Silencing of the SNARE protein NAPA sensitizes cancer cells to cisplatin by inducing ERK1/2 signaling, synoviolin ubiquitination and p53 accumulation

We found earlier that NAPA represents an anti-apoptotic protein that promotes resistance to cisplatin in cancer cells by inducing the degradation of the tumor suppressor p53. In the present study, we investigated the cellular mechanism underlying the degradation of p53 by NAPA. Knockdown of NAPA using short-hairpin RNA was shown to induce p53 accumulation and to sensitize HEK293 cells to cisplatin. On the other hand, this sensitization effect was not found in H1299 lung carcinoma cells which lack p53. Expression of exogenous p53 in H1299 cells was increased following knockdown of NAPA and these cells showed increased sensitivity to cisplatin-induced apoptosis. Notably, knockdown of NAPA induced the ubiquitination and degradation of the E3 ubiquitin ligase synoviolin and the accumulation of p53 in unstressed HEK293 cells. Conversely, NAPA overexpression decreased the ubiquitination and degradation of synoviolin, and reduced p53 protein level. Knockdown of NAPA disrupted the interaction between synoviolin and proteins that form the endoplasmic reticulum-associated degradation (ERAD) complex and in turn decreased the ability of this complex to ubiquitinate p53. In addition, knockdown of NAPA induced the activation of the MAPK kinases ERK, JNK and p38, but only inhibition of ERK reduced synoviolin ubiquitination and p53 accumulation. These results indicate that NAPA promotes resistance to cisplatin through synoviolin and the ERAD complex which together induce the degradation of p53 and thus prevent apoptosis. Based on these findings, we propose that the combination of cisplatin and knockdown of NAPA represents a novel and attractive strategy to eradicate p53-sensitive cancer cells.