| The folate receptor alpha (FR-alpha) is a cell surface protein that expressed in tissue selective manner and is capable of transporting folate, antifolates and folate conjugated molecules and nanoparticles into the cells. In contrast to the normal tissues where its expression is limited to the apical surface of epithelial tissues and isolated from the bloodstream, FR-alpha is over-expressed in certain gynecological tumors such as ovarian cancer where it is highly accessible via the circulation. Therefore, FR-alpha is a promising means for selective tumor targeting.; We have shown that the glucocorticoid (GR) and progesterone (PR) receptors positively regulate FR-alpha gene expression in cell culture and in tumor xenograft models. The expression of FR-alpha is enhanced further by combination of GR agonist with well-tolerated HDAC inhibitors such as valproic acid (VPA). Time course experiments and the use of protein synthesis inhibitor cycloheximide, showed that GR, PR-A and PR-B up-regulate FR-alpha expression indirectly in a ligand dependent manner. The promoter analysis of the FR-alpha gene showed the involvement of the G/C-rich Sp1 binding sites of the P4 promoter for the action of GR and PR and regulation by both receptors was optimal in the proper initiator context. Interestingly, the classical GR and PR receptor antagonist RU486 also activates FR-alpha expression but only through PR-B. Similar observations were made for PR regulation of the genes encoding p27, thymidine kinase 1, and p21. The results support the concept of increasing FR-alpha expression selectively in the receptor-positive tumors by brief treatment with a nontoxic dose of a GR and PR agonist, alone or in combination with a well-tolerated HDAC inhibitor, to increase the efficacy of various FR-alpha-dependent therapeutic and diagnostic applications. In addition, our findings contradict the current view of Sp1-dependent gene regulation by PR and point to the existence of one or more PR target genes whose promoter and cell context(s) must be key determinants of the agonistic activity of RU486 on a large group of important Sp1-dependent downstream target genes. |
