Netrin-1 induces apoptosis in human cervical tumor cells via the TAp73 alpha tumor suppressor

Netrins and their receptors deleted in colon cancer (DCC), neogenin, UNC5, and integrins are involved in axon guidance, epithelial morphogenesis, vascular pattering, cancer cell survival, invasion, tumor angiogenesis, and metastasis. Here, we considered the possible contribution of the p53-related apoptosis mediators p63 and p73 in the mechanisms underlying the antagonism between netrin-1 and DCC at the cell death control. We have showed that ectopic expression and external addition of netrin-1 in HeLa and HEK-293 cells with inactive p53 lead to impaired cell viability and induction of apoptosis. These responses were associated with up-regulation of the proapoptotic protein TAp73 alpha, decreased Bcl-2/Bax ratio, and caspase-3 cleavage, with no change in protein levels of the antiapoptotic NH2-terminal-truncated Delta Np73 alpha isoform, p73 adapter Yap-1 and p73 E3 ubiquitin ligase Itch, and p63, as well as the transcripts encoding p63, TAp73 alpha, and Delta Np73 alpha. However, the proteasome inhibitor MG132 potentiated, while DCC counteracted, netrin-1-induced TAp73 alpha. Consistently, netrin-1 expression correlated with stabilization of the TAp73 alpha protein and lower levels of TAp73 alpha ubiquitination that was conversely enhanced by DCC, in a netrin-dependent manner. Our data indicate that netrin-1 selectively up-regulates TAp73 alpha, by preventing its ubiquitination and degradation. Targeted repression of p73 alpha by shRNA reversed TAp73 alpha and the apoptosis induced by netrin-1, and exacerbated the growth of HeLa tumor xeno-grafts. Apoptosis induced by cisplatin was markedly enhanced in netrin-1 or DCC-expressing cells. Collectively, our data reveal that the transcriptionally active TAp73 alpha, tumor suppressor is implicated in the apoptosis induced by netrin-1 in a p53-independent and DCC/ubiquitin-proteasome dependent manner