Oxysterols synergize with statins by inhibiting SREBP-2 in ovarian cancer cells

Objective: Determine mechanisms responsible for enhanced statin efficacy in a novel statin combination we name STOX (STatin-OXysterol).

Methods: Ovarian cancer cell lines were treated with combinations of statins and oxysterols. Cell viability was determined by a modified MTT assay. Apoptosis was evaluated by immunoblotting of PARP and DAPI-mediated visualization of apoptotic nuclei. STOX effects on the expression of genes of the mevalonate pathway were assessed by real-time qPCR and immunoblotting. siRNA-mediated gene silencing was used to test the involvement of oxysterol-mediated repression of SREBP-2 in STOX synergy. The impact of statin-mediated inhibition of protein prenylation and on cholesterol homeostasis was evaluated.

Results: Oxysterols dramatically enhance cytotoxicity of statins in ovarian cancer cells through increased apoptosis. Decreased expression of SREBP-2 down-regulates the mevalonate pathway and prevents the active statin-induced sterol feedback, enhancing statin toxicity. Comparison of two ovarian cancer cell lines reveals two distinct mechanisms of statin induced toxicity, namely, dependence on protein geranylgeranylation and/or perturbation of cellular cholesterol levels.

Conclusions: We provide evidence of statins' mechanisms of cytotoxicity in different ovarian cancer cells and discovered a new approach to significantly enhance the anti-tumor activity of statins. These observations provide a potential new path to improve statins as a treatment against ovarian cancer with obtainable dosages.