Abstract 4735: Epigenetic modification of the PI3K/Akt/HIF1α pathway in lymphoma cells by HDAC inhibition

Abstract

Abstract Hypoxia-inducible factors (HIFs) strongly influence many cellular process such as erythropoiesis, angiogenesis, energy metabolism, pH regulation, cell migration and tumor invasion. Under normoxic conditions, HIF-1α possesses a very short half-life and is rapidly degraded by the ubiquitin proteasome pathway and remains inactive. In hypoxia, HIF-1α is stabilized and forms a complex with HIF-1α that allows HIF-1 to function as a transcription factor. HIF-1α is activated only during hypoxia under normal physiologic conditions. By contrast, HIF-1α is frequently activated in cancer cells, including under normoxic conditions by oncogene products or impaired activity of tumor suppressor genes. Recent studies have shown that the expression of HIF1α is increased in a variety of human tumors a significant fraction of diffuse large B-cell lymphoma (DLBCL) and follicular lymphoma patients, implicating a potential role of dysregulated HIF activation. Constitutive expression of HIF-1α enhances vascularization, increases glucose metabolism, and induces the expression of anti-apoptotic proteins in cancer tissues, thus making it an attractive target for cancer therapy. We hypothesized that epigenetic modification of lymphoma cells by histone deacetylase (HDAC) inhibition may modify HIF-1α expression and lead to cell death. We therefore studied the role of the pan HDACi PCI-24781 (PCI) in HIF1α mediated cell death. Western blot analysis of cells treated with 0.125μM to 0.5μM of PCI show significant down regulation of HIF1α protein in all NHL cell lines. To confirm that down regulation of HIF1α is an important factor in cell apoptosis induced by PCI, we incubated cells with the small molecule inhibitor of HIF1α, PX-478 in combination with PCI. Combination of PCI and PX-478 at concentrations of 0.5μM and 10μM respectively, showed an increase in apoptosis as compared to either agent alone in all the DLBCL cell lines. Moreover, knocking down HIF1α in SUDHL4 and OCI-LY3 cell lines using shRNA followed by treatment with PCI also showed the similar effect. We further show that down regulation of HIF1α by PCI is mediated via the PI3K/Akt pathway and inhibition of the PI3K/Akt pathway by LY294002 inhibits HIF1α protein expression. Furthermore pre-incubation of cells with LY inhibits PCI-induced apoptosis. By contrast, constitutive activation of AKT using Myr-Akt-CA shows an increase in HIF1α protein and partial attenuation of PCI-induced apoptosis.We conclude that the activation of the PI3K/Akt/HIF-1α pathway plays a critical role in mediating hypoxia-induced drug resistance leading to unfavorable treatment outcome in lymphoma. Our study suggests a mechanism by which lymphoma cells become resistant to HDAC inhibitors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4735. doi:1538-7445.AM2012-4735