Ovarian cancer remains a deadly disease for countless women. Recent evidence demonstrates that ovarian cancer cell clusters, spheroids, are important in promoting ovarian recurrence and metastasis. Often these spheroids are resistant to therapy. Therefore, we were interested in identifying drugs that could target ovarian cancer spheroids. To do this, we wanted to identify pathways that were activated in cells growing in three-dimensional (3D) clusters. Analysis of gene expression from 2D and 3D growing cells identified STAT3 as a pathway enriched in 3D growing cells. STAT3 is a transcription factor that is activated by tyrosine phosphorylation. In ovarian cancer cells, activation usually occurs from interleukin 6 (IL-6) binding to the IL-6 receptor and recruiting the signal transducer molecule (GP130, IL6ST). This activates Jak kinases that then tyrosine phosphorylates STAT3. Once activated, STAT3 dimerizes, translocates into the nucleus, and regulates the expression of target genes. Using shRNA targeting STAT3 and the upstream signaling molecule, GP130, we found that reduction of both STAT3 and its upstream signaling molecule GP130 reduced the growth of spheroids. This suggested that targeting STAT3 or an upstream signaling molecule might be a viable treatment option. Atovaquone, used normally for the treatment of malaria or the prevention of pneumocystis pneumonia, inhibits STAT3 activation by inhibiting the expression of GP130 on the cell surface. Therefore, we hypothesized that atovaquone would have efficacy in ovarian cancer spheroids. Treatment of ovarian spheroids with atovaquone reduced their growth and viability. Importantly, atovaquone inhibited STAT3 tyrosine phosphorylation and target gene expression, demonstrating that atovaquone is inhibiting STAT3 in ovarian spheroids. Preliminary data suggest that overexpression of STAT3 partially rescues the effects of atovaquone on spheroid growth. As spheroids are also involved in ovarian cancer metastasis, we are currently assessing the effects of atovaquone on spheroid-mediated ovarian cancer metastasis. Taken together, our data suggest that atovaquone may be a beneficial treatment for ovarian cancer patients.
Citation Format: Kayli E. Neil, Sarah J. Lacroix, David F. Walker, Suhu Liu, David A. Frank, Sarah R. Walker. Atovaquone targets STAT3 in ovarian cancer spheroids [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research; 2019 Sep 13-16, 2019; Atlanta, GA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(13_Suppl):Abstract nr B14.