Evidence for a Specific Uptake and Retention Mechanism for 25-Hydroxyvitamin D (25OHD) in Skeletal Muscle Cells

Abstract

Little is known about the mechanism for the prolonged residence time of 25-hydroxyvitamin D (25OHD) in blood. Several lines of evidence led us to propose that skeletal muscle could function as the site of an extravascular pool of 25OHD. In vitro studies investigated the capacity of differentiated C2 murine muscle cells to take up and release 25OHD, in comparison with other cell types and the involvement of the membrane protein megalin in these mechanisms. When C2 cells are differentiated into myotubes, the time-dependent uptake of labeled 25OHD is 2-3 times higher than in undifferentiated myoblasts or nonmuscle osteoblastic MG63 cells (P < .001). During in vitro release experiments (after 25OHD uptake), myotubes released only 32% ± 6% stored 25OHD after 4 hours, whereas this figure was 60% ± 2% for osteoblasts (P < .01). Using immunofluorescence, C2 myotubes and primary rat muscle fibers were, for the first time, shown to express megalin and cubilin, endocytotic receptors for the vitamin D binding protein (DBP), which binds nearly all 25OHD in the blood. DBP has a high affinity for actin in skeletal muscle. A time-dependent uptake of Alexafluor-488-labeled DBP into mature muscle cells was observed by confocal microscopy. Incubation of C2 myotubes (for 24 hours) with receptor-associated protein, a megalin inhibitor, led to a 40% decrease in 25OHD uptake (P < .01). These data support the proposal that 25OHD, after uptake into mature muscle cells, is held there by DBP, which has been internalized via membrane megalin and is retained by binding to actin.

Vitamin D, muscle and bone: Integrating effects in development, aging and injury

“…Another question is whether vitamin D's effects in bone and muscle are truly integrated or rather independent. In support of the former, osteoblast VDR modulates the effect of muscle loading on bone and vitamin D, sequestered within muscle fibers, may diffuse to adjacent bone under the influence of local stimuli (Abboud et al, 2013). Furthermore, vitamin D regulates paracrine factors considered to facilitate bone-muscle crosstalk -osteocalcin and, possibly, IL6 and myostatin (Morrison et al, 1989;Schleithoff et al, 2006;Szulc et al, 2012).…”

“…Regulated by local and systemic vitamin D signals, bone is the major depot for calcium, an essential mineral in muscle contraction, morphology and plasticity. Conversely, skeletal muscle is emerging as a major storage site for vitamin D from where it may diffuse back into the circulation or possibly, into adjacent bone, following specific signals (Abboud et al, 2013;Girgis et al, 2014d). Thus, vitamin D may have complementary effects in bone and muscle, further supporting the integration of these two tissues.…”

Modifiable factors of vitamin D status among a Brazilian osteoporotic population attended a public outpatient clinic

“…More recently, in animal studies, it was shown that after uptake into mature muscle cells, 25(OH)D is held there by vitamin D binding protein. This sequestration by the skeletal muscle could protect 25(OH)D from hepatic degradation and thus could provide a functional store to maintain vitamin D status (48). However, more data are needed to clarify these complex relationships.…”

The clinical use of vitamin D metabolites and their potential developments: a position statement from the European Society for Clinical and Economic Aspects of Osteoporosis and Osteoarthritis (ESCEO) and the International Osteoporosis Foundation (IOF)

“…Nonetheless, serum/plasma 25(OH)D does not reflect the storage of vitamin D within the tissues, which can be converted to calcitriol by tissutal 1a-hydroxylase, and the whole vitamin D bioavailability, which also depends on the presence of catabolic metabolites and protein transport in the blood [5]. Recent studies have demonstrated a direct uptake of 25(OH)D by skeletal myocytes, which would internally retain 25(OH)D binded to VDBP [6]. This While the hydroxylation of the side chain of 25(OH)D leads to the production of inactive metabolites, alternative modifications form metabolites, such as the epimer 25(OH)-3-epi-D, capable of binding and activating the VDR [7].…”