The effect of hypoxia on the expression and secretion of major adipokines by human preadipocytes has been examined. Hypoxia (1% O₂) led to an increase in the HIF-1α transcription factor subunit in cultured preadipocytes, as did incubation with the hypoxia mimetic CoCl₂. Leptin mRNA was essentially undetectable in preadipocytes incubated under normoxia (21% O₂), but exposure to 1% O₂, or CoCl₂, for 4 or 24 h resulted in an induction of leptin gene expression (measured by real-time PCR). Immunoreactive leptin was not detected in the medium from normoxic preadipocytes, but was present in the medium from the hypoxic cells. Hypoxia stimulated expression of the GLUT-1 facilitative glucose transporter gene and the vascular endothelial growth factor (VEGF) gene in preadipocytes, as in adipocytes. PPARγ and aP2 mRNA levels, markers of adipocyte differentiation, were reduced by hypoxia in both cell types. In marked contrast to adipocytes, interleukin-6 (IL-6), angiopoietin-like protein 4, and plasminogen activator inhibitor-1 expression by preadipocytes was not stimulated by low O₂ tension. Consistent with the gene expression results, VEGF release into the medium from preadipocytes was increased by hypoxia, but there was no change in IL-6 secretion. It is concluded that hypoxia induces human preadipocytes to synthesize and secrete leptin. Preadipocytes and adipocytes differ in their responsiveness to low O₂ tension, maturation of the response to hypoxia developing on differentiation.

Previous studies by Tisdale et al. have reported that zinc-α₂-glycoprotein (ZAG (AZGP1)) reduces body fat content and improves glucose homeostasis and the plasma lipid profile in Aston (ob/ob) mice. It has been suggested that this might be mediated via agonism of β₃- and possibly β₂-adrenoceptors. We compared the effects of dosing recombinant human ZAG (100 μg, i.v.) and BRL35135 (0.5 mg/kg, i.p.), which is in rodents a 20-fold selective β₃- relative to β₂-adrenoceptor agonist, given once daily for 10 days to male C57Bl/6 Lep ^ob /Lep ^ob mice. ZAG, but not BRL35135, reduced food intake. BRL35135, but not ZAG, increased energy expenditure acutely and after sub-chronic administration. Only BRL35135 increased plasma concentrations of glycerol and non-esterified fatty acid. Sub-chronic treatment with both ZAG and BRL35135 reduced fasting blood glucose and improved glucose tolerance, but the plasma insulin concentration 30 min after administration of glucose was lowered only by BRL35135. Both ZAG and BRL35135 reduced β₁-adrenoceptor mRNA levels in white adipose tissue, but only BRL35135 reduced β₂-adrenoceptor mRNA. Both ZAG and BRL35135 reduced β₁-adrenoceptor mRNA levels in brown adipose tissue, but neither influenced β₂-adrenoceptor mRNA, and only BRL35135 increased β₃-adrenoceptor and uncoupling protein-1 (UCP1) mRNA levels in brown adipose tissue. Thus, ZAG and BRL35135 had similar effects on glycaemic control and shared some effects on β-adrenoceptor gene expression in adipose tissue, but ZAG did not display the thermogenic effects of the β-adrenoceptor agonist, nor did it increase β₃-adrenoceptor or UCP1 gene expression in brown adipose tissue. ZAG does not behave as a typical β_3/2-adrenoceptor agonist.