Modulation of cell proliferation by cytokeratins K10 and K16

The members of the large keratin family of cytoskeletal proteins are expres sed in a carefully regulated tissue- and differentiation-specific manner. A lthough these proteins are thought to be involved in imparting mechanical i ntegrity to epithelial cells, the functional significance of their complex differential expression is stilt unclear, Here we provide new data suggesti ng that the expression of particular keratins may influence cell proliferat ion. Specifically, we demonstrate that the ectopic expression of K10 inhibi ts the proliferation of human keratinocytes in culture, while K16 expressio n appears to promote the proliferation of these cells. Other keratins, such as K13 or K14, do not significantly alter this parameter. K10-induced inhi bition Is reversed by the coexpression of K16 but not that of K14. These re sults are coherent with the observed expression pattern of these proteins i n the epidermis: basal, proliferative keratinocytes express K14; when they terminally differentiate, keratinocytes switch off K14 and start K10 expres sion, whereas in response to hyperproliferative stimuli, K16 replaces K10. The characteristics of this process indicate that K10 and K16 act on the re tinoblastoma (Rb) pathway, as (i) K10-induced inhibition is hampered by cot ransfection with viral oncoproteins which interfere with pRb but not,vith p 53; (ii) K10-mediated cell growth arrest is rescued by the coexpression of specific cyclins, cyclin-dependent kinases (CDKs), or cyclin-CDK complexes; (iii) K10-induced inhibition does not take place in Rb-deficient cells but is restored in these cells by cotransfection with pRb or p107 but not p130 ; (iv) K16 efficiently rescues the cell growth arrest induced by pRb in HaC aT cells but not that induced by p107 or p130; and (v) pRb phosphorylation and cyclin D1 expression are reduced in K10-transfected cells and increased in K16-transfected cells. Finally, using K10 deletion mutants, we map this inhibitory function to the nonhelical terminal domains of K10, hypervariab le regions in which keratin-specific functions are thought to reside, and d emonstrate that the presence of one of these domains is sufficient to promo te cell growth arrest.