The RAS-RAF-MEK-extracellular signal-regulated kinase (ERK) pathway plays a pivotal role in various cellular responses, including cellular growth, differentiation, survival and motility. Constitutive activation of the ERK pathway has been linked to the development and progression of human cancers. Here, we reported that mitogen-activated protein kinase phosphatase (MKP)-3, a negative regulator of ERK1/2, lost its expression particularly in the protein level, was significantly correlated with high ERK1/2 activity in primary human ovarian cancer cells using quantitative reverse transcription-polymerase chain reaction and western blot analyses. Intriguingly, the loss of MKP3 protein was associated with ubiquitination/proteosome degradation mediated by high intracellular reactive oxygen species (ROS) accumulation such as hydrogen peroxide in ovarian cancer cells. Functionally, short hairpin RNA knock down of endogenous MKP3 resulted in increased ERK1/2 activity, cell proliferation rate, anchorage-independent growth ability and resistance to cisplatin in ovarian cancer cells. Conversely, enforced expression of MKP3 in MKP3-deficient ovarian cancer cells significantly reduced ERK1/2 activity and inhibited cell proliferation, anchorage-independent growth ability and tumor development in nude mice. Furthermore, the enforced expression of MKP3 succeeded to sensitize ovarian cancer cells to cisplatin-induced apoptosis in vitro and in vivo. These results suggest a molecular mechanism by which the accumulation of ROS during ovarian cancer progression may cause the degradation of MKP3, which in turn leads to aberrant ERK1/2 activation and contributes to tumorigenicity and chemoresistance of human ovarian cancer cells.

Background: Epigenetic gene silencing is one of the major causes of carcinogenesis. Its widespread occurrence in cancer genome could inactivate many cellular pathways including DNA repair, cell cycle control, apoptosis, cell adherence, and detoxification. The abnormal promoter methylation might be a potential molecular marker for cancer management.

Endometrial and ovarian cancers are the most common and the most lethal gynecologic malignancies worldwide, respectively. By performing differential expression analysis using annealing control primer-based reverse transcription (RT)-polymerase chain reaction (PCR) on pooled complementary DNA (cDNA) from 45 endometrial and 36 ovarian cancers and their non-tumor samples, reduced expression of the follistatin-like 1 (FSTL1) was identified. Downregulation of FSTL1 was further confirmed on individual samples and cell lines by quantitative real-time RT-PCR and western blotting. For in vitro functional study, full-length cDNA of FSTL1 was cloned and transiently transfected into the ovarian cancer cell line Ovca420 and endometrial cancer cell line AN3CA. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay and cell count demonstrated significantly slower proliferation rate. By terminal uridine deoxynucleotidyl transferase dUTP nick end labeling and flow cytometric analysis, higher apoptotic activity and a remarkable increase in sub-G(1) cell population were observed in transfected cells, suggesting that FSTL1 induced apoptosis in cancer cells. Subsequent messenger RNA and protein expression analysis on downstream apoptotic molecules revealed upregulation and/or activation of FAS, FASLG, TRADD, Caspase-3, Caspase-7 and PARP by FSTL1 transfection, suggesting that FSTL1-induced apoptosis may be initiated mainly by FAS/FASLG death receptor-ligand binding. Cell migration and invasion assays demonstrated a remarkably lower cell migration and invasion capability in FSTL1-transfected cells in relation to downregulation of matrix metallopeptidase-2. Our findings suggested that a tumor suppressor role of FSTL1 may be important in ovarian and endometrial carcinogenesis.

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