The Role of FOXO3 in Mechanisms of Doxorubicin Resistance in Hepatocellular Carcinoma

Abstract

Trans-arterial chemoembolization (TACE) with doxorubicin is commonly used to treat hepatocellular carcinoma (HCC), but has limited efficacy due to a high level of resistance. The factors that determine the sensitivity to TACE-doxorubicin are unknown. FOXO3 is a multifunctional transcription factor that plays a role in determining cell fate in response to stress. It frequently functions as a tumor suppressor but can also promote tumor pathogenesis. FOXO3 is also known to be a mediator of doxorubicin sensitivity in many types of tumor cells, while in others it can promote resistance. The role of FOXO3 in HCC and in doxorubicin resistance in HCC is unknown. FOXO3 function is largely determined by post-translational modification (PTM). Two FOXO3 PTMs, acetylation and serine 574 (S574)-phosphorylation, are known to promote its apoptotic function. Contrary to expectations, expression of FOXO3 was increased in HCC compared to surrounding liver. Cytosolic FOXO3 was significantly greater in TACE-resistant HCC as compared to treatment-naïve tumors. In Huh7 hepatoma cells, doxorubicin induced acetylation and S574-phosphorylation of FOXO3, and these modifications promoted doxorubicin-induced cell death by suppressing the pro-survival function of FOXO3. Resveratrol, an activator of SIRT deacetylase enzymes, inhibited these doxorubicin-induced PTMs and increased doxorubicin resistance. The expression of SIRT6, a known FOXO3 deacetylase, was also increased in TACE-resistant HCC tumors and correlated with cytosolic FOXO3. SIRT6 also blocked doxorubicin-induced S574-phosphorylation of FOXO3 and increased resistance to doxorubicin in Huh7 cells. Therefore, targeting SIRT6 and/or manipulating FOXO3 modifications may prove useful in enhancing the chemotherapy sensitivity of HCC.