Retinoic acid metabolism in cancer: potential feasibility of retinoic acid metabolism blocking therapy.

Retinoic acid (RA) is an active metabolite of vitamin A, which is an essential signaling molecule involved in cell fate decisions, such as differentiation, proliferation, and apoptosis, in a wide variety of cell types. Accumulated data have demonstrated that expression of RA-metabolizing enzymes, CYP26A1, B1, and C1 (cytochrome P450, family 26A1, B1, and C1, respectively), protects cells and tissues from exposure to RA through restriction of RA access to transcriptional machinery by converting RA to rapidly excreted derivatives. CYP26 enzymes play similar but separate roles in limiting the consequences of fluctuations in nutritional vitamin A. Recently, we found that RA depletion caused by expression of CYP26A1 promotes malignant behaviors of tumor cells derived from various tissues, implicating CYP26A1 as a candidate oncogene. We also showed that the expression levels of CYP26 enzymes are elevated in various types of cancer. We have provided evidence for oncogenic and cell survival properties of CYP26 enzymes, indicating that these molecules are possible therapeutic targets for CYP26-expressing malignancies.