Sex hormones are already known to play an important role in maintaining healthy bones – but the relationship may be a two-way street. New evidence suggests that bones are important for controlling testosterone levels.
Gerard Karsenty at Columbia University in New York City and his colleagues applied osteoblasts – bone cells involved in building new bone – to cultures of cells taken from either the testes or ovaries of mice. They found that testis cells treated with bone cells increased their production of the hormone testosterone threefold. The cells from the ovaries, on the other hand, showed no change in their production of the hormones progesterone and oestradiol.
Karsenty's team then focused in closer by looking at osteocalcin – a hormone produced by osteoblasts. They found that testis cells treated with an active form of the hormone released testosterone – and the more of the hormone they got, the more testosterone they produced. Injecting live mice with the hormone similarly boosted levels of testosterone in their bloodstream.
To find out if this effect on testosterone production might affect mice's fertility, the team knocked out the gene for osteocalcin in a group of the rodents. These modified mice had significantly smaller testes and lower sperm counts than their normal counterparts – and when the group bred them with normal females, they found that the litter size was around half normal.
Mice and men
Rebecca Sokol at the University of Southern California in Los Angeles is intrigued by the findings. "I am particularly surprised by the absence of effects in female mice," she says.
Karsenty is stumped too. "We were flabbergasted," he says. "Don't ask me why it only affects males because I don't know."
He thinks the hormone might also boost fertility in men, as many hormones have been found to have the same effects in mice and humans. His team are currently exploring the possibility of osteocalcin as a treatment option for infertile men.
Bone is already known to release hormones. A few years ago, the same group found that osteocalcin plays a role in maintaining the body's glucose levels (Cell DOI: 10.1016/j.cell.2007.05.047).
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