Signaling via β1 integrins and mitogen-activated protein kinase determines human epidermal stem cell fate in vitro

Human epidermal stem cells express higher levels of beta 1 integrins and are more adhesive than keratinocytes that are destined to differentiate. To investigate whether high beta 1 integrin expression and adhesiveness are essential for maintaining keratinocytes in the stem cell compartment, we introduced a dominant-negative beta 1 integrin mutant, CD8 beta 1, into cultured human keratinocytes, thereby interfering with beta 1 integrin function. Surface beta 1 integrin levels, adhesiveness, and mitogen-activated protein (MAP) kinase activation on fibronectin were reduced, and exit from the stem cell compartment was stimulated. Adhesiveness and proliferative potential were restored by overexpressing wild-type beta 1 integrin or by constitutive MAP kinase activation. Conversely, a dominant-negative MAP kinase kinase 1 mutant decreased adhesiveness and stem cell number in the absence of CD8 beta 1, MAP kinase activation by alpha 6 beta 4-mediated adhesion and mitogens was normal in CD8 beta 1 cells, and constitutive MAP kinase activation did not affect adhesion and proliferation of control keratinocytes, We conclude that beta 1 integrins and MAP kinase cooperate to maintain the epidermal stem cell compartment in vitro.