The Heat Shock Protein 70 is a Novel Target for Nobiletin in Human Colon Cancer Cells

Abstract

Nobiletin is a unique flavonoid mainly found in citrus fruits, and has been reported to inhibit colon carcinogenesis in multiple rodent models. However, the direct molecular targets of nobiletin are unknown. Heat shock protein 70 (HSP70) contributes to cancer cell survival and resistance to chemotherapies, thereby inhibition of HSP70 is a promising strategy in cancer chemoprevention. Using affinity chromatography, we demonstrated that nobiletin bound to HSP70 at its ATP‐binding domains in human colon cancer cells and specifically inhibited its ATPase activity. Down regulation of HSP70 by siRNAs significantly increased cancer cells’ sensitivity to nobiletin treatments, which were evidenced by increased apoptosis and decreased colony formation capacity of cancer cells. The association between HSP70 and HSP90 is critical for the stability of their client proteins, such as EGFR, AKT, and STAT3, which contributes to carcinogenesis. We found that nobiletin disrupted the association between HSP70 and HSP90, and combined treatment with nobiletin and HSP90 inhibitors promoted the degradation of those client proteins. Moreover, combined treatment with nobiletin and HSP90 inhibitors resulted in enhanced inhibition on cancer cell growth by inducing apoptosis and cell cycle arrest. Most interestingly, our results also demonstrated that the major colonic metabolites of nobiletin (generated in the colon of mice fed with nobiletin) showed similar inhibitory effects against HSP70‐mediated signaling pathways. Overall, our results demonstrated, for the first time, that HSP70 is a promising target of nobiletin and its colonic metabolites in inhibiting human colon cancer cell growth, which warrants further investigation on their role in colon cancer chemoprevention.Support or Funding InformationThis study was supported by funding from NIH, AICR and USDA.