Early events in the induction of apoptosis in ovarian carcinoma cells by CD437: activation of the p38 MAP kinase signal pathway

Retinoids have great potential in the areas of cancer therapy and chemoprevention. 6-[3-(1-admantyl)]-4-hydroxyphenyl]2-naphthalene carboxylic acid (CD437) is a conformationally restricted synthetic retinoid that has been reported to induce growth arrest and apoptosis in ovarian tumor cell lines but the entire mechanism for apoptotic induction has not been fully defined. We set out to identify the early events of CD437-induced apoptosis of the CA-OV-3 cell line and determine if these occur in a CA-OV-3 cell line resistant to CD437 (CA-CD437R). Using inhibitors for the MAP kinase cascade, we determined that MEK and p38 inhibitors could block CD437-induced apoptosis of the CA-OV-3 cell line. Moreover, treatment of CA-OV-3 and CA-CD437R cells with CD437 resulted in increased phosphorylation and activity of p38 independent of caspase-3 activation. Furthermore, p38 induced the phosphorylation of MEF2 in both CA-OV-3 and CA-CD437R cells after CD437 treatment. Finally, GFP-TR3 protein translocated to the cytosol and associated with mitochondria in both cell lines in response to CD437 treatment. This leads to depolarization of mitochondria and subsequent induction of apoptosis only in CA-OV-3 cells. These results identify a number of initial molecular events in the induction of apoptosis by CD437 in CA-OV-3 cells and demonstrate that the alteration in CA-CD437R cells, which results in resistance to CD437 maps downstream of these early events after TR3 translocation but prior to mitochondrial depolarization.