Effects of chromogranin expression on inositol 1,4,5-trisphosphate-induced intracellular Ca2+ mobilization

We show here that expression of chromogranins in non-neuroendocrine NIH3T3 cells significantly increased the amount of IP3-mediated intracellular Ca2+ mobilization in these cells, whereas suppression of them in neuroendocrine PC12 cells decreased the amount of mobilized Call. We have therefore investigated the relationship between the IP3-induced intracellular Ca2+ mobilization and secretory granules. The level of IP3-mediated Ca2+ release in CGA-expressing NIH3T3 cells was 40% higher than in the control cells, while that of CGB-expressing cells was 134% higher, reflecting the number of secretory granules formed. Suppression of CGA and CGB expression in PC12 cells resulted in 41 and 78% reductions in the number of secretory granules, respectively, while the extents Of IP3-induced Ca2+ release in these cells were reduced 40 and 69%, respectively. The newly formed secretory granules of NIH3T3 cells contained all three isoforms of the IP(3)Rs. Comparison of the concentrations of the IP3R isoforms expressed in the ER and nucleus of chromogranin-expressing and nonexpressing NIH3T3 cells did not show significant differences, indicating that chromogranin expression did not affect the expression of endogenous IP(3)Rs. Nonetheless, the IP3R concentrations in secretory granules of chromogranin-expressing NIH3T3 cells were 3.5-4.7-fold higher than those of the ER, similar to the levels found in secretory granules of neuroendocrine chromaffin cells, thus suggesting that the IP(3)Rs targeted to the newly formed secretory 4, Granules are newly induced by chromogranins without affecting the expression of intrinsic IP(3)Rs. These results strongly suggest that the extent of IP3-induced intracellular Call mobilization in secretory cells is closely related to the number of secretory granules.