Jürgen Zapf

Direct effects of leptin on brown and white adipose tissue.

Leptin is thought to exert its actions on energy homeostasis through the long form of the leptin receptor (OB-Rb), which is present in the hypothalamus and in certain peripheral organs, including adipose tissue. In this study, we examined whether leptin has direct effects on the function of brown and white adipose tissue (BAT and WAT, respectively) at the metabolic and molecular levels. The chronic peripheral intravenous administration of leptin in vivo for 4 d resulted in a 1.6-fold increase in the in vivo glucose utilization index of BAT, whereas no significant change was found after intracerebroventricular administration compared with pair-fed control rats, compatible with a direct effect of leptin on BAT. The effect of leptin on WAT fat pads from lean Zucker Fa/ fa rats was assessed ex vivo, where a 9- and 16-fold increase in the rate of lipolysis was observed after 2 h of exposure to 0.1 and 10 nM leptin, respectively. In contrast, no increase in lipolysis was observed in the fat pads from obese fa/fa rats, which harbor an inactivating mutation in the OB-Rb. At the level of gene expression, leptin treatment for 24 h increased malic enzyme and lipoprotein lipase RNA 1.8+/-0.17 and 1.9+/-0.14-fold, respectively, while aP2 mRNA levels were unaltered in primary cultures of brown adipocytes from lean Fa/fa rats. Importantly, however, no significant effect of leptin was observed on these genes in brown adipocytes from obese fa/fa animals. The presence of OB-Rb receptors in adipose tissue was substantiated by the detection of its transcripts by RT-PCR, and leptin treatment in vivo and in vitro activated the specific STATs implicated in the signaling pathway of the OB-Rb. Taken together, our data strongly suggest that leptin has direct effects on BAT and WAT, resulting in the activation of the Jak/STAT pathway and the increased expression of certain target genes, which may partially account for the observed increase in glucose utilization and lipolysis in leptin-treated adipose tissue.

Activity in fetal bovine serum that stimulates erythroid colony formation in fetal mouse livers is insulinlike growth factor I.

In the present study, the erythropoietic activity of fetal serum was characterized. Using fetal bovine serum (FBS) as a source of the erythropoietic activity and serum-free cultures of fetal mouse livers (FMLC assay) as a detection system, we found that FBS stimulated colony formation from late erythroid progenitor cells (CFU-E) in a dose-dependent fashion. The slope of the dose-response curve, however, was significantly different from that for erythropoietin (Ep), the best-characterized erythropoietic activity so far. The erythropoietic activity of FBS was found in the 120-160- and 40-70-kD range at neutral pH. In the presence of 1 M acetic acid, however, the erythropoietic activity had an apparent molecular mass between 3 and 13 kD. From ion exchange experiments with DEAE-cellulose, the isoionic point of the activity was estimated to about pH 5. Furthermore, the erythropoietic activity of FBS was found to be co-eluted on Sephadex G-150 with the binding proteins of insulinlike growth factors (IGF). The IGF I concentration determined by radioimmunoassay was 70 ng IGF I/ml. The Ep activity of FBS was less than 5 mU/ml when determined with the posthypoxic polycythemic mouse assay for Ep. These results suggest that the erythropoietic activity of FBS is related to IGF and not to Ep. The erythropoietic activity of FBS was abolished by an antiserum against IGF I. Furthermore, IGF I was a factor of approximately 40 more potent than IGF II in stimulating erythroid colony formation. All of these findings suggest that the erythropoietic activity of FBS is IGF I.

Triiodothyronine induces over-expression of alpha-smooth muscle actin, restricts myofibrillar expansion and is permissive for the action of basic fibroblast growth factor and insulin-like growth factor I in adult rat cardiomyocytes.

Effects of triiodothyronine (T3) on the expression of cytoskeletal and myofibrillar proteins in adult rat cardiomyocytes (ARC) were followed during two weeks of culture in the presence of 20% T3-depleted (stripped) FCS. Control cultures expressed mainly beta-myosin heavy chain (MHC) mRNA. T3 caused a switch to alpha-MHC expression and a dose-dependent increase of alpha-smooth muscle (alpha-sm) actin mRNA and protein. In parallel, the number of alpha-sm actin immunoreactive cells increased from 1% in controls to 29 and 62% in ARC treated with 5 and 100 nM T3. In the presence of T3, cells exhibited a higher beating rate than controls. The distribution of myofibrils in T3-treated cells was restricted to the perinuclear area with a sharp boundary. Only 5% of the control cells but 30 and 62% of the T3-treated (5 and 100 nM) ARC showed this restricted myofibrillar phenotype. Basic fibroblast growth factor (bFGF) which restricts myofibrillar growth and upregulates alpha-sm actin in ARC cultured with normal FCS had no effect on alpha-sm actin in ARC cultured in stripped FCS, but potentiated the effect of T3. In contrast, insulin-like growth factor I (IGF I), which suppresses alpha-sm actin and stimulates myofibrillogenesis in the presence of normal FCS suppressed T3-induced alpha-sm actin expression in stripped FCS. Thus, T3 appears to be permissive for the action of bFGF and IGF I on alpha-sm actin expression.