Activation of p53 Function in Carcinoma Cells by the α6β4 Integrin

Abstract

The interaction of integrins with extracellular matrix is known to promote cell survival by inhibiting apoptotic signaling. In contrast, we demonstrate here that the ␣ 6 ␤ 4 integrin induces apoptosis in carcinoma cells by stimulating p53 function. Specifically, we show that expression of ␣ 6 ␤ 4 in carcinoma cells that lack this integrin stimulates an increase in the transactivating function of p53 as demonstrated by the ability of this integrin to up-regulate the expression of a p53-sensitive reporter gene as well as the endogenous p53 response gene, bax. In addition, we report that ␣ 6 ␤ 4 triggers apoptosis in carcinoma cells that express wild-type but not mutant p53 and that these ␣ 6 ␤ 4 functions are inhibited by a dominant negative p53 construct. Importantly, we provide a link between integrin signaling and p53 activation by demonstrating that the clustering of ␣ 6 ␤ 4 with a ␤ 4 integrin-specific antibody promotes p53-dependent apoptosis in cells that express both ␣ 6 ␤ 4 and wild-type p53. These studies are the first to demonstrate that a specific integrin can promote apoptosis by activating p53. Moreover, given the ability of ␣ 6 ␤ 4 to stimulate invasion (Shaw, L. M., Rabinovitz, I., Wang, H. F., Toker, A., and Mercurio, A. M. (1997) Cell 91, 949 -960), these studies suggest that the ability of ␣ 6 ␤ 4 to promote carcinoma progression will be enhanced in tumor cells that express mutant, inactive forms of p53.Integrins are the primary receptors used by cells to interact with extracellular matrices. Although initial studies had emphasized the functional contribution of integrins to cell adhesion and migration, a significant finding was the observation that integrins are essential for cell survival (2, 3). Specifically, epithelial cells, endothelial cells, and fibroblasts are prone to growth arrest and apoptosis when deprived of integrin-mediated contact with the extracellular matrix (4, 5). To date, several integrins including ␣ 5 ␤ 1 (6 -7), ␣ v ␤ 3 (8 -10), and ␣ 6 ␤ 1 (11) have been implicated in the promotion of cell growth and survival.Arguably, one of the most complex integrins in terms of both structure and function is ␣ 6 ␤ 4 . This integrin is distinguished structurally from other integrins on the basis of the unusually large cytoplasmic domain of its ␤ 4 subunit (12-14). Aside from its involvement in cell adhesion and migration (1,(15)(16)(17)(18)(19)(20), the ␣ 6 ␤ 4 integrin can promote growth arrest and apoptosis in some carcinoma cells. Specifically, we reported that ␣ 6 ␤ 4 expression induces the growth arrest and apoptosis of the RKO colon carcinoma cell line (21), a finding that has been substantiated in other carcinoma cell lines (22-23), as well as in endothelial cells (24). These findings, however, conflict with considerable evidence that supports a role for ␣ 6 ␤ 4 in promoting carcinoma invasion and progression (1,19). In order to understand how ␣ 6 ␤ 4 can deliver these apparently conflicting signals, we analyzed the mechanism by which ␣ 6 ␤ 4 promotes apoptosis in more detail. S...

p53 activation in response to microtubule disruption is mediated by integrin-Erk signaling

“…The e ect was transient: 18 h after plating onto ®bronectin the cells showed higher reporter gene activity, but by 36 h the activity went down. Along with recent observations showing induction of p53-mediated apoptosis in response to expression of alpha 6 beta 4 in carcinoma cells which lack this integrin (Bachelder et al, 1999) our results support the idea that activation of integrin-ERK pathways can trigger, at least in some cell contexts, p53 up-regulation. It should be noted, however, that while activation of p53 following ®bronectin plating was transient, disruption of microtubules caused sustained activation of p53 that was observed during all the period of treatment with colcemid.…”

The Calcium/Calcineurin Pathway Promotes Hemidesmosome Stability through Inhibition of β4 Integrin Phosphorylation

“…CN inhibitor-induced ␤4 phosphorylation and HD disruption may further contribute to the dissemination of the tumor by facilitating migration. Interestingly, ␤4 is known to activate p53, thus promoting apoptosis (36), and therefore, it would be valuable to determine whether phosphorylation can modify this capability and add to the effects of ATF3.…”

Integrins regulate the apoptotic response to DNA damage through modulation of p53

“…In these cells, there is some evidence for pathways linking integrin ␣6␤4 to activation of p53. Bachelder et al (21) showed that overexpression of ␣6␤4 integrin in suspended carcinoma cells, where the integrin would be unoccupied by any ECM ligand, activates p53 and induces apoptosis. They also reported that antibody crosslinking of unoccupied integrins accelerated apoptosis; however, this effect is unlikely to reflect interactions of ␣6␤4 with basement membranes, because adhesion to basement membranes promotes epithelial cell survival.…”

“…In epithelial and endothelial cells, integrin ligation regulates cell survival such that detachment from the ECM rapidly induces apoptosis (17,18). In some of these cases, cell death results from increased levels of p53 or Bax caused by detachment or inhibition of integrins or by overexpression of unligated integrins (19)(20)(21). Correspondingly, binding of some carcinomas to the ECM protects against apoptosis initiated by DNA damage (22).…”