Gab1 and SHP-2 promote Ras/MAPK regulation of epidermal growth and differentiation

In epidermis, Ras can influence proliferation and differentiation; however, regulators of epidermal Ras function are not fully characterized, and Ras effects on growth and differentiation are controversial. EGF induced Ras activation in epidermal cells along with phosphorylation of the multisubstrate docking protein Gab1 and its binding to SHP-2. Expression of mutant Gab1(Y627F) deficient in SHP-2 binding or dominant-negative SHP-2(C459S) reduced basal levels of active Ras and downstream MAPK proteins and initiated differentiation. Differentiation triggered by both Gab1(Y627F) and SHP-2(C459S) could be blocked by coexpression of active Ras, consistent with Gab1 and SHP-2 action upstream of Ras in this process. To study the role of Gabl and SHP-2 in tissue, we generated human epidermis overexpressing active Gabl and SHP-2. Both proteins stimulated proliferation. In contrast, Gab(Y627F) and SHP-2(C459S) inhibited epidermal proliferation and enhanced differentiation. Consistent with a role for Gabl and SHP-2 in sustaining epidermal Ras/MAPK activity, Gabl (-/-) murine epidermis displayed lower levels of active Ras and MAPK with postnatal Gabl -/- epidermis, demonstrating the hypoplasia and enhanced differentiation seen previously with transgenic epidermal Ras blockade. These data provide support for a Ras role in promoting epidermal proliferation and opposing differentiation and indicate that Gabl and SHP-2 promote the undifferentiated epidermal cell state by facilitating Ras/MAPK signaling.