Evidence for Posttranscriptional Regulation of the Keratins Expressed during Hyperproliferation and Malignant Transformation in Human Epidermis

Keratin K6 is a protein that is expressed in human skin under conditions of hyperproliferation (e.g., wound-healing, psoriasis, and cell culture) and malignant transformation (e.g., squamous cell carcinomas). When induced, the appearance of K6 is rapid: if skin tissue is placed in radiolabeled culture medium, this protein can be detected within an hour. The regulation of K6 seems to be controlled partly by a posttranscriptional mechanism: At least two K6 genes are actively transcribed both in vivo, when the protein is not made, as well as in vitro, when abundant levels of the protein are expressed. Substantial levels of K6a and K6b RNAs can be detected in skin by Northern Blot analysis, and these RNAs are largely, if not fully translatable in vitro. In situ hybridizations reveal that the RNAs are distributed throughout the living layers of the epidermis. The rapid induction of K6 expression through a posttranscriptional regulatory mechanism suggests that this keratin may play an important role in designing a cytoskeletal architecture that is compatible with the hyperproliferative state. PITHELIAL cells manifest their common protective role by constructing an extensive cytoskeletal network comprised of three filamentous structures. Two of these, the 6-nm actin microfilaments and the 23-rim microtubules, are ubiquitous throughout the eukaryotic kingdom. The third, the 8-nm keratin filaments, are unique to epithelial cells and seem to be specifically tailored to suit the particular structural needs of each higher eukaryotic epithelial cell. Whereas the actins and tubulins have been highly evolutionarily conserved, the keratins form a highly divergent family of proteins (40-70 kD) which are differentially expressed in different epithelia and at different stages of differentiation and development (for review, see references 13, 35, and 44). The keratins can be subdivided into two distinct groups by sequence homologies (21). Type I keratins are acidic (pKi 4.5-5.5) and generally small (40-56.5 kD), whereas type II keratins are basic (pKi 6.5-7.5) and larger (53-67 kD) (35). At least one member of each type is expressed at all times, suggesting the importance of both sequences in filament assembly (4). Indeed, no single keratin seems competent for filament formation (5, 22, 31, 43). The epidermis of the skin serves the greatest protective role of all epithelia, and thus, it is not surprising that keratin synthesis is especially abundant in this tissue. Even in the basal layer, much of the total protein is keratin (8). When a basal cell undergoes a commitment to terminally differentiate and begins to migrate towards the skin surface, keratin synthesis increases as other metabolic processes come to a halt (16, 23). The fully differentiated cells of the outermost stratum corneum layer are merely cellular skeletons that are packed with bundles of keratin filaments (30). Keratin synthesis in the epidermis is complex. In the basal epidermal layer, keratins of 58 and 50 kD are produced (11, 36, 45). These keratins have been designated K5 (type II) and K14 (type I) according to the nomenclature of Moll et al. (35). In response to an as yet unidentified trigger of terminal differentiation, a suprabasal epidermal cell begins to make two additional type II keratins, K1 (67 kD) and K2 (65 kD), and two type I keratins, K10 (56.5 kD) and Kll (56 kD) (4, 11, 49, 53). Throughout the suprabasal layers of the skin, these keratins are the predominant proteins that are synthesized. When skin is injured, or if it otherwise undergoes hyperproliferation or malignant transformation, it induces the production of a type II keratin, K6 (56 kD), and a type I keratin, K16 (48 kD) (35, 51). Normally, however, these keratins are not expressed (4, 35). Whether the synthesis of these hyperproliferation-associated keratins may in some way be important for producing a cytoskeleton that is more compatible with either (a) a faster rate of cell division or (b) an increase in the number of cell cycles an epidermal cell undergoes during its lifetime, has yet to be determined. The biochemical mechanisms underlying the differential expression of keratins in epidermis and during hyperproliferation remain largely unexplored. It is known that there are different mRNAs for most if not all of these keratins (2, 10, 11), and these mRNAs are encoded largely if not solely by separate genes (15, 24, 27, 32, 33, 39, 41, 48). However, studies to investigate changes in keratin expression at the © The Rockefeller University Press, 0021-9525/86/11/1945/11 $1.00 The Journal of Cell Biology, Volume 103, November 1986 1945-1955 1945 on A ril 0, 2017 D ow nladed fom Published November 1, 1986