A M Barreca | PubMed Health

PubMed2005

The IGF system and bone.

All components of the IGF systems and all regulatory mechanisms known are present and active in bone tissue. Osteoblasts synthesize IGF-I and IGF-II which have mitogenic effects on bone cells. Although the relative productions of IGF-I and IGF-II by osteoblasts are different depending on different bone districts and experimental conditions, it seems now that IGF-II is, in general, more expressed than IGF-I. The synthesis of IGFs is down-regulated by many locally produced growth factors, particularly trasforming growth factor beta (TGF-beta) and cortisol, and this probably accounts for the osteoporotic effects of this steroid, whereas PTH is stimulatory. Similarly to other tissues, IGFs action on bone is not only limited to cell replication but also to differentiated functions, such as production of collagen and matrix apposition. Binding proteins 2-5 have been demonstrated to be present in bone, the most expressed being IGFBP-4 and -5. Unlike IGFBP-4 which has an inhibiting effect on IGF actions, IGFBP-5 has a potentiating effect, both in vivo and in vitro, probably by binding directly to sites which are independent of the IGF receptor. The involvement of IGFBP proteases has also been demonstrated in human osteoblasts which are stimulated by IGF-II and TGF-beta. In a rat osteoblast cell culture that we studied, IGF-II and IGFBP-2 were the most abundant peptides of the IGF system released in cell medium, IGFBP-5 was abundantly expressed but bound to cell surface and cell matrix and IGF-I and IGFBP-3 were found at very low concentrations. All these peptides reach the maximum concentration in the first stage of maturation and gradually decrease in the following stages. Also IGFBP proteases, namely MMP-2, are important actors of the systems, being involved in the inactivation of IGFBPs in the late stages of maturation.

Animals↗

PubMed1997

Effect of the somatostatin analog, octreotide, and of other hormones on the release of the acid-labile subunit of the 150 kDa complex by rat hepatocyte in primary culture.

OBJECTIVE: In normal subjects, the major form of circulating IGF is the GH-dependent 150 kDa complex. The liver appears to be the main source of the three components of the 150 kDa complex and, in particular, hepatocytes synthesize the insulin-like growth factor (IGF) peptide and the acid-labile subunit (ALS), whereas Kupffer and sinusoidal endothelial cells produce IGF-binding protein-3 (IGFBG-3). We have studied the effects of the somatostatin analog octreotide, IGF-II des(1-3)IGF-I, transforming growth factor (TGF)-beta 1 and tri-iodothyronine (T3) on ALS secretion into the medium conditioned by rat hepatocytes in primary culture. METHODS: The regulation of ALS release was evaluated in the conditioned medium of adult rat hepatocytes exposed to increasing concentrations of test substances or to vehicle alone (control), after gel filtration in basic conditions, by immunoblot using an antiserum generated against the N-terminal 34 amino acids of human ALS. RESULTS: The results demonstrate that: 1) octreotide in vitro produces a dose-dependent inhibition of both basal and GH-stimulated ALS secretion into the hepatocyte conditioned medium; 2) the release of ALS by adult rat hepatocytes is not affected by the presence during the incubation of des(1-3)IGF-I or IGF-II; 3) an inhibitory effect, although only with very high doses, can be observed after treatment with TGF-beta 1; and 4) a small but significant increase of ALS released into the medium can be seen when hepatocytes are treated with T3. CONCLUSIONS: Evaluation of the effect of substances known to affect the production of IGF peptides, the IGFBPs, or both, on adult rat hepatocytes in primary culture revealed no powerful stimulator, but instead a potent inhibitor of ALS release/synthesis. Our data suggest that the effect of somatostatin on the 150 kDa complex is mediated not only by the reduction in GH concentration, but also by a direct inhibition of ALS release or synthesis.

Acids↗