Anyone concerned about their bones is likely to make sure they have plenty of vitamin D, either by getting enough sunshine, eating fish, or taking supplements. Yet scientists know surprisingly little about how the compound works. A new study has finally shed some light on this process, showing how the vitamin takes part in a delicate balancing act between cells that tear down our bones and cells that rebuild them.
Vitamin D is a familiar player in bone health. Without sufficient amounts of this hormone, our frames become frail with disorders such as osteoporosis or rickets. But vitamin D has some puzzling effects in the lab. As might be expected, it reduces the number of bone-dissolving cells, or osteoclasts, in mice with a condition resembling osteoporosis. When added to cultured bone marrow, however, vitamin D ramps up the production of these bone destroyers.
So physiologist Kyoji Ikeda and colleagues at the National Center for Geriatrics and Gerontology in Obu, Japan, and Chugai Pharmaceutical Co., in Gotemba, Japan, tried to tease apart the effects. By adding vitamin D to a petri dish full of osteoclast precursors, they discovered that the hormone blocked a signal called RANK ligand that tells these cells to become osteoclasts. That's how vitamin D cuts back on osteoclasts.
Vitamin D performs the opposite trick--boosting osteoclast numbers--via a more circuitous route. When the researchers performed a similar experiment with precursors of bone building cells, the precursors boosted their production of RANK ligand, the group reports in the February Journal of Clinical Investigation. The findings confirm and explain vitamin D's competing effects in the body, say the researchers.
"This [report] actually works out a substantial part of the [vitamin D] mechanism," says endocrinologist T. John Martin of St. Vincent's Institute of Medical Research in Melbourne, Australia. Vitamin D therapy might be particularly useful once the balance has tipped toward bone loss, he says, or if vitamin D analogs can be made to target the precursors of bone-dissolving cells specifically.
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