Extracellular Calcium-sensing Receptor Activation Induces Vitamin D Receptor Levels in Proximal Kidney HK-2G Cells by a Mechanism That Requires Phosphorylation of p38α MAPK

Abstract

2؉ ] e concentration in the physiological range is capable of directly increasing renal proximal VDR expression, and the induction mechanism requires activation of the CaSR and signal mediation by the p38␣ MAP kinase pathway.Extracellular ionized calcium [Ca 2ϩ ] e is a critical mediator of cell signaling for the storage and release of both parathyroid hormone (PTH) 2 and calcitonin. Homeostasis of [Ca 2ϩ ] e is important to proper neuromuscular contractions, cellular integrity, and the deposition of mineral in skeletal structures (1). A multi-organ system coordinates and maintains [Ca 2ϩ ] e homeostasis within a narrow physiological range (2). Activation of the an extracellular calcium sensing receptor (CaSR) when [Ca 2ϩ ] e concentrations are high maintains the storage form of PTH in the parathyroid gland (3,4). Release of PTH from the parathyroid gland is a direct result of a decrease in serum [Ca 2ϩ ] e (5, 6). PTH is a potent stimulus of 1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ) biosynthesis in the kidney proximal convoluted tubule by its induction of 1␣-hydoxylation (7). Resultant increase in [Ca 2ϩ ] e by 1,25(OH) 2 D 3 regulates PTH at two levels; where CaSR signaling shuts down PTH secretion (8, 9) and 1,25(OH) 2 D 3 through transactivation of its nuclear receptor (VDR) inhibits prepro-PTH transcription (10).[Ca 2ϩ ] e is also a potent mediator of the balance between cellular proliferation and differentiation while VDR mediates biological functions of 1,25(OH) 2 D 3 and is expressed in many different cell types (11). Past work has focused on the role of VDR in mineral homeostasis, with VDR activators used mainly to treat hyperparathyroidism secondary to chronic kidney disease (12). A more recent perspective includes the use of vitamin D analogs in combination with calcimimetics and phosphate binders in the management of hyperparathyroid disorders (13). It is therefore important to identify the mechanisms whereby CaSR signaling and VDR activation coincide in the tissues that co-express these two receptors. We previously reported for the first time that [Ca 2ϩ ] e is a direct regulator of VDR in proximal human kidney . The present study was designed to better explore the mechanism by which [Ca 2ϩ ] e influences VDR increase in these cells.PTH exerts several actions in the renal proximal tubule that include control of phosphate transport and induction of 1,25(OH) 2 D 3 -1␣-hydroxylase (CYP27B1) mRNA (15-17). The receptor for PTH is a another member of the GPCR family capable of coordinating bidirectional signaling through pathways involving adenylate cyclase or activation of protein kinase C depending on the concentration of PTH and the specific cell type (18,19). In proximal tubule epithelial cells, PTH stimulates 1␣-hydroxylation, mediates 1,25(OH) 2 D 3 -24-hydroxylase (CYP24) down-regulation and represses 1,25(OH) 2 D 3 -receptor (VDR) transcription all by increasing cAMP and activating the cAMP-dependent response element-binding protein (CREB) (20 -22). However, the set of ...

Ca2+/Calmodulin-dependent Protein Kinase II Promotes Cell Cycle Progression by Directly Activating MEK1 and Subsequently Modulating p27 Phosphorylation

“…However, components of the signaling pathway that lie between the Ca 2ϩ /CaMKII signaling and the cell cycle machinery have been undefined. This is particularly relevant, since potential connections between Ca 2ϩ /CaMKII signaling and multiple signaling pathways have been reported in many cell types; elevated extracellular Ca 2ϩ can activate the MAPK pathways (12), the CaMKII-MAPK pathways regulate the neuronal cell fate determination (13), and the Wnt-5a-mediated Ca 2ϩ signaling activates the TAK1-NLK pathway via CaMKII (14). Our results also show that hCaMKIIN␣ deactivates MEK/ ERK and causes the inhibition of S-phase progression of the cell cycle (11).…”

Agonists and Allosteric Modulators of the Calcium-Sensing Receptor and Their Therapeutic Applications

“…As is the case for other GPCRs, the activated CaR is capable of activating multiple types of G proteins from different G protein subfamilies, primarily G␣ q/11 and G␣ i . This leads to a range of cellular responses, including stimulation of phospholipase C␤ (PLC␤), production of inositol 1,4,5-triphosphate (IP 3 ), release of intracellular Ca 2ϩ (Ca 2ϩ i ), stimulation of mitogen-activated protein kinases (MAPKs), and an inhibition of adenylate cyclase, causing a decrease in cAMP levels (Brown and MacLeod, 2001;Maiti et al, 2008) (Fig. 1).…”

Vitamin D and the Calcium-Sensing Receptor

“…P38 is one of several MAPKs activated by the CaSR in various cell types. Since knocking down the CaSR with siRNA prevented the high Ca 2þ o -evoked increase in VDR expression, the latter was CaSR-mediated [92]. The use of siRNA to prove the CaSR's role in the concomitant inhibition of the expression of CYP27B1 was not reported [90].…”

“…Maiti and Beckman used a proximal tubular cell line, HK-2G, in which they had shown that high extracellular calcium inhibits the expression of CYP27B1 [90], to demonstrate that high Ca 2þ o up-regulates the expression of the VDR [24]. This effect was mediated by a p38 mitogen-activated protein kinase (MAPK)-dependent mechanism [92]. P38 is one of several MAPKs activated by the CaSR in various cell types.…”