Expression and regulation of alkaline phosphatases in human breast cancer MCF-7 cells

The effect of retinoic acid and dexamethasone on alkaline phosphatase (AP) expression was investigated in human breast cancer MCF-7 cells. Cellular AP activity was induced significantly by retinoic acid or dexamethasone in a time-dependent and dose-dependent fashion. A marked synergistic induction o f AP activity was observed when the cells were incubated with both agents s imultaneously. Two AP isozymes, tissue-nonspecific (TNAP) and intestinal (I AP), were shown to be expressed in MCF-7 cells as confirmed by the differen tial rate of thermal inactivation of these isozymes and RT-PCR. Based on th e two-isozyme thermal-inactivation model, the specific activities for TNAP and IAP in each sample were analyzed. TNAP activity was induced only by ret inoic acid and IAP activity was induced only by dexamethasone. Whereas dexa methasone conferred no significant effect on TNAP activity, retinoic acid w as shown to inhibit IAP activity by approximate to 50%. Interestingly, TNAP was found to be the only isozyme activity superinduced when the cells were costimulated with retinoic acid and dexamethasone. Northern blot and RT-PC R analysis were then used to demonstrate that the steady-state TNAP mRNA le vel was also superinduced, which indicates that the superinduction is regul ated at the transcriptional or post-transcriptional levels. In the presence of the glucocorticoid receptor antagonist RU486, the dexamethasone-mediate d induction of IAP activity was blocked completely as expected. However, th e ability of RU486 to antagonize the action of glucocorticoid was greatly c ompromised in dexamethasone-mediated superinduction of TNAP activity. Furth ermore, in the presence of retinoic acid, RU486 behaved as an agonist, and conferred superinduction of TNAP gene expression in the same way as dexamet hasone. Taken together, these observations suggest that the induction of IA P activity by dexamethasone and the superinduction of TNAP by dexamethasone were mediated through distinct regulatory pathways. In addition, retinoic acid plays an essential role in the superinduction of TNAP gene expression by enabling dexamethasone to exert its agonist activity, which otherwise ha s no effect.