Molybdate increases intracellular 3',5'-guanosine cyclic monophosphate and stabilizes vitamin D receptor association with tubulin-containing filaments.

With a recently developed method we detected rapid and sequential reorganization of vitamin D receptors (VDR), including their temporary association with fibers, and we showed that calcitriol induces cGMP accumulation around reorganizing VDRs. In this report we first identified the VDR-associated fibers as microtubules: they show immunoreactivity with tubulin antisera and were sensitive to tubulin-disruptive agents. Tubulin-disruptive agents also prevented calcitriol-induced alignment and intranuclear accumulation of VDR and cGMP, but did not prevent the initial cGMP accumulation in the cytoplasm. Then we studied the effect of molybdate on VDR reorganization and on cGMP accumulation. Sodium molybdate inhibits steroid receptor transformation into a DNA binding form through interaction with the steroid binding region of the receptor. The mechanism of molybdate effect on steroid receptors is not well understood and the interaction of molybdate with guanylate cyclase has not been investigated. We found in cells pretreated with molybdate that the addition of calcitriol resulted in a prolonged and accentuated association of VDR and cGMP with the microtubules. Furthermore, both immunocytology and radioimmunoassay demonstrated that molybdate is a highly potent inducer of guanylate cyclase. Neither calcitriol nor molybdate effect on guanylate cyclase were prevented by methylene blue pretreatment, suggesting that they activate particulate guanylate cyclase. Pretreatment of cells with dibutyryl-cGMP mimicked molybdate effect on VDR reorganization. The effect of molybdate on cGMP may participate in molybdate stabilization of steroid receptors. We suggest that rapid cGMP accumulation after steroid exposure plays a role in facilitation of intracellular transport of the steroid receptor through interaction with microtubules.