Induction of osteoblast aggregation, detachment, and altered integrin expression by bear serum.

Animal models have long been used to elucidate the mechanisms responsible for osteoporosis in humans. The American black bear, an animal that does not experience extensive bone loss normally associated with long-term immobilization (when hibernating), may provide an insight into the nature of the pathogenesis of the disease. Circulating growth and differentiation factors present in the serum may facilitate continued proliferation of bone-forming cells. The aim of our study was to determine the effects of bear serum on human osteoblasts when cultured for extended periods of time. Unexpectedly, exposure to the bear serum in vitro led to the detachment of osteoblasts from the surface of the culture plate after 3 d of incubation. The osteoblasts pulled off the polystyrene surface in sheets and aggregated into floating conglomerations of viable cells. In contrast, osteoblasts cultured in fetal calf serum maintained adherence to the surface of the culture plate. Detachment of osteoblasts propagated in bear serum was time dependent and was associated with an increased expression of integrins compared with osteoblasts propagated in fetal calf serum, as indicated by reverse transcriptase-polymerase chain reaction and immunostaining.

Enhancement of osteoblast proliferative capacity by growth factor-like molecules in bear serum.

The use of animal serum in cell culture is vital for providing the nutrient factors required to promote proliferation and function. Fetal calf serum has become the preferred choice because of its abundance, reasonable cost, and ability to sustain human cells in vitro. Although a wide variety of serum sources have been tested and used, little is known about the ability of serum obtained from the American black bear (Ursus americanus) to support human cell growth in culture. The American black bear, an animal comparable in size to humans, is unique in that it hibernates for mo at a time but does not experience extensive bone loss normally associated with extended immobility. The aim of this study was to analyze the effect of bear serum on human osteoblast cultures. We discovered that three of the eight bear serum samples induced significantly higher proliferation rates in osteoblasts than did fetal calf serum over a 24-h period. Osteoblasts incubated in bear serum displayed higher messenger ribonucleic acid levels for phenotype markers osteocalcin and type I collagen than did those incubated in fetal calf serum. The mitogenic activity of the bear serum was reduced when heated at 56 degrees C for 30 min before use in culture. The molecular weight of the mitogenic factors was found to be primarily greater than 50 kDa. The present work demonstrates the capability of serum from American black bears to support human osteoblast proliferation in vitro.