Rapid accumulation of cyclic GMP near activated vitamin D receptors.

The mechanisms of early calcitriol (1 alpha,25-dihydroxycholecalciferol) effects, including its receptor activation process as well as its "nongenomic" effects, are poorly understood. Calcitriol causes a rapid accumulation of cGMP, dependent on the presence of normal vitamin D receptors (VDRs). We recently developed an immunocytology method based on rapid microwave fixation suitable to detect the locations of agonist-induced intracellular cGMP accumulation. With the same technique we found that calcitriol induces stepwise and rapid reorganization of VDRs. Here we used this technique to study the subcellular compartmentalization of cGMP accumulation after exposure of cells to various steroid-related agonists and to study the spatial relationship between cGMP accumulation and VDRs. Calcitriol (10 nM) within 15 sec caused clumping of VDRs and accumulation of cGMP around VDR clumps; thereafter (up to 5 min), the cGMP accumulation surrounded VDRs throughout their stepwise reorganization. In fibroblasts from subjects with mutations affecting VDR function, we found disruptions of the calcitriol-induced patterns of cGMP accumulation analogous to the disruptions of VDR reorganization. The colocalization of cGMP accumulation with reorganizing VDRs at early moments after calcitriol addition indicates transduction of the cGMP increase by VDRs inside the cell, rather than by components in the plasma membrane. Other steroid-related agonists caused compartmentalized and sequential changes in cGMP accumulation that seemed specific for each class of agonist. Our findings suggest that compartmentalized cGMP accumulation is an early and common step during activation of steroid-related receptors.