Inhibition of Protein Kinase B (PKB) and PKCζ Mediates Keratin K10-Induced Cell Cycle Arrest

Abstract

) and atypical PKC causes sequestration of these kinases within the cytoskeleton and inhibits their intracellular translocation. As a consequence, the expression of K10 impairs the activation of PKB and PKC. We also demonstrate that this inhibition impedes pRb phosphorylation and reduces the expression of cyclins D1 and E. Functional and biochemical data also demonstrate that the interaction between K10 and these kinases involves the non-␣-helical amino domain of K10 (NTerm). Together, these results suggest new and essential roles for the keratins as modulators of specific signal transduction pathways.Keratins form the intermediate filament cytoskeleton of all epithelial cells. Although it has been demonstrated that this structure is essential for providing cell resilience in epithelia (11, 13), there is little information on individual keratin-specific functions that can account for the complex cell type-and differentiation-specific expression patterns observed in this protein family. Keratin expression is highly regulated in the epidermis. Proliferative basal cells express the keratin pair comprising K5 and K14 (K5-K14 pair). However, when keratinocytes begin terminal differentiation, they move upward, become postmitotic, and switch to the expression of keratin pair K1-K10. Under hyperproliferative conditions (e.g., tumors and wound healing), keratinocytes downregulate K1-K10 and express K6-K16. We recently showed that the ectopic expression of keratin K10 inhibits the proliferation of human keratinocytes, while K16 expression stimulates the process (23). In agreement with this, K10 impairs skin tumor development when ectopically expressed in transgenic mice (29) and K16 overexpression, or ectopic expression, in transgenic mice leads to aberrant epidermal keratinization and hyperproliferation (19,30).Keratin K10-induced inhibition of cell proliferation occurs through a process linked to the retinoblastoma protein (pRb) and the molecular machinery controlling cell cycle progression during G 1 (23). The different cellular distributions of keratins (cytoplasmic) and pRb (nuclear) nonetheless suggest that this effect must take place through the K10-mediated impairment of a pathway leading to the functional inactivation of pRb rather than by the direct physical interaction of these two molecules. We selected the ras oncogene-dependent pathway on the basis of a number of findings that imply that K10 and ras could have antagonistic functions in keratinocyte proliferation and differentiation. First, Ha-ras signaling has been implicated in the control of cell cycle progression in a pRb-dependent manner (18, 28), similar to that described for K10 (23). Second, whereas mutations in Ha-ras are early and essential events in mouse skin carcinogenesis protocols (3), K10 is lost during early stages of tumor development. Third, although K10 is expressed in postmitotic, terminally differentiating epidermal cells, transgenic mice expressing a mutant Ha-ras gene from a K10 promoter develop generalized hyperkeratosis of the skin a...

Thymic epithelial cell expansion through matricellular protein CYR61 boosts progenitor homing and T-cell output

“…CYR61-induced TEC proliferation was independent of thymocytes and was associated with the activation of the Akt signalling pathway. This is in accordance with previous studies showing that a reduced activity of Akt in TEC was responsible for altered thymic architecture and early involution 30 , as well as defects of keratinocyte proliferation 28,29 . Moreover, Akt was recently shown to be a target of CYR61 in lung epithelial cells 49 but also in PBMC, ovarian cancer, gastric cancer, cardiomyocytes and fibroblast-like synoviocytes 20,50-52 .…”

“…S4). Previous studies showed that reduced Akt activity in TEC was responsible for altered thymic architecture, early involution and defects in keratinocyte proliferation [28][29][30] . In order to determine the implication of Akt in CYR61-mediated TEC proliferation, 2DG-FTOC were cultured with recombinant CYR61 in the presence of Akt inhibitor MK2206.…”

Unique and overlapping functions of pRb and p107 in the control of proliferation and differentiation in epidermis

“…4) might be of a particular relevance. Our recent work has demonstrated that this keratin may impose a cell cycle arrest in keratinocytes in a pRb-dependent manner (Paramio et al, 1999;Paramio et al, 2001a;Santos et al, 2002). Here, we show that K10-expressing cells do not exhibit a proliferative arrest.…”

Abnormal urethra formation in mouse models of Split-hand/split-foot malformation type 1 and type 4

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