Regulation of Bcl-x_Lexpression in human keratinocytes by cell–substratum adhesion and the epidermal growth factor receptor

Abstract

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 was 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.

EGF receptor signaling inhibits keratinocyte apoptosis: evidence for mediation by Bcl-XL

“…Heparin-binding EGF-like growth factor (HB-EGF) is included in Figure 3a (Nass et al, 1996). Moreover, Rodeck and colleagues have recently reported results similar to our own in keratinocytes (Rodeck et al, 1997a). Our studies are the ®rst to demonstrate a functional role for Bcl-X L in EGFR-dependent KC survival.…”

“…However, we found that protection from PD153035-induced apoptosis was never complete (Figure 4). Rodeck and colleagues reported a similar result, in that keratinocyte apoptosis developed only gradually after mAb-mediated blockade of ErbB-1 signaling, becoming detectable only after 4 to 5 days of mAb treatment (Rodeck et al, 1997b). These ®ndings suggest at least three possibilities: (i) that loss of ErbB signaling triggers multiple downstream cell death pathways, only some of which can be blocked by Bcl-X L ; (ii) that Bcl-X L exerts only a modulatory or stochastic e ect on a ®nal common pathway of cell death, rather than an all-or-nothing e ect; or (iii) that the ErbB pathway interacts with other signaling systems to maintain cell survival.…”

“…These ®ndings suggest at least three possibilities: (i) that loss of ErbB signaling triggers multiple downstream cell death pathways, only some of which can be blocked by Bcl-X L ; (ii) that Bcl-X L exerts only a modulatory or stochastic e ect on a ®nal common pathway of cell death, rather than an all-or-nothing e ect; or (iii) that the ErbB pathway interacts with other signaling systems to maintain cell survival. Rodeck's ®nding that anti-ErbB-1-induced apoptosis was dramatically increased by subsequent trypsinization (Rodeck et al, 1997b) favors the third possibility, and suggests that cell contacts provide strong survival signals to keratinocytes. In fact, deprivation of cell-substratum contact has been shown to induce dramatic apoptosis in keratinocytes, accompanied by reduced expression of Bcl-X L (Frisch and Francis, 1994;Mitta et al, 1997).…”

Inhibition of EGFR-mediated phosphoinositide-3-OH kinase (PI3-K) signaling and glioblastoma phenotype by Signal-Regulatory Proteins (SIRPs)

“…Bcl-X L levels returned to baseline 72 h following irradiation in U87/SIRPa1 cells, while cells expressing vector without insert showed sustained elevation in bcl-X L protein levels 24 ± 72 h following g-irradiation ( Figure 3d). Modulation of Bcl-X L protein levels has also been shown to in¯uence sensitivity to apoptosis in EGFR-containing keratinocytes exposed to cell death signals (Rodeck et al, 1997).…”

Detachment-induced upregulation of XIAP and cIAP2 delays anoikis of intestinal epithelial cells

“…Anoikis of various types of non-malignant epithelial cells such as those of intestine, skin and breast is executed by several, rather than one, proapoptotic mechanisms (Rodeck et al, 1997;Rytomaa et al, 1999;Gilmore et al, 2000;Rosen et al, 2000Rosen et al, , 2002Puthalakath et al, 2001;Reginato et al, 2003). The reasons why several death-promoting events are required for the induction of this form of apoptosis have not so far been identified.…”