Mutation of the p53 tumor suppressor frequently occurs in lung cancer, and can be as high as 75‑90% in small‑cell lung cancer. Mutant p53 (mtp53) can inhibit the wild‑type p53 protein, disrupting its tumor suppressor functions. In addition, mutant p53 often acquires the functions of an oncogene. Post‑translational modification of the p53 protein is important for its transcriptional and tumor suppressive functions. We previously revealed that high levels of mutant p53 expression were associated with reduced expression of the deacetylation enzyme sirtuin 3 (SIRT3) in lung cancer tissues. Given this negative correlation between p53 and SIRT3 expression, and given that SIRT3 is a deacetylase, we speculated that SIRT3 participates in the post‑translational modification of mutant p53, regulating its stability and function, thereby inhibiting the growth of lung cancer cells. Light microscopy, MTT and flow cytometric assays revealed that SIRT3 overexpression inhibited growth and promoted apoptosis in NCI‑H446 human small cell lung cancer cells. SIRT3 overexpression also resulted in necroptosis, and this could be partially reversed following cell treatment with the necroptosis inhibitor necrostatin‑1 (Nec‑1), which could restore certain cells to survive. Western blotting assays revealed that SIRT3 overexpression resulted in the reduced expression and half‑life of mutant p53, indicating that SIRT3 decreases mutant p53 stability. Proteasome inhibitor experiments revealed that the decrease in mutant p53 stability was a result of increased proteasomal degradation of the protein. Immunoprecipitation studies revealed that ubiquitination of mutant p53 was elevated in SIRT3‑overexpressing cells, indicating that SIRT3 affected ubiquitination‑mediated protein degradation. In the present study, it was therefore revealed that SIRT3 can inhibit the growth of human small‑cell lung cancer cells by promoting apoptosis and necroptosis. It was also revealed that SIRT3 expression could regulate the stability of mutant p53 by controlling ubiquitination‑mediated proteasomal degradation of the protein. SIRT3 expression may therefore play an important role in the growth of mutant p53‑associated lung cancer.
Keywords: SCLC; mutant p53; SIRT3; apoptosis; necroptosis.