Oncogene (2011) 30, 127–138 & 2011 Macmillan Publishers Limited All rights reserved 0950-9232/11 www.nature.com/onc REVIEW Keratins in health and cancer: more than mere epithelial cell markers V Karantza1,2 1Department of Medicine, University of Medicine and Dentistry of New Jersey, Robert Wood Johnson Medical School, Piscataway, NJ, USA and 2Division of Medical Oncology, The Cancer Institute of New Jersey, New Brunswick, NJ, USA Keratins are the intermediate filament (IF)-forming 1998). Eukaryotic cells contain three main kinds of proteins of epithelial cells. Since their initial characteriza- cytoskeletal filaments, namely microfilaments, inter- tion almost 30 years ago, the total number of mammalian mediate filaments (IFs) and microtubules. Microfila- keratins has increased to 54, including 28 type I and 26 ments are composed of 6-nm intertwined actin chains type II keratins. Keratins are obligate heteropolymers and are responsible for resisting tension and maintaining and, similarly to other IFs, they contain a dimeric central cellular shape, forming cytoplasmic protuberances and a-helical rod domain that is flanked by non-helical head participating in cell–cell and cell–matrix interactions and tail domains. The 10-nm keratin filaments participate (Pollard and Cooper, 2009). The 10 nm in diameter IFs in the formation of a proteinaceous structural framework are more stable than actin filaments, organize the within the cellular cytoplasm and, as such, serve an internal three-dimensional cellular structure and, simi- important role in epithelial cell protection from mechan- larly to microfilaments, also function in cell shape ical and non-mechanical stressors, a property extensively maintenance by bearing tension (Herrmann et al., 2009). substantiated by the discovery of human keratin mutations Finally, the microtubules are 23 nm in diameter hollow predisposing to tissue-specific injury and by studies in cylinders, most commonly comprising of 13 protofila- keratin knockout and transgenic mice. More recently, ments, which in turn are polymers of alpha- and beta- keratins have also been recognized as regulators of other tubulin, and play key roles in intracellular transport and cellular properties and functions, including apico-basal the formation of the mitotic spindle (Glotzer, 2009). polarization, motility, cell size, protein synthesis and In contrast to actin filaments and microtubules, IFs membrane traffic and signaling. In cancer, keratins are are encoded by a large family of genes expressed in a extensively used as diagnostic tumor markers, as epithelial tissue- and differentiation state-specific manner and are malignancies largely maintain the specific keratin patterns classified into discrete categories based on their rod associated with their respective cells of origin, and, in domain amino-acid sequence. Types 1 and 2 IFs are many occasions, full-length or cleaved keratin expression found primarily in epithelial cells and include the acidic (or lack there of) in tumors and/or peripheral blood and basic keratins, respectively; type 3 IFs include carries prognostic significance for cancer patients. Quite vimentin, desmin and glial fibrillary acidic protein; type intriguingly, several studies have provided evidence for 4 IFs assemble into neurofilaments; and type 5 IFs are active keratin involvement in cancer cell invasion and the nuclear lamins (Herrmann et al., 2007). All IFs have metastasis, as well as in treatment responsiveness, and a dimeric central rod domain, which is a coiled coil of have set the foundation for further exploration of the role two parallel a-helices flanked by head and tail domains of keratins as multifunctional regulators of epithelial of variable lengths. Antiparallel molecular dimers tumorigenesis. (referred to as tetramers) polymerize in a staggered Oncogene (2011) 30, 127–138; doi:10.1038/onc.2010.456; manner to make apolar protofilaments (3 nm in published online 4 October 2010 diameter), which in turn associate to protofibrils (4–5 nm in diameter), and then to the 10-nm IFs Keywords: keratins; cancer; invasion; diagnosis; (Steinert et al., 1993), which are among the most prognosis; drug resistance chemically stable cellular structures, resisting high temperature, high salt and detergent solubilization. In this review, we will focus on keratins, the IFs of Introduction epithelial cells and will review their functional role in the normal epithelium and their emergent significance in the The cytoskeleton is a proteinaceous structural frame- pathophysiology and treatment of epithelial malignancies. work within the cellular cytoplasm (Fuchs and Cleveland, Correspondence: Dr V Karantza, The Cancer Institute of New Jersey, Keratins in health Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, Room 3550, 195 Little Albany Street, Epithelial cell IFs New Brunswick, NJ 08903, USA. E-mail: [email protected] Keratins are the IF-forming proteins of epithelial cells Received 7 July 2010; revised 23 August 2010; accepted 27 August 2010; and account for the majority of IF genes in the human published online 4 October 2010 genome. Two-dimensional isoelectric focusing and Keratins in cancer V Karantza 128 characteristically induced after skin injury; K1/K10, K15, K9 and K2 are all expressed in keratinocytes; K3 and K12 are the keratins of corneal epithelium; K4 and K13 are characteristic of mucosal stratified squamous Figure 1 Keratin structure. Keratins form obligate heteropoly- epithelial cells; and K25–K28 and K71–K75 are mers (between one type I and one type II keratin) and share a hair-follicle-specific keratins, whereas K31–K40 and common structure consisting of a central coiled-coil a-helical rod K81–K86 are keratins of the hair fiber (Moll et al., 2008). domain that is flanked by non-helical head and tail domains. The a-helical rod domain is subdivided into four subdomains (coils IA, IB, IIA and IIB), which are connected with three linkers (L1, L12 and L2). Most post-translationally modified sites are found within Protectors of epithelial cell integrity and much more, and the head and tail domains (conserved phosphorylation sites on so on human K8 and K18 are shown). Keratins serve an important role in epithelial cell protection from mechanical and non-mechanical stres- sors (Coulombe and Omary, 2002), a function that has sodium dodecyl sulfate–polyacrylamide gel electrophor- been particularly well characterized in the skin, cornea esis were initially used to profile the keratins of normal and liver by both the discovery of human keratin human epithelial tissues, cell cultures and tumors (Moll mutations predisposing to tissue-specific injury (Omary et al., 1982), and resulted in the first comprehensive et al., 2009) and the development of keratin knockout keratin catalog, which included 19 members and (KO) and transgenic mouse models (Vijayaraj et al., separated them into type I (or acidic, K9–K19) and 2007). For example, mutations in keratins K5 or K14, type II (or basic to neutral, K1–8) keratins, and the which are expressed in the basal cells of stratified recognition that keratin filaments form by heterotypic epithelial, lead to physical trauma-induced fragility and pairing of type I and type II proteins at equimolar lysis of these cells resulting in intraepithelial blisters and amounts (Quinlan et al., 1984). Additional keratins were epidermolysis bullosa simplex (Bonifas et al., 1991; subsequently identified (Moll et al., 1990; Takahashi Coulombe et al., 1991; Lane et al., 1992); mutations in et al., 1995), including a large number of hair follicle- the cornea-specific keratins K3 or K12 result in the specific keratins (Winter et al., 1998). Given the fragility of the anterior corneal epithelium and intrae- completion of the human genome sequence, a consensus pithelial microcyst formation (Meesmann’s corneal nomenclature for mammalian keratin genes and pro- dystrophy) (Irvine et al., 1997); K8 (Ku et al., 2001) or teins is now in place (Schweizer et al., 2006) and includes K18 (Ku et al., 1997) mutations are found as a 28 type I (20 epithelial and 11 hair) keratins and 26 type predisposition to chronic (Omary et al., 2009) and acute II (20 epithelial and six hair) keratins. Similarly to other (Strnad et al., 2010) liver disease. Similarly, K5-null IFs, keratins contain a central a-helical domain of about mice survive for only a few hours after birth and exhibit 310 amino acids, which is composed of subdomains 1A, extensive skin blistering (Peters et al., 2001), whereas 1B, 2A and 2B connected by linkers L1, L12 and L2 K14-null mice show a similar, but less severe, phenotype (Figure 1). The non-helical head and tail domains and die within 3–4 days (Lloyd et al., 1995); K12 KO consist of subdomains V1 and H1 and H2 and V2, mice show corneal erosions (Kao et al., 1996); K8 respectively (Herrmann et al., 2007). deficiency results in liver hemorrhage and embryonic Keratins are expressed in all types of epithelial cells lethality in C57BL/6 mice (Baribault et al., 1993) (simple, stratified, keratinized and cornified) (Moll et al., together with mechanical fragility and susceptibility to 1982; Bragulla and Homberger, 2009). K8 and its hepatocyte injury during liver perfusion (Loranger et al., obligate partner K18 constitute the primary keratin 1997), but causes colorectal hyperplasia and inflamma- pair in many simple epithelial cells, such as hepatocytes, tion, rectal prolapse and mild liver injury in the FVB pancreatic acinar and islet cells, and proximal tubular strain (Baribault et al., 1994; Habtezion et al., 2005), kidney epithelial cells, and are also found in pseudos- whereas K18 KO mice exhibit late-onset, subclinical tratified (for example, respiratory) and complex (for liver pathology (Magin et al., 1998), suggesting different example, glandular) epithelia (Figure 2) and the K8 and K18 roles in inflammatory bowel and liver urothelium; K7 and K19 are expressed in other simple diseases, although K8 mutations do not appear to be epithelial cells, such as duct-lining cells, intestinal cells associated with human inflammatory bowel disease and mesothelial cells, in pseudostratified epithelia and (Buning et al., 2004; Ku et al., 2007).