P53 Inhibits α6β4 Integrin Survival Signaling by Promoting the Caspase 3–Dependent Cleavage of Akt/PKB

Abstract

Although the interaction of matrix proteins with integrins is known to initiate signaling pathways that are essential for cell survival, a role for tumor suppressors in the regulation of these pathways has not been established. We demonstrate here that p53 can inhibit the survival function of integrins by inducing the caspase-dependent cleavage and inactivation of the serine/threonine kinase AKT/PKB. Specifically, we show that the α6β4 integrin promotes the survival of p53-deficient carcinoma cells by activating AKT/PKB. In contrast, this integrin does not activate AKT/PKB in carcinoma cells that express wild-type p53 and it actually stimulates their apoptosis, in agreement with our previous findings (Bachelder, R.E., A. Marchetti, R. Falcioni, S. Soddu, and A.M. Mercurio. 1999. J. Biol. Chem. 274:20733–20737). Interestingly, we observed reduced levels of AKT/PKB protein after antibody clustering of α6β4 in carcinoma cells that express wild-type p53. In contrast, α6β4 clustering did not reduce the level of AKT/PKB in carcinoma cells that lack functional p53. The involvement of caspase 3 in AKT/PKB regulation was indicated by the ability of Z-DEVD-FMK, a caspase 3 inhibitor, to block the α6β4-associated reduction in AKT/PKB levels in vivo, and by the ability of recombinant caspase 3 to promote the cleavage of AKT/PKB in vitro. In addition, the ability of α6β4 to activate AKT/PKB could be restored in p53 wild-type carcinoma cells by inhibiting caspase 3 activity. These studies demonstrate that the p53 tumor suppressor can inhibit integrin-associated survival signaling pathways.

Increased cell survival, migration, invasion, and Akt expression in PTHrP-overexpressing LoVo colon cancer cell lines

“…This effect of integrin α6β4 on apoptosis is linked to the p53 status of the cell. In cells expressing wildtype p53, such as LoVo cells [58], integrin α6β4 stimulates p53-dependent caspase-3 activity, resulting in the cleavage and inactivation of Akt [59,60] and a loss of the pro-survival effects. However, we report that there is no significant difference in total Akt levels between cells with elevated integrin α6β4 levels (PTHrP-overexpressing cells) vs. parental LoVo cells, with lower integrin α6β4 levels.…”

Activation and activities of the p53 tumour suppressor protein

“…Loss of survival signalling can augment p53-mediated apoptosis (Gottlieb et al, 1994;Abrahamson et al, 1995;Canman et al, 1995;Lin and Benchimol, 1995;Prisco et al, 1997), and the ability of p53 to negatively regulate the IGF pathway (Buckbinder et al, 1995) and inhibit intergrin-associated survival signalling may further sensitize cells to p53-induced death (Bachelder et al, 1999). The NF-κB transcription factor has lately been shown to play an important role in p53-mediated apoptosis (Ryan et al, 2000), in contrast to the anti-apoptotic effect of NF-κB induced in response to TNF (Van Antwerp et al, 1996;Phillips et al, 1999).…”

p53 can inhibit cell proliferation through caspase-mediated cleavage of ERK2/MAPK

“…21,22 Thus, we evaluated whether ERK2 can be a caspase-3 target by an in vitro cleavage assay. As shown in Figure 4f, recombinant caspase-3 can cleave an in vitrotranslated ERK2 protein and yield a major product of approximately 39 kDa.…”

“…The hurdle in detecting cleavage products in vivo is not unusual even during apoptosis. 22,32 Indeed, the appearance of proteolytic products is mainly correlated with the production of active and stable enzymes from pro-enzymes (e.g., caspase cascade activation). Instead, our data show that the ERK2 cleavage has an inhibitory effect on its activity, supporting the existence of a subsequent rapid degradation mechanism of the 39 kDa polypeptide.…”