Leptin mediates its effects on food intake through the hypothalamic form of its receptor OB-R. Variants of OB-R are found in other tissues, but their function is unknown. Here, an OB-R variant was found in human hepatic cells. Exposure of these cells to leptin, at concentrations comparable with those present in obese individuals, caused attenuation of several insulin-induced activities, including tyrosine phosphorylation of the insulin receptor substrate-1 (IRS-1), association of the adapter molecule growth factor receptor-bound protein 2 with IRS-1, and down-regulation of gluconeogenesis. In contrast, leptin increased the activity of IRS-1-associated phosphatidylinositol 3-kinase. These in vitro studies raise the possibility that leptin modulates insulin activities in obese individuals.

Vesicular stomatitis virus (VSV) exhibits a remarkably robust and pantropic infectivity, mediated by its coat protein, VSV-G. Using this property, recombinant forms of VSV and VSV-G-pseudotyped viral vectors are being developed for gene therapy, vaccination, and viral oncolysis and are extensively used for gene transduction in vivo and in vitro. The broad tropism of VSV suggests that it enters cells through a highly ubiquitous receptor, whose identity has so far remained elusive. Here we show that the LDL receptor (LDLR) serves as the major entry port of VSV and of VSV-G-pseudotyped lentiviral vectors in human and mouse cells, whereas other LDLR family members serve as alternative receptors. The widespread expression of LDLR family members accounts for the pantropism of VSV and for the broad applicability of VSV-G-pseudotyped viral vectors for gene transduction.

Hypoxic stress induces the expression of genes associated with increased energy flux, including the glucose transporters Glut1 and Glut3, several glycolytic enzymes, nitric oxide synthase, tyrosine hydroxylase, erythropoietin and vascular endothelial growth factor (VEGF). Induction of these genes is mediated by a common basic helix-loop-helix-PAS transcription complex, the hypoxia-inducible factor-1α (HIF-1α)/ aryl hydrocarbon nuclear translocator (ARNT). Insulin also induces some of these genes; however, the underlying mechanism is unestablished. We report here that insulin shares with hypoxia the ability to induce the HIF-1α/ARNT transcription complex in various cell types. This induction was demonstrated by electrophoretic mobility shift of the hypoxia response element (HRE), and abolished by specific antisera to HIF-1α and ARNT, and by transcription activation of HRE reporter vectors. Furthermore, basal and insulininduced expression of Glut1, Glut3, aldolase A, phosphoglycerate kinase and VEGF was reduced in cells having a defective ARNT. Similarly, the insulin-induced activation of HRE reporter vectors and VEGF was impaired in these cells and was rescued by re-introduction of ARNT. Finally, insulin-like growth factor-I (IGF-I) also induced the HIF-1α/ARNT transcription complex. These observations establish a novel signal transduction pathway of insulin and IGF-I and broaden considerably the scope of activity of HIF-1α/ARNT.

No abstract

Phospholipase C-y (PLC--y) is a substrate of the fibroblast growth factor receptor (FGFR; encoded by theflg gene) and other receptors with tyrosine kinase activity. It has been demonstrated that the src homology region 2 (SH2 domain) of PLC--y and of other signalling molecules such as GTPase-activating protein and phosphatidylinositol 3-kinase-associated p85 direct their binding toward tyrosine-autophosphorylated regions of the epidermal growth factor or platelet-derived growth factor receptor. In this report, we describe the identification of Tyr-766 as an autophosphorylation site of flg-encoded FGFR by direct sequencing of a tyrosine-phosphorylated tryptic peptide isolated from the cytoplasmic domain of FGFR expressed in Escherichia coli. The same phosphopeptide was found in wild-type FGFR phosphorylated either in vitro or in living cells. Like other growth factor receptors, tyrosine-phosphorylated wild-type FGFR or its cytoplasmic domain becomes associated with intact PLC-y or with a fusion protein containing the SH2 domain of PLC-,y. To delineate the site of association, we have examined the capacity of a 28-amino-acid tryptic peptide containing

No abstract

Apoptosis prevents osteoporosis G randmothers everywhere know well that estrogen defi cits lead to osteoporosis. Now, the molecular basis for this debilitating bone loss is fi nally identifi ed. Estrogen is needed to kill off bone-destroying osteoclasts, show Takashi Nakamura, Shigeaki Kato (University of Tokyo, Japan), and colleagues. The root cause of osteoporosis has been diffi cult to pin down, in part because bones are not frail in female mice lacking estrogen receptors. These mice make extra androgen, which builds bone in male mice and might compensate for bone loss in the mutant females. To avoid the androgen rise, Kato's group knocked out estrogen receptors only in mature osteoclasts, which accumulate in osteoporotic bones. These female mutants developed rickety bones due to losses within the central bone shafts. The authors then isolated osteoclasts to determine why they are so abundant in diseased bone. Microarray analyses revealed that estrogen induced apoptotic proteins, including Fas ligand, that were not induced in the estrogen-blind osteoclasts. Men who have estrogen receptor mutations develop osteoporosis. But male mice were not affected by the loss of estrogen receptors in osteoclasts. Perhaps the androgen-headed pathway is more dominant in mice than in humans. Currently, potential drugs to treat osteoporosis are screened through mice whose ovaries have been removed. Screens for the induction of Fas ligand in cultures of estrogen-blind osteoclasts should be much simpler.

No abstract