Ovarian granulosa cell tumours (GCT) are hormonally active cancers characterised by indolent growth and late, invasive relapse. Our research has established that several nuclear receptors, including the anti-proliferative peroxisome proliferator-activated receptor gamma (PPARγ), are upregulated in GCT. Subsequent studies using two GCT-derived cell lines (KGN and COV434), revealed that PPARγ activity in these cell lines is transrepressed by constitutive NF-κB signalling. NF-κB induces a key effector protein, X-linked inhibitor of apoptosis protein (XIAP). Small molecule inhibitors of XIAP and PPARγ agonists have been developed as potential anti-tumour therapeutic agents. We investigated what effects XIAP inhibition and PPARγ activation have on GCT cells. We hypothesise that XIAP antagonism sensitises GCT cells to pro-apoptotic strategies such as PPARγ activation.
We analysed PPARγ and XIAP expression in GCT, other ovarian cancer subtypes and normal ovarian tissues using tissue microarray and immunohistochemistry. Strong immunostaining for PPARγ and XIAP was observed in stage 1 and recurrent GCT as well as other ovarian cancers but not in normal ovaries. Utilizing apoptosis and proliferation assays, we observed that combined PPARγ activation and XIAP inhibition in GCT-derived cells significantly increases apoptosis, decreases cell proliferation and viability. We examined using transactivation assays, the effect of small molecule inhibition of XIAP on PPARγ activity. PPARγ signalling in GCT-derived cells was restored when treated with PPARγ agonist (rosiglitazone) and XIAP inhibitor (smac mimetic).
The overexpression of PPARγ and XIAP in GCT warrants investigation as potential targets for anti-tumour therapy. Removal of NF-κB transrepression of PPARγ leads to increase apoptosis in GCT cells. Given a similar effect is observed with XIAP inhibition on PPARγ transactivation, we are currently determining if XIAP regulates NF-κB activity as part of a regulatory loop. This will improve understanding of the molecular mechanisms in GCT pathophysiology and enable identification of new targets for therapeutic strategies.