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Abstract
Prostate-specific antigen (PSA) was recently shown to be an insulin-like growth factor-binding protein (IGFBP)-3 protease. However, only IGFBPs-2 and -4 have been identified in conditioned medium of prostate epithelial cells. Using cultures of human prostate epithelial and stromal fibroblastic cells, we examined conditioned medium and cell extracts for evidence of IGFBP-3 expression and secretion. Western ligand blotting of conditioned medium from epithelial or stromal cultures revealed the presence of IGFBPs in the molecular weight range 36–48 kDa, suggestive of IGFBP-3. Western immunoblots of these media confirmed the presence of IGFBP-3. Northern analyses of extracts of both stromal and epithelial cells showed a 2·5 kb band, the size of IGFBP-3 mRNA. We conclude that prostate cells express IGFBP-3 and that local proteolysis by PSA could modify this binding protein's actions in the prostate.
Journal of Endocrinology (1994) 141, 535–540
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Abstract
IGF-I and -II have potent effects on proliferation and differentiation of osteoblasts in vitro. These cells secrete both IGFs and expression of these peptides is regulated by several of the hormones and growth factors that promote bone resorption and/or formation. However, the physiological role(s) of IGFs in the remodelling process of adult bone is still unclear. Some confusion may arise from results influenced, in part, by differences in the state of osteoblast development of in vitro cultures. Several laboratories have demonstrated that murine osteoblast cultures progress from proliferating preosteoblasts, to mature differentiated osteoblasts that form an extracellular matrix, to cultures that form a mineralized matrix. We have recently documented changes in IGF-binding protein expression and secretion in these cultures. To complement and extend this work, we have examined IGF-I expression and secretion and IGF-II expression during in vitro osteoblast development.
Steady-state mRNA levels of both IGF-I and -II increased from the earliest time examined, day 5 in culture, to a maximum at day 11 and, thereafter, declined. IGF-I secreted into the medium also changed in a biphasic manner, but IGF-II could not be quantitated due to the sensitivity of our assay. Secretion of IGF-I was lowest between days 8 and 14. IGF-I secretion on day 5 was significantly greater than day 8. Similarly, IGF-I secretion from day 17 to 26 was also greater than observed for days 8 to 14. If differentiation of the cells was inhibited, this late rise in IGF-I secretion was abolished. We conclude that IGF-I is an autocrine mitogen of the proliferating preosteoblasts. Further, we also suggest that the rise in IGF-I secretion, late in osteoblast development, may lead to sequestration of this mitogen in the extracellular matrix for release during a subsequent remodelling cycle.
Journal of Endocrinology (1995) 144, 251–259
