P. Blumberg et al., INCREASED TURNOVER OF SMALL PROTEOGLYCANS SYNTHESIZED BY HUMAN OSTEOBLASTS DURING CULTIVATION WITH ASCORBATE AND BETA-GLYCEROPHOSPHATE, Calcified tissue international, 60(6), 1997, pp. 554-560
The small proteoglycan decorin had been localized previously at the d-
band in the gap zone of collagen fibrils in nonmineralizing tissues. I
n bone matrix this zone is proteoglycan free and is at least in some s
pecies the place where mineralization along collagen fibrils starts. T
o study the metabolism of the small proteoglycans decorin and biglycan
under mineralizing conditions, osteoblasts from human nasal bone were
cultured for several weeks in the presence or absence of beta-glycero
phosphate and ascorbate. An immediate consequence of the treatment was
a reduced expression of decorin, as judged by immune precipitation, w
hereas the biosynthesis of biglycan was not affected. Pulse-chase expe
riments were performed with osteoblasts embedded in floating type I co
llagen gels. In the presence of beta-glycerophosphate and ascorbate, a
more rapid turnover of both proteoglycans was noted; the one of bigly
can reached statistical significance. Indirect evidence for an enhance
d rate of proteoglycan endocytosis was obtained. This effect was not s
een in cultured skin fibroblasts. Thus, osteoblasts respond rapidly to
mineralizing conditions with alterations of small proteoglycan biosyn
thesis and turnover.