Temporal regulation of hyaluronan and proteoglycan metabolism by human bone cells in vitro

Osteoblasts elaborate a dynamic extracellular matrix that is constructed and mineralized as bone is formed. This matrix is primarily composed of collagen, along with noncollagenous proteins which include glycoproteins and proteoglycans. After various times in culture, human bone cells were labeled with [³⁵S]sulfate, [³H] leucine/proline, or [³H]glucosamine and the metabolism of hyaluronan and four distinct species of proteoglycans (PGs) was assayed in the medium, cell layer, and intracellular pools. These cells produce hyaluronan (M_r ∼ 1,400,000; a chondroitin sulfate PG (CSPG), M_r ∼ 600,000; a heparan sulfate PG (HSPG), M_r ∼ 400,000; and two dermatan sulfate PGs with M_r ∼ 270,000 (biglycan, PG I) and M_r ∼ 135,000 (decorin, PG II) that distribute between the medium and cell layer. Two days following subculture, 12 h [³⁵S]sulfate steady-state labeling yielded a composition of 24, 27, 31, and 18% for total CSPG, HSPG, biglycan, and decorin, respectively. While HSPG and decorin levels and distribution between medium and cell layer remained relatively constant during steady-state labeling at different times in culture, CSPG and biglycan levels increased dramatically at late stages of growth, and their distribution changed throughout culture. These results were independent of cell density, media depletion, and labeling pool effects. In contrast, hyaluronan synthesis was uncoupled from PG synthesis and apparently density-dependent. Pulse chase labeling at different stages of culture showed that the CSPG and decorin behaved as secretory PGs. Both HSPG and biglycan underwent catabolism, with HSPG possessing a t_1/2 of 8 h and biglycan a t_1/2 of 4 h. While the rate of HSPG turnover did not appreciably change between early and late culture, that of biglycan decreased. The mRNA for decorin was constant, while that of biglycan changed during culture. These results suggest that each PG possesses a distinct pattern of cellular and temporal distribution that may reflect specific stages in matrix formation and maturation.