AJP - Endocrinology and Metabolism, Vol 268, Issue 4 E565-E571, Copyright © 1995 by American Physiological Society
Ascorbate concentration in osteoblastic cells is elevated by transforming growth factor-beta
J. X. Wilson and S. J. Dixon
Department of Physiology, Faculty of Dentistry, University of Western Ontario, London, Canada.
Transforming growth factor-beta modulates the proliferation,
differentiation, and synthetic activity of osteoblasts, but its mechanisms
of action are not fully understood. Because ascorbate also influences
osteoblast differentiation and is a cofactor for collagen synthesis, the
present study examined the effect of transforming growth factor-beta on the
initial rate of transport and steady-state concentration of ascorbate in an
osteoblastic cell line. UMR-106 rat osteosarcoma cells accumulated reduced
vitamin C from culture medium. Virtually all accumulation of ascorbate was
accomplished by a saturable Na(+)-dependent transport mechanism.
Transforming growth factor-beta increased the initial rate of ascorbate
transport, measured in either attached or suspended cells. Within 24 h, the
growth factor also increased the steady-state intracellular concentration
of ascorbate, without significantly changing cell volume or the DNA or
protein content of cultures. These data provide evidence that
Na(+)-ascorbate cotransport activity controls ascorbate concentration in
osteoblasts. Furthermore, the results indicate that both the transport rate
and steady-state concentration of ascorbate in these cells are regulated by
transforming growth factor-beta.
