Vitamin D receptor and vitamin D action in muscle.

Abstract

Vitamin D has been known for nearly a century to be essential for bone health because it can prevent and cure endemic rickets and osteomalacia (1–3). Currently, the complex implications of deficiency or excess vitamin D on bone modeling and remodeling are well documented by a wealth of preclinical and clinical studies (reviewed in 4). The vitamin D receptor (VDR) as well as the key activating (CYP27B1) or inactivating (CYP24A1) enzymes are, however, expressed in many cells. Moreover, a very large number of genes—from zebra fish up to mice and man—are under direct or indirect control of the active hormone, 1,25(OH)2D3. Therefore, it looks attractive to hypothesize that the vitamin D endocrine system would have many extraskeletal effects. Such hypothesis fits with the very broad spectrum of activity of most ligands of nuclear receptors (1–3). Poor vitamin D status has also been associated with most major diseases of mankind, ranging from immune diseases and infections, cancer, diabetes, and the metabolic syndrome, as well as cardiovascular risk factors and events, energy homeostasis, bile acid metabolism, to (last but not least) muscle function and falls (1, 3, 5). However, following an extensive evaluation of these data, the Institute of Medicine concluded, that “Scientific evidence indicates that calcium and vitamin D play key roles in bone health. The current evidence, however, does not support other benefits” and “There is inconsistent evidence that supplemental vitamin D reduces falls in postmenopausal women and older men” (6). This conclusion was highly debated because several meta-analyses concluded that vitamin D supplementation of elderly subjects decreased the risk of falls by approximately 20% and improved proximal muscle strength of severely vitamin D–deficient (serum 25OHD 10 ng/ml) subjects (7, 8). Vitamin D’s direct action on muscle became doubtful when a careful analysis of VDR expression in adult human muscle as well as mouse skeletal and cardiac muscle, performed in H. DeLuca’s laboratory (9), vitamin D receptor showed only very low gene expression (10 000-fold lower than in the intestine) without detectable protein expression even with a highly specific anti-VDR antibody, in contrast with older publications using less stringent antibodies (9). Therefore, any potential effect of vitamin D metabolites on muscle could at most be indirect given that both genomic and potential nongenomic effects require the presence of VDR. In the current issue of Endocrinology, Girgis et al (10) demonstrate that VDR protein (using the same highly specific antibody) can be clearly demonstrated in adult mouse skeletal muscle, although only when using a hyperosmolar lysis buffer. Such buffer releases VDR from its tight binding to DNA, as it has been demonstrated previously that even unliganded VDR mainly resides in the nucleus. The gene and protein expression of VDR was higher in muscle cell precursors (in vitro) compared with adult mature muscle and was also higher in muscle from younger than in older animals (in vivo). Nevertheless, the expression was still several-fold lower in skeletal muscle than in the intestine. Moreover, the authors confirmed the expression as well as the functionality of cyp27b1 in muscle cell cultures. The activity of the vitamin D receptor was also confirmed because both 25OHD3 and 1,25(OH)2D3 stimulated the expression of VDR and Cyp24a1, both known targets of VDR action, as well as the uptake of 25OHD in muscle. Negative controls for all these end points were obtained in VDR-null mouse tissue. All the data are in line with decreasing VDR expression, as well as decreasing