The MEK1/2 Pathway as a Therapeutic Target in High-Grade Serous Ovarian Carcinoma

SIMPLE SUMMARY: High-grade serous ovarian carcinoma (HGSOC) has poor prognosis for patients due to its high rate of recurrence and acquired resistance to therapy. MEK1/2-ERK1/2 signaling pathway that controls cell proliferation and survival is active in the majority of HGSOC cases, but its functional impact is unclear. We suggest that inhibition of MEK1/2 with specific inhibitor trametinib may exert anti-cancer effects upon HGSOC cells. Here we demonstrate that trametinib treatment of HGSOC cells indeed prominently inhibits cell proliferation and tumor growth, and that cisplatin-resistant cells displaying high MEK1/2 activity are particularly sensitive to trametinib. However, we also discovered that trametinib treatment of HGSOC cells has no cytotoxic effects and promotes cancer stem-like characteristics. We therefore suggest to use MEK1/2 inhibitors with other treatment strategies targeting cancer stem-like cells, like aldehyde dehydrogenase 1 inhibition that might show together strong synergy. ABSTRACT: 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.