calcimycin and Polycystic-Kidney-Diseases

Polycystic kidney disease (PKD) is associated with alterations in developmental processes that severely affect kidney integrity, often leading to fatal consequences. It has been suggested that dysfunctional calcium (Ca2+) regulation associated with the PKD phenotype is consequent to mutations affecting the pkd1 gene. Previously, it has been observed that blocking calcium along with cAMP allowed tubular epithelial cells to enter the proliferative phase that culminated in a cyst-like phenotype. In this regard, mouse metanephroi, (embryonic day 13.5, E13.5) were used to study morphological and ultrastructural effects of calcium replenishment on 8-bromocyclic 3'5'cyclic adenosine monophosphate (8-Br-cAMP)-induced cyst-like tubular dilations. Phase contrast microscopy of 8-Br-cAMP-treated metanephroi exhibited numerous dilated tubules that continued to increase in size for 4 days in culture. The effects of 8-Br-cAMP on renal tubular epithelia were assessed by histopathological and electron microscopic analyses. Transmission electron microscopy revealed changes such as increased vacuolation, swollen mitochondria, chromatin condensation, and disrupted cell membrane in tubular epithelia of 8-Br-cAMP-treated metanephroi. Concurrent treatments with calcium-channel agonists (calcium ionophore A23187 and phorbol-12-myristate-13-acetate) and 8-Br-cAMP abolished cAMP-induced morphometric and ultrastructural alterations. Calcium replenishment rescued tubular epithelial cells from mitogenic effects of cAMP and restored normal morphology at cellular and sub-cellular levels as verified by histopathological and ultrastructural examinations.

Topics: 8-Bromo Cyclic Adenosine Monophosphate; Animals; Calcimycin; Calcium; Calcium Channel Agonists; Kidney; Mice; Mice, Inbred BALB C; Microscopy, Electron, Transmission; Microscopy, Phase-Contrast; Organ Culture Techniques; Polycystic Kidney Diseases; Tetradecanoylphorbol Acetate

cAMP can be either mitogenic or anti-mitogenic, depending on the cell type. We demonstrated previously that cAMP inhibited the proliferation of normal renal epithelial cells and stimulated the proliferation of cells derived from the cysts of polycystic kidney disease (PKD) patients. The protein products of the genes causing PKD, polycystin-1 and polycystin-2, are thought to regulate intracellular calcium levels, suggesting that abnormal polycystin function may affect calcium signaling and thus cause a switch to the cAMP growth-stimulated phenotype. To test this hypothesis, we disrupted intracellular calcium mobilization by treating immortalized mouse M-1 collecting duct cells and primary cultures of human kidney epithelial cells with calcium channel blockers and by lowering extracellular calcium with EGTA. Calcium restriction for 3-5 h converted both cell types from a normal cAMP growth-inhibited phenotype to an abnormal cAMP growth-stimulated phenotype, characteristic of PKD. In M-1 cells, we showed that calcium restriction was associated with an elevation in B-Raf protein levels and cAMP-stimulated, Ras-dependent activation of B-Raf and ERK. Moreover, the activity of Akt, a negative regulator of B-Raf, was decreased by calcium restriction. Inhibition of Akt or phosphatidylinositol 3-kinase also allowed cAMP-dependent activation of B-Raf and ERK in normal calcium. These results suggest that calcium restriction causes an inhibition of the phosphatidylinositol 3-kinase/Akt pathway, which relieves the inhibition of B-Raf to allow the cAMP growth-stimulated phenotypic switch. Finally, M-1 cells stably overexpressing an inducible polycystin-1 C-terminal cytosolic tail construct were shown to exhibit a cAMP growth-stimulated phenotype involving B-Raf and ERK activation, which was reversed by the calcium ionophore A23187. We conclude that disruption of calcium mobilization in cells that are normally growth-inhibited by cAMP can derepress the B-Raf/ERK pathway, thus converting these cells to a phenotype that is growth-stimulated by cAMP.

Topics: Animals; Blotting, Western; Calcimycin; Calcium; Calcium Channel Blockers; Cell Division; Cell Line; Cells, Cultured; Cyclic AMP; Cyclic AMP-Dependent Protein Kinases; Dose-Response Relationship, Drug; Egtazic Acid; Enzyme Activation; Epithelial Cells; Humans; Ionophores; Kidney; Membrane Proteins; Mice; Mitogen-Activated Protein Kinases; Models, Biological; Phenotype; Phosphatidylinositol 3-Kinases; Polycystic Kidney Diseases; Proteins; Proto-Oncogene Proteins B-raf; Proto-Oncogene Proteins c-raf; Time Factors; Transfection; TRPP Cation Channels; Verapamil