrosis factor-receptor 1, and tumor necrosis factor-related apoptosis-inducing ligand receptor 2/death receptor 5 were clustered into lipid rafts in leukemic Jurkat cells following Aplidin treatment, the presence of Fas being essential for apoptosis. Preformed membrane-bound Fas ligand (FasL) as well as downstream signaling molecules, including Fas-associated death domain-containing protein, procaspase-8, procaspase-10, c-Jun amino-terminal kinase, and Bid, were also translocated into lipid rafts, connecting death receptor extrinsic and mitochondrial intrinsic apoptotic pathways. Blocking Fas/FasL interaction partially inhibited Aplidin-induced apoptosis. Aplidin was rapidly incorporated into membrane rafts, and drug uptake was inhibited by lipid raft disruption. Actin-linking proteins ezrin, moesin, RhoA, and RhoGDI were conveyed into Fas-enriched rafts in drug-treated leukemic cells. Disruption of lipid rafts and interference with actin cytoskeleton prevented Fas clustering and apoptosis. Thus, Aplidin-induced apoptosis involves Fas activation in both a FasL-independent way and, following Fas/FasL interaction, an autocrine way through the concentration of Fas, membrane-bound FasL, and signaling molecules in membrane rafts. These data indicate a major role of actin cytoskeleton in the formation of Fas caps and highlight the crucial role of the clusters of apoptotic signaling molecule-enriched rafts in apoptosis, acting as concentrators of death receptors and downstream signaling molecules and as the linchpin from which a potent death signal is launched.     

Akt isoforms regulate intermediate filament protein levels in epithelial carcinoma cells

Keratin 8 and 18 are simple epithelial intermediate filament (IF) proteins, whose expression is differentiation- and tissue-specific, and is maintained during tumorigenesis. Vimentin IF is often co-expressed with keratins in cancer cells. Recently, IF have been proposed to be involved in signaling pathways regulating cell growth, death and motility. The PI3K/Akt pathway plays a pivotal role in these processes. Thus, we investigated the role of Akt (1 and 2) in regulating IF expression in different epithelial cancer cell lines. Over-expression of Akt1 increases K8/18 proteins. Akt2 up-regulates K18 and vimentin expression by an increased mRNA stability. To our knowledge, these results represent the first indication that Akt isoforms regulate IF expression and support the hypothesis that IFs are involved in PI3K/Akt pathway.     

Insights into the dynamic properties of keratin intermediate filaments in living epithelial cells

The properties of keratin intermediate filaments (IFs) have been studied after transfection with green fluorescent protein (GFP)-tagged K18 and/or K8 (type I/II IF proteins). GFP-K8 and -K18 become incorporated into tonofibrils, which are comprised of bundles of keratin IFs. These tonofibrils exhibit a remarkably wide range of motile and dynamic activities. Fluorescence recovery after photobleaching (FRAP) analyses show that they recover their fluorescence slowly with a recovery t(1/2) of approximately 100 min. The movements of bleach zones during recovery show that closely spaced tonofibrils (<1 microm apart) often move at different rates and in different directions. Individual tonofibrils frequently change their shapes, and in some cases these changes appear as propagated waveforms along their long axes. In addition, short fibrils, termed keratin squiggles, are seen at the cell periphery where they move mainly towards the cell center. The motile properties of keratin IFs are also compared with those of type III IFs (vimentin) in PtK2 cells. Intriguingly, the dynamic properties of keratin tonofibrils and squiggles are dramatically different from those of vimentin fibrils and squiggles within the same cytoplasmic regions. This suggests that there are different factors regulating the dynamic properties of different types of IFs within the same cytoplasmic regions.     

Intermediate filaments interact with dormant ezrin in intestinal epithelial cells

Ezrin connects the apical F-actin scaffold to membrane proteins in the apical brush border of intestinal epithelial cells. Yet, the mechanisms that recruit ezrin to the apical domain remain obscure. Using stable CACO-2 transfectants expressing keratin 8 (K8) antisense RNA under a tetracycline-responsive element, we showed that the actin-ezrin scaffold cannot assemble in the absence of intermediate filaments (IFs). Overexpression of ezrin partially rescued this phenotype. Overexpression of K8 in mice also disrupted the assembly of the brush border, but ezrin distributed away from the apical membrane in spots along supernumerary IFs. In cytochalasin D-treated cells ezrin localized to a subapical compartment and coimmunoprecipitated with IFs. Overexpression of ezrin in undifferentiated cells showed a Triton-insoluble ezrin compartment negative for phospho-T567 (dormant) ezrin visualized as spots along IFs. Pulse-chase analysis showed that Triton-insoluble, newly synthesized ezrin transiently coimmunoprecipitates with IFs during the first 30 min of the chase. Dormant, but not active (p-T567), ezrin bound in vitro to isolated denatured keratins in Far-Western analysis and to native IFs in pull-down assays. We conclude that a transient association to IFs is an early step in the polarized assembly of apical ezrin in intestinal epithelial cells.     

Spontaneous losses of control of cytokeratin gene expression in transformed, non-epithelial human cells occurring at different levels of regulation

Intermediate filaments (IFs) of the cytokeratin (CK) type are cytoskeletal elements typical for epithelial differentiation. However, in diverse transformed culture lines of non-epithelial origin, rare cells emerge spontaneously, which synthesize, in addition to their vimentin IFs, CKs 8 and 18. We enriched such cells by cloning and studied the level(s) of regulation at which these changes occur. We found that in SV40-transformed fibroblasts the CK 18 gene is constitutively transcribed into translatable mRNA but that the protein is rapidly degraded in the absence of its complex partner, CK 8. In contrast, cells immunocytochemically positive for CK IFs contained both CKs 8 and 18, which apparently stabilized in heterotypic complexes. These findings and related observations of active genes for CKs 8 and/or 18 in several other transformed non-epithelial cell lines indicate that the genes for CKs 18 and, less frequently, 8 can be active in diverse different non-epithelial cell lines; synthesis of type I and type II CK pair partners can be uncoupled; control of CK IF formation can take place at different levels. We suggest that the intrinsic instability of the inactive state of these genes is responsible for the occurrence of CKs 8 and 18 in certain non-epithelial tissues and tumors, a caveat in tumor diagnosis.     

Proteomics of MUC1‐containing lipid rafts from plasma membranes and exosomes of human breast carcinoma cells MCF‐7

Apically expressed human MUC1 is known to become endocytosed and either to re‐enter the secretory pathway for recycling to the plasma membrane or to be exported by the cells via the formation of multi‐vesicular bodies and the release of exosomes. By using recombinant fusion‐tagged MUC1 as a bait protein we followed an anti‐myc affinity‐based approach for isolating subpopulations of lipid rafts from the plasma membranes and exosomes of MCF‐7 breast cancer cells. MUC1⁺ lipid rafts were not only found to contain genuine raft proteins (flotillin‐1, prohibitin, G protein, annexin A2), but also raft‐associated proteins linking these to the cytoskeleton (ezrin/villin‐2, profilin II, HSP27, γ‐actin, β‐actin) or proteins in complexes with raft proteins, including the bait protein (HSP60, HSP70). Major overlaps were revealed for the subproteomes of plasma membranous and exosomal lipid raft preparations, indicating that MUC1 is sorted into subpopulations of rafts for its trafficking via flotillin‐dependent pathways and export via exosomes.     

Dissection of keratin dynamics: different contributions of the actin and microtubule systems

It has only recently been recognized that intermediate filaments (IFs) and their assembly intermediates are highly motile cytoskeletal components with cell-type- and isotype-specific characteristics. To elucidate the cell-type-independent contribution of actin filaments and microtubules to these motile properties, fluorescent epithelial IF keratin polypeptides were introduced into non-epithelial, adrenal cortex-derived SW13 cells. Time-lapse fluorescence microscopy of stably transfected SW13 cell lines synthesizing fluorescent human keratin 8 and 18 chimeras HK8-CFP and HK18-YFP revealed extended filament networks that are entirely composed of transgene products and exhibit the same dynamic features as keratin systems in epithelial cells. Detailed analyses identified two distinct types of keratin motility: (I) Slow (approximately 0.23 microm/min), inward-directed, continuous transport of keratin filament precursor particles from the plasma membrane towards the cell interior, which is most pronounced in lamellipodia. (II) Fast (approximately 17 microm/min), bidirectional and intermittent transport of keratin particles in axonal-type cell processes. Disruption of actin filaments inhibited type I motility while type II motility remained. Conversely, microtubule disruption inhibited transport mode II while mode I continued. Combining the two treatments resulted in a complete block of keratin motility. We therefore conclude that keratin motility relies both on intact actin filaments and microtubules and is not dependent on epithelium-specific cellular factors.     

Intermediate filaments formed de novo from tail-less cytokeratins in the cytoplasm and in the nucleus

The roles of the different molecular domains of intermediate filament (IF) proteins in the assembly and higher order organization of IF structures have recently been studied by various groups but with partially controversial results. To examine the requirement of the aminoterminal (head) and the carboxyterminal (tail) domain of cytokeratins (CKs) for de novo IF formation in the living cell, we have constructed cDNAs coding for intact as well as head- and/or tail-less human CKs 8 and 18 and the naturally tail-less human CK 19, all under the control of the human beta-actin promoter. After transient and stable transfections of mouse 3T3-L1 cells, which are devoid of any CKs, we have studied, with such constructs, the resulting gene products by gel electrophoresis and immunolocalization techniques. By light and electron microscopy we show that extended cytoplasmic IF meshworks are formed from pairs of the type II CK 8 with the type I CKs 18 or 19 as well as from pairs of tail-less CK 8 with tail-less CKs 18 or 19 in the transfected cells, proving that the absence of the tail domain in both types of CKs does not prevent the de novo formation of regular IFs. Most surprisingly, however, we have observed spectacular alterations in the nucleocytoplasmic distribution of the IFs formed from tail-less CKs. In many of the transfected cells, a large part, or all, of the detectable CKs was found to occur in extensive IF bundles in the nucleoplasm. Intranuclear accumulations of CK deposits, however mostly nonfibrillar, were also observed when the cells had been transfected with cDNAs encoding tail-less CKs also lacking their head domains, whereas CKs deleted only in the head domain were found exclusively in the cytoplasm. The specific domain requirements for the assembly of cytoplasmic IF bundles are discussed and possible mechanisms of intranuclear accumulation of IFs are proposed.     

Interaction of the bullous pemphigoid antigen 1 (BP230) and desmoplakin with intermediate filaments is mediated by distinct sequences within their COOH terminus

The bullous pemphigoid antigen 1 (BP230) and desmoplakin (DP) are members of the plakin protein family of cytolinkers. Despite their homology, their COOH termini selectively bind distinct intermediate filaments (IFs). We studied sequences within their COOH termini required for their interaction with the epidermal keratins K5/K14, the simple epithelial keratins K8/K18, and type III IF vimentin by yeast three-hybrid, cell transfection, and overlay assays. The results indicate that BP230 interacts with K5/K14 but not with K8/K18 or vimentin via a region encompassing both the B and C subdomains and the COOH extremity, including a COOH-terminal eight-amino-acid stretch. In contrast, the C subdomain with the COOH-terminal extremity of DP interacts with K5/K14 and K8/K18, and its linker region is able to associate with K8/K18 and vimentin. Furthermore, the potential of DP to interact with IF proteins in yeast seems to be regulated by phosphorylation of Ser 2849 within its COOH terminus. Strikingly, BP230 and DP interacted with cytokeratins only when both type I and type II keratins were present. The head and tail domains of K5/K14 keratins were dispensable for their interaction with BP230 or DP. On the basis of our findings, we postulate that (1) the binding specificity of plakins for various IF proteins depends on their linker region between the highly homologous B and C subdomains and their COOH extremity and (2) the association of DP and BP230 with both epidermal and simple keratins is critically affected by the tertiary structure induced by heterodimerization and involves recognition sites located primarily in the rod domain of these keratins.     

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.