Epigenetic Regulation of GDF2 Suppresses Anoikis in Ovarian and Breast Epithelia

Anoikis, a cell death mechanism triggered upon cell-matrix detachment, is regarded as a physiological suppressor of metastasis that can be regulated by a diverse array of signals. The protein encoded by GDF2 is BMP9 and is a member of the bone morphogenetic protein family and the transforming growth factor (TGF) β superfamily with emerging yet controversial roles in carcinogenesis. In an attempt to identify the function of growth and differentiation factor 2 (GDF2) in epithelial systems, we examined the signaling machinery that is involved and cell fate decisions in response to GDF2 in ovarian and breast epithelia. We find that GDF2 can robustly activate the SMAD1/5 signaling axis by increasing complex formation between the type I receptor serine threonine kinases activin receptor-like kinase (ALK) 3 and ALK6 and the type II receptor serine threonine kinase BMPRII. This activation is independent of cross talk with the SMAD2-transforming growth factor β pathway. By activating SMAD1/5, epithelial cells regulate anchorage-independent growth by increasing anoikis sensitivity that is dependent on GDF2's ability to sustain the activation of SMAD1/5 via ALK3 and ALK6. Consistent with a role for GDF2 in promoting anoikis susceptibility, the analysis of cell lines and patient data suggests epigenetic silencing of GDF2 in cancer cell lines and increased promoter methylation in patients. These findings collectively indicate an antimetastatic role for GDF2 in ovarian and breast cancer. The work also implicates loss of GDF2 via promoter methylation-mediated downregulation in promotion of carcinogenesis with significant relevance for the use of epigenetic drugs currently in clinical trials.