Regulation of Bcl-x(L) expression in human keratinocytes by cell-substratum adhesion and the epidermal growth factor receptor

Cell-substratum adhesion is an essential requirement for survival of human neonatal keratinocytes in vitro, Similarly, activation of the epidermal growth factor receptor (EGF-R) has recently been implicated not only in cell cycle progression but also in survival of normal keratinocytes, The mechanisms by which either cell-substratum adhesion or EGF-R activation protect keratinocytes from programmed cell death are poorly understood, Here we describe that blockade of the EGF-R and inhibition of substratum adhesion share a common downstream event, the down-regulation of the cell death protector Bcl-x(L), Expression of Bcl-x(L) protein was down-regulated during forced suspension culture of keratinocytes, concurrent with large-scale apoptosis, Similarly, EGF-R blockade,vas accompanied by down-regulation of Bcl-x(L) steady-state mRNA and protein levels to an extent comparable to that observed in forced suspension culture, However, down-regulation of Bcl-x(L) expression by EGF-R blockade was not accompanied by apoptosis; in this case, a second signal, generated by passaging, was required to induce rapid and large-scale apoptosis. These findings are consistent with the conclusions that (i) Bcl-x(L) represents a shared molecular target for signaling through cell-substrate adhesion receptors and the EGF-R, and (ii) reduced levels of Bcl-x(L) expression through EGF-R blockade lower the tolerance of keratinocytes for cell death signals generated by cellular stress.