Data from Netrin-1 Induces Apoptosis in Human Cervical Tumor Cells via the TAp73α 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 TAp73α, decreased Bcl-2/Bax ratio, and caspase-3 cleavage, with no change in protein levels of the antiapoptotic NH₂-terminal–truncated ΔNp73α isoform, p73 adapter Yap-1 and p73 E3 ubiquitin ligase Itch, and p63, as well as the transcripts encoding p63, TAp73α, and ΔNp73α. However, the proteasome inhibitor MG132 potentiated, while DCC counteracted, netrin-1–induced TAp73α. Consistently, netrin-1 expression correlated with stabilization of the TAp73α protein and lower levels of TAp73α ubiquitination that was conversely enhanced by DCC, in a netrin-dependent manner. Our data indicate that netrin-1 selectively up-regulates TAp73α by preventing its ubiquitination and degradation. Targeted repression of p73α by shRNA reversed TAp73α and the apoptosis induced by netrin-1, and exacerbated the growth of HeLa tumor xenografts. Apoptosis induced by cisplatin was markedly enhanced in netrin-1 or DCC-expressing cells. Collectively, our data reveal that the transcriptionally active TAp73α tumor suppressor is implicated in the apoptosis induced by netrin-1 in a p53-independent and DCC/ubiquitin-proteasome dependent manner. [Cancer Res 2008;68(20):8231–9]