BMP2 and BMP6 control p57Kip2 expression and cell growth arrest/terminal differentiation in normal primary human epidermal keratinocytes

Functional studies of the canonical Bone Morphogenetic Protein (BMP) signalling pathway in human epidermal keratinocytes have been limited to the immortalized and p53-mutated HaCaT cells and are primarily dependent on BMP6 treatment in mouse epidermal keratinocytes. Despite these insightful analyses, the molecular mechanism underlying the role of BMP signalling in the precise balance between growth arrest and terminal differentiation of keratinocytes still remains not clearly defined. The current study first investigated the hitherto uncharacterized status and functions of BMP signalling in normal human keratinocytes by using three independent strains of primary interfollicular epidermal keratinocytes. Then we provided data demonstrating the role of BMP2 compared to BMP6 in the inhibition of growth and induction of subsequent terminal differentiation of these cells. A second relevant finding is based on the clonal analysis of colony types present in untreated and BMP2/6-treated cultures in absence of EGF. BMP treatment results in the clonal transition from proliferative to abortive colonies, suggesting that BMP signalling most likely inhibits stem cell proliferation and triggers cell cycle exit from transit amplifying cells. Third, we showed evidence that, of the three members of the Cip/Kip family of cyclin-dependent kinase inhibitors, only p57Kip2 and p21Cip1 have a BMP2/6-induced expression. One mechanism of inhibition of cell proliferation involves p57Kip2 as an immediate early response, in contradistinction with p21Cip1 which largely depends on de novo protein synthesis for its effect to proceed. All together, these results clarify the BMP signalling status in normal primary human keratinocytes and support a new mechanism of inhibition of the proliferation of interfollicular epidermal keratinocytes coupled with induction of their terminal differentiation following BMP2 or BMP6 addition.