Abstract 3968: Mutation and phosphorylation of p53 may switch TGF-beta from a tumor suppressor to a tumor promoter

Transforming growth factor-β (TGF-β) is a tumor suppressor during early tumor outgrowth, but becomes a tumor promoter at later stages by enhancing tumor cell motility, invasion and metastasis. Therefore, anti-TGF-β cancer therapy should exercise great caution due to the dual nature of the TGF-β signaling. However, it is still unclear when the TGF-β inhibitor should be applied to abrogate the tumor-promoting activity of TGF-β signaling. Thus, studies aiming to unravel key mechanisms by which TGF-β signaling is switched from tumor suppressor to tumor promoter will greatly benefit the design of efficient clinical treatment of cancer with TGF-β inhibitors. Recently, it was reported that the oncogenic activity of TGF-β in the MDA-MB-231 breast cancer cells was due to the presence of the mutant p53. TGF-β-induced cell migration and metastasis were enhanced by the sequestration of the tumor suppressor p63 into a ternary complex containing the mutant p53 and the intracellular TGF-β signaling mediators Smad2/3. The phosphorylation of mutant p53 at Ser6/Ser9 by Ras/MAPK signaling was essential for the formation of the ternary complex and appeared to be a crucial hallmark for the switch of TGF-β to be a tumor promoter. To better understand the molecular mechanism driving the switch of TGF-β pathway to become a tumor promoter, we performed a study using spontaneously immortalized and non-tumorigenic human mammary epithelial MCF-10A cell line (10A). We found that ectopic expression of both oncogenic H-RasV12 and mutant p53 (R175H) in 10A enhanced phosphorylation of p53 (P-p53) at Ser6/9 and TGF-β-induced formation of p53/P-Smad2 complex. Additionally, the cellular migration of those transformed 10A was more sensitive to TGF-β when compared with 10A expressing H-RasV12 alone. These results indicate that mutant p53 may induce TGF-β to become a tumor promoter in our model derived from 10A. Notably, we found there might be a PI3K/AKT signaling-dependent fashion for the switch of TGF-β to become a tumor promoter as 10A with ectopic expression of Simian virus 40 T antigens also expressed a high level of P-p53 at Ser6/9 with active PI3K/AKT signaling in contrast to active MAPK signaling. Furthermore, the level of P-p53 at Ser9 was elevated in the 10A and the p53-null PC-3 prostate cancer cells with ectopic expression of both constitutively active AKT (*AKT) and mutant p53 R175H. Importantly, TGF-β-induced interaction among p63, p53 R175H and P-Smad2 was significantly increased in the absence of a strong MAPK signaling activity in those transformed cells. Taken together, our studies may provide a novel mechanism for switching TGF-β to play the oncogenic role. In addition, we can use the model systems with key molecular alterations such as mutant p53 together with oncogenic Ras or *AKT to test the efficacy of TGF-β inhibitors as a novel cancer therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3968.