High-grade serous ovarian carcinoma (HGSOC) is the deadliest of gynecological cancers due to its high recurrence rate and acquired chemoresistance. RAS/MEK/ERK pathway activation is linked to cell proliferation and therapeutic resistance, but the role of MEK1/2-ERK1/2 pathway in HGSOC is poorly investigated. We evaluated MEK1/2 pathway activity in clinical HGSOC samples and ovarian cancer cell lines using immunohistochemistry, immunoblotting, and RT-qPCR. HGSOC cell lines were used to assess immediate and lasting effects of MEK1/2 inhibition with trametinib in vitro. Trametinib effect on tumor growth in vivo was investigated using mouse xenografts. MEK1/2 pathway is hyperactivated in HGSOC and is further stimulated by cisplatin treatment. Trametinib treatment causes cell cycle arrest in G1/0-phase and reduces tumor growth rate in vivo but does not induce cell death or reduce fraction of CD133+ stem-like cells, while increasing expression of stemness-associated genes instead. Transient trametinib treatment causes long-term increase in a subpopulation of cells with high aldehyde dehydrogenase (ALDH)1 activity that can survive and grow in non-adherent conditions. We conclude that MEK1/2 inhibition may be a promising approach to suppress ovarian cancer growth as a maintenance therapy. Promotion of stem-like properties upon MEK1/2 inhibition suggests a possible mechanism of resistance, so a combination with CSC-targeting drugs should be considered.

Rationale: High-grade serous ovarian carcinoma (HGSOC) is the deadliest of gynecological cancers due to high rate of recurrence and acquired chemoresistance. Mutation and activation of the RAS/MAPK pathway has been linked to cancer cell proliferation and therapeutic resistance in numerous cancers. While RAS mutations are not commonly observed in HGSOC, less is known about downstream pathway activation. We therefore sought to investigate the role of MEK1/2 signaling in ovarian cancer.Methods: MEK1/2 pathway activity was evaluated in clinical HGSOC tissue samples and ovarian cancer cell lines by using tissue microarray-based immunohistochemistry, immunoblotting, and RT-qPCR. OVCAR8 and PEO4 HGSOC cell lines were used to assess the effect of MEK1/2 inhibition on cell viability, proliferation rate, and stem-like characteristics. Xenografts were used in mice to investigate the effect of MEK1/2 inhibition on tumor growth in vivo. A drug washout experimental model was used to study the lasting effects of MEK1/2 inhibition therapy.Results: MEK1/2 signaling is active in a majority of HGSOC tissue samples and cell lines. MEK1/2 is further stimulated by cisplatin treatment, suggesting that MEK1/2 activation may play a role in chemotherapy resistance. The MEK1/2 inhibitor, trametinib, drastically inhibits MEK1/2 downstream signaling activity, causes prominent cell cycle arrest in the G1/0-phase in cell cultures, and reduces the rate of tumor growth in vivo, but does not induce cell death. Cells treated with trametinib display a high CD133 + fraction and increased expression of stemness-associated genes.Transient trametinib treatment causes long-term increases in a high ALDH1 activity subpopulation of cells that possess the capability of surviving and growing in non-adherent conditions. Conclusions: MEK1/2 inhibition in HGSOC cells efficiently inhibits proliferation and tumorgrowth and therefore may be a promising approach to suppress ovarian cancer cell growth. MEK1/2 inhibition promotes stem-like properties, thus suggesting a possible mechanism of resistance and that a combination with CSC-targeting drugs should be considered.