Abnormalities of insulin-like growth factor-I signaling and impaired cell proliferation in osteoblasts from subjects with osteoporosis.

IGF-I regulates bone acquisition and maintenance, even though the cellular targets and signaling pathways responsible for its action in human bone cells are poorly understood. Whether abnormalities in IGF-I action and signaling occur in human osteoblasts under conditions of net bone loss has not been determined. Herein we carried out a comparative analysis of IGF-I signaling in primary cultures of human osteoblasts from osteoporotic and control donors. In comparison with control cells, osteoporotic osteoblasts showed increased tyrosine phosphorylation of the IGF-I receptor in the basal state and blunted stimulation of receptor phosphorylation by IGF-I. Augmentation of basal IGF-I receptor phosphorylation was associated with coordinate increases in basal tyrosine phosphorylation of insulin receptor substrate (IRS)-2 and activation of Erk, which were also minimally responsive to IGF-I stimulation. By contrast, phosphorylation levels of IRS-1, Akt, and glycogen synthase kinase-3 were similar in the basal state in control and osteoporotic osteoblasts and showed marked increases after IGF-I stimulation in both cell populations, even though these responses were significantly lower in the osteoporotic osteoblasts. The IGF-I signaling abnormalities in osteoporotic osteoblasts were associated with reduced DNA synthesis both under basal conditions and after stimulation with IGF-I. Interestingly, treatment of the osteoporotic osteoblasts with the MAPK kinase inhibitor PD098059 reduced the elevated levels of Erk phosphorylation and increased basal DNA synthesis. Collectively, our data show that altered osteoblast proliferation in human osteoporosis may result from dysregulation of IGF-I receptor signaling, including constitutive activation of the IRS-2/Erk signaling pathway, which becomes unresponsive to IGF-I, and defective induction of the IRS-1/Akt signaling pathway.

Intrauterine growth restriction in humans is associated with abnormalities in placental insulin-like growth factor signaling.

The IGFs promote the growth and development of the feto-placental unit during gestation, and impairment of their placental actions may result in altered intrauterine growth of the fetus. In this study, proteins involved in IGF signaling were investigated in human placentas from pregnancies complicated by intrauterine growth restriction (IUGR) compared with those from normal pregnancies. IUGR placentas exhibited 33% reduction in the protein content of IGF-I receptors, but no changes in insulin receptor protein levels. In addition, insulin receptor substrate-2 (IRS-2) protein levels were reduced in IUGR placentas, with no changes in IRS-1 or Shc protein content, and this was associated with a parallel decrease in IRS-2-associated phosphatidyl inositol 3-kinase. Akt protein expression was also reduced in IUGR, whereas phosphorylation of Akt and its substrate glycogen synthase kinase-3 was unchanged. Finally, in IUGR placentas there was impaired activation of multiple members of the MAPK family, because phosphorylation of p38 and c-Jun N-terminal kinase was reduced 70%. In conclusion, human placentas from pregnancies complicated by IUGR are characterized by decreased IGF-I receptor content, selective impairment of the IRS-2/ phosphatidyl inositol 3-kinase pathway, and reduced p38 and c-Jun N-terminal kinase activation. The observed abnormalities in IGF-I signaling may contribute to altered fetal growth and development in human IUGR.

Dehydroepiandrosterone stimulates glucose uptake in human and murine adipocytes by inducing GLUT1 and GLUT4 translocation to the plasma membrane.

Dehydroepiandrosterone (DHEA) has been shown to modulate glucose utilization in humans and animals, but the mechanisms of DHEA action have not been clarified. We show that DHEA induces a dose- and time-dependent increase in glucose transport rates in both 3T3-L1 and human adipocytes with maximal effects at 2 h. Exposure of adipocytes to DHEA does not result in changes of total GLUT4 and GLUT1 protein levels. However, it does result in significant increases of these glucose transporters in the plasma membrane. In 3T3-L1 adipocytes, DHEA increases tyrosine phosphorylation of insulin receptor substrate (IRS)-1 and IRS-2 and stimulates IRS-1- and IRS-2-associated phosphatidylinositol (PI) 3-kinase activity with no effects on either insulin receptor or Akt phosphorylation. In addition, DHEA causes significant increases of cytosolic Ca(2+) concentrations and a parallel activation of protein kinase C (PKC)-beta(2). The effects of DHEA are abrogated by pretreatment of adipocytes with PI 3-kinase and phospholipase C gamma inhibitors, as well as by inhibitors of Ca(2+)-dependent PKC isoforms, including a specific PKC-beta inhibitor. Thus, DHEA increases glucose uptake in both human and 3T3-L1 adipocytes by stimulating GLUT4 and GLUT1 translocation to the plasma membrane. PI 3-kinase, phospholipase C gamma, and the conventional PKC-beta(2) seem to be involved in DHEA effects.