HUMAN RETINOIC ACID (RA) 4-HYDROXYLASE (CYP26) IS HIGHLY SPECIFIC FORALL-TRANS-RA AND CAN BE INDUCED THROUGH RA RECEPTORS IN HUMAN BREAST AND COLON-CARCINOMA CELLS

We report on the isolation of a cytochrome P450 (CYP)-like retinoic ac id (RA) 4-hydroxylase cDNA from T-47D human breast cancer cells that i s identical to the recently cloned hCYP26, which is involved in the me tabolic breakdown of PA, Northern analysis showed that this novel huma n CYP26 is induced within 1 h upon PA treatment in RA-sensitive T-47D breast carcinoma cells but not in RA-resistant MDA-MB-231 breast cance r cells and HCT 116 colon cancer cells. Stable introduction of differe nt RA receptor (RAR) subtypes in HCT 116 cells showed that CYP26 expre ssion is dependent on RAR alpha and RAR gamma and, to a lesser extent, on RAR beta and closely paralleled RA metabolism, suggesting that it represents the major RA 4-hydroxylase in these human cells. Furthermor e, stable introduction of all three RAR subtypes in HCT 116 cells resu lted in restored PA sensitivity as assayed by growth inhibition. Inter estingly, CYP26 activity was efficiently inhibited by liarozole, an in hibitor of PA metabolism, leading to enhanced growth inhibition by PA, The RA-induced CYP26 was shown to be highly specific for the hydroxyl ation of all-trans-PA and did not recognize the 13-cis and 9-cis isome rs, This substrate specificity is promising for finding retinoids that are not recognized by this enzyme and, therefore, could be more effec tive in growth inhibition of susceptible cancer cells.