Megalin and Vitamin D Metabolism-Implications in Non-Renal Tissues and Kidney Disease

Megalin is an endocytic receptor abundantly expressed in proximal tubular epithelial cells and other calciotropic extrarenal cells expressing vitamin D metabolizing enzymes, such as bone and parathyroid cells. The receptor functions in the uptake of the vitamin D-binding protein (DBP) complexed to 25 hydroxyvitamin D-3 (25(OH)D-3), facilitating the intracellular conversion of precursor 25(OH)D-3 to the active 1,25 dihydroxyvitamin D3 (1,25(OH)(2)D-3). The significance of renal megalin-mediated reabsorption of 25(OH)D-3 and 1,25(OH)(2)D-3 has been well established experimentally, and other studies have demonstrated relevant roles of extrarenal megalin in regulating vitamin D homeostasis in mammary cells, fat, muscle, bone, and mesenchymal stem cells. Parathyroid gland megalin may regulate calcium signaling, suggesting intriguing possibilities for megalin-mediated cross-talk between calcium and vitamin D regulation in the parathyroid; however, parathyroid megalin functionality has not been assessed in the context of vitamin D. Within various models of chronic kidney disease (CKD), megalin expression appears to be downregulated; however, contradictory results have been observed between human and rodent models. This review aims to provide an overview of the current knowledge of megalin function in the context of vitamin D metabolism, with an emphasis on extrarenal megalin, an area that clearly requires further investigation.

Automated summary

Megalin is an endocytic receptor that plays a crucial role in vitamin D metabolism, facilitating the conversion of precursor 25(OH)D-3 to the active form 1,25(OH)(2)D-3. It is expressed in renal and extrarenal cells, regulating vitamin D homeostasis in various tissues. Despite downregulation of megalin expression in chronic kidney disease models, contradictory results have been observed between humans and rodents.