Vitamin D: Newer Concepts of Its Metabolism and Function at the Basic and Clinical Level.

The interest in vitamin D continues unabated with thousands of publications contributing to a vast and growing literature each year. It is widely recognized that the vitamin D receptor (VDR) and the enzymes that metabolize vitamin D are found in many cells, not just those involved with calcium and phosphate homeostasis. In this mini review I have focused primarily on recent studies that provide new insights into vitamin D metabolism, mechanisms of action, and clinical applications. In particular, I examine how mutations in vitamin D metabolizing enzymes-and new information on their regulation-links vitamin D metabolism into areas such as metabolism and diseases outside that of the musculoskeletal system. New information regarding the mechanisms governing the function of the VDR elucidates how this molecule can be so multifunctional in a cell-specific fashion. Clinically, the difficulty in determining vitamin D sufficiency for all groups is addressed, including a discussion of whether the standard measure of vitamin D sufficiency, total 25OHD (25 hydroxyvitamin) levels, may not be the best measure-at least by itself. Finally, several recent large clinical trials exploring the role of vitamin D supplementation in nonskeletal diseases are briefly reviewed, with an eye toward what questions they answered and what new questions they raised

Alterations in gut transport of minerals and in binding proteins during simulated weightlessness

The structural components of the skeleton develop and are maintained in a 1 g environment, shaped by the mechanical load to which they are constantly exposed. Altering such a mechanical load by reducing the gravitational force imposed on the system, as in space flight, has profound effects on the skeleton and permits an exploration of the molecular events which regulate normal skeletal homeostasis. The objective was to determine whether simulated weightlessness reduced intestinal calcium transport, and if so, to determine the molecular mechanisms for such an effect. A nonstressful tail suspension in which the rats gained weight normally while suspended was used to simulate weightlessness. A significant change in intestinal calcium transport was not demonstrated. However, a cyclic change in bone formation with suspension was shown. Based on these observations, the objective changed to determination of the hormonal regulation of bone formation during simulated weightlessness

Myosin 1a Regulates Osteoblast Differentiation Independent of Intestinal Calcium Transport.

Myosin 1A (Myo1a) is a mechanoenzyme previously thought to be located exclusively in the intestinal epithelium. It is the principle calmodulin-binding protein of the brush border. Based on earlier studies in chickens, we hypothesized that Myo1a facilitates calcium transport across the brush border membrane of the intestinal epithelium, perhaps in association with the calcium channel Trpv6. Working with C2Bbe1 cells, a human intestinal epithelial cell line, we observed that overexpression of Myo1a increased, whereas the antisense construct blocked calcium transport. To further test this hypothesis, we examined mice in which either or both Myo1a and Trpv6 had been deleted. Although the Trpv6-null mice had decreased intestinal calcium transport, the Myo1a-null mouse did not, disproving our original hypothesis, at least in mice. Expecting that a reduction in intestinal calcium transport would result in decreased bone, we examined the skeletons of these mice. To our surprise, we found no decrease in bone in the Trpv6-null mouse, but a substantial decrease in the Myo1a-null mouse. Double deletions were comparable to the Myo1a null. Moreover, Myo1a but not Trpv6 was expressed in osteoblasts. In vitro, the bone marrow stromal cells from the Myo1a-null mice showed normal numbers of colony-forming units but marked decrements in the formation of alkaline phosphatase-positive colonies and mineralized nodules. We conclude that Myo1a regulates osteoblast differentiation independent of its role, if any, in intestinal calcium transport, whereas Trpv6 functions primarily to promote intestinal calcium transport with little influence in osteoblast function

The temporal response of bone to unloading

Rats were suspended by their tails with the forelimbs bearing the weight load to simulate the weightlessness of space flight. Growth in bone mass ceased by 1 week in the hindlimbs and lumbar vertebrae in growing rats, while growth in the forelimbs and cervical vertebrae remained unaffected. The effects of selective skeletal unloading on bone formation during 2 weeks of suspension was investigated using radio iostope incorporation (with Ca-45 and H-3 proline) and histomorphometry (with tetracycline labeling). The results of these studies were confirmed by histomorphometric measurements of bone formation using triple tetracycline labeling. This model of simulated weightlessness results in an initial inhibition of bone formation in the unloaded bones. This temporary cessation of bone formation is followed in the accretion of bone mass, which then resumes at a normal rate by 14 days, despite continued skeletal unloading. This cycle of inhibition and resumption of bone formation has profound implication for understanding bone dynamics durng space flight, immobilization, or bed rest and offers an opportunity to study the hormonal and mechanical factors that regulate bone formation

Disruption of Vitamin D and Calcium Signaling in Keratinocytes Predisposes to Skin Cancer.

1,25 dihydroxyvitamin D (1,25(OH)2D), the active metabolite of vitamin D, and calcium regulate epidermal differentiation. 1,25(OH)2D exerts its effects through the vitamin D receptor (VDR), a transcription factor in the nuclear hormone receptor family, whereas calcium acts through the calcium sensing receptor (Casr), a membrane bound member of the G protein coupled receptor family. We have developed mouse models in which the Vdr and Casr have been deleted in the epidermis ((epid) Vdr (-∕-) and (epid) Casr (-∕-)). Both genotypes show abnormalities in calcium induced epidermal differentiation in vivo and in vitro, associated with altered hedgehog (HH) and β-catenin signaling that when abnormally expressed lead to basal cell carcinomas (BCC) and trichofolliculomas, respectively. The Vdr (-∕-) mice are susceptible to tumor formation following UVB or chemical carcinogen exposure. More recently we found that the keratinocytes from these mice over express long non-coding RNA (lncRNA) oncogenes such as H19 and under express lncRNA tumor suppressors such as lincRNA-21. Spontaneous tumors have not been observed in either the (epid) Vdr (-∕-) or (epid) Casr (-∕-). But in mice with epidermal specific deletion of both Vdr and Casr ((epid) Vdr (-∕-)/(epid) Casr (-∕-) [DKO]) tumor formation occurs spontaneously when the DKO mice are placed on a low calcium diet. These results demonstrate important interactions between vitamin D and calcium signaling through their respective receptors that lead to cancer when these signals are disrupted. The roles of the β-catenin, hedgehog, and lncRNA pathways in predisposing the epidermis to tumor formation when vitamin D and calcium signaling are disrupted will be discussed