OVCA2 is downregulated and degraded during retinoid-induced apoptosis

Retinoids, the natural and synthetic derivatives of vitamin A, have been shown to regulate the growth and differentiation of a wide variety of cell types and consequently have enormous potential as chemotherapeutic agents. We have previously identified 2 genes, termed OVCAI and OVCA2, which are located in a small region showing a high frequency of allelic loss in breast and ovarian tumors and share a common exon. Recent studies have suggested that expression of OVCAI may be influenced by retinoids. Therefore, we analyzed the expression of OVCAI and OVCA2 in cells in response to treatment with all-trans retinoic acid (RA) and N.(4-hydroxyphenyl)retinamide (4HPR), or under conditions of low serum and confluence, to determine further the roles of OVCAI and OVCA2 in cell growth, apoptosis and differentiation. We show that OVCA2 mRNA and protein are ubiquitously expressed and that they are downregulated in the lung cancer cell line Calu-6 after treatment with RA and 4HPR. In addition, we observed that OVCA2 protein is proteolytically degraded in response to RA and 4HPR treatment in a time- and dose-dependent manner in the promyelocytic leukemia cell line HL60. In contrast, expression of the candidate tumor suppressor OVCAI was not downregulated by these treatments. Furthermore, we demonstrate that OVCA2 is evolutionarily conserved and shows regional homology with dihydrofolate reductases (DHFRs), specifically with hydrolase folds found in α-β hydrolases. Our results are in contrast to a previous report and show that OVCA2, not OVCAI mRNA and protein, is downregulated in response to RA and 4HPR.