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Bone loss with aging may at least in part be due to inadequate bone formation. In this study, we examined whether the proliferation of osteoblast-like cells in vitro in response to local and systemic factors might be attenuated with age. A total of 36 cultures of osteoblast-like cells were obtained from outgrowths of human trabecular bone. Parathyroid hormone, growth hormone, calcitonin, transforming growth factor beta, insulin-like growth factor I, and platelet-derived growth factor BB dose dependently increased DNA synthesis in all cultures. Increases in DNA synthesis with each of these factors were significantly negatively correlated with donor age in cultures obtained from the iliac crest bone of 50- to 70-year-old women. Cells from 61- to 70-year-old donors required approximately 10-fold higher concentrations of growth factors and hormones to yield comparable increases in DNA synthesis than cells from 51- to 60-year-old donors. A significant negative correlation between age and mitogenic responsiveness to platelet-derived growth factor and growth hormone, but not toward the other factors, was also observed in cultures from the femoral head trabecular bone of 60- to 90-year-old women. Our findings suggest that bone loss with aging may be partially due to a decreased capacity of osteoblasts to proliferate in response to systemic or locally released osteotropic factors.

Genes for gentamicin-3-acetyltransferases [ACC(3)] of types III and IV have been cloned from various R-plasmids. In two R-plasmids, pWP14a (AAC(3)-III) and pWP7b [AAC(3)-IV], resistance genes have been found directly adjacent to a single copy of an IS element, IS140. Nucleotide sequence determination of the AAC(3)-IV gene from plasmid pWP7b and of part of IS140 from three different sources suggested that the -35 region of the AAC(3)-IV promoter was part of the IS element. A similarly built-up promoter was found in pWP14a. It was found also, that a hygromycin B phosphotransferase was expressed from a locus neighbouring the AAC(3)-IV gene in pWP7b which was under the control of the same promoter. In two other R-plasmids, pWP113a and pWP116a, the AAC(3)-III gene was found in different genetic environments, namely close to Tn3-like structures.

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