Effects of metformin and vanadium on leptin secretion from cultured rat adipocytes

Objective: We have reported that glucose utilization regulates leptin expre ssion and secretion from isolated rat adipocytes. In this study, we employe d two antidiabetic agents that act to increase glucose uptake by peripheral tissues, metformin and vanadium, as pharmacological tools to examine the e ffects of altering glucose utilization on leptin secretion in primary cultu res of rat adipocytes. Research Methods and Procedures: Isolated adipocytes (100 mu L of packed ce lls per well) were anchored in a defined matrix of basement membrane compon ents (Matrigel) with media containing 5.5 mM glucose and incubated for 96 h ours with metformin or vanadium. Leptin secretion, glucose utilization, and lactate production were assessed. Results: Metformin (0.5 and 1.0 mM) incr eased glucose uptake in the presence of 0.16 nM insulin by 37 +/- 10% (p < 0.005) and 62 +/- 8% (p < 0.0001) over insulin alone, respectively. Metform in from 0.5 to 5.0 mM increased lactate production by 105 +/- 43% (p < 0.02 5) to 202 +/- 52% (p < 0.0025) and at 1.0 and 5.0 mM increased the proporti onal rate of glucose conversion to lactate by 78 +/- 18% (p < 0.005) and 16 6 +/- 41% (p < 0.0025), respectively. At concentrations less than 0.5 mM, m etformin did not affect leptin secretion, but at 0.5 mM, the only concentra tion that significantly increased glucose utilization without increasing gl ucose conversion to lactate, leptin secretion was modestly stimulated (by 2 0 +/- 9%; p < 0.05). Concentrations from 1.0 to 25 mM inhibited leptin secr etion by 25 +/- 8% (p < 0.005) to 89 +/- 4% (p < 0.0001). Across metformin doses, leptin secretion was inversely related to the percentage of glucose taken up and released as lactate (r = -0.74; p < 0.0001). Vanadium (5 to 20 mu M) increased glucose uptake from 20 +/- 7% (p < 0.01) to 34 +/- 13% (p < 0.02) and increased lactate production at 5 mu M by 17 +/- 8% (p < 0.025) and 10 mu M by 61 +/- 20% (p < 0.02) but did not alter the conversion of g lucose to lactate, Vanadium (5 to 50 mu M) inhibited leptin secretion by 33 +/- 6% (p < 0.0025) to 61 +/- 8% (p < 0.0001). Discussion: Both metformin and vanadium increase glucose uptake and inhibit leptin secretion from cultured adipocytes. The inhibition of leptin secret ion by metformin is related to an increase in the metabolism of glucose to lactate. The inhibition by vanadium most likely involves direct effects on cellular phosphatases. We hypothesize that the effect of glucose utilizatio n to stimulate leptin production involves the metabolism of glucose to a fa te other than anaerobic lactate production, possibly oxidation or lipogenes is.