Novel and reversible mechanisms of smoking-induced insulin resistance in humans

Abstract

Smoking is the most common cause of preventable morbidity and mortality in the United States, in part because it is an independent risk factor for the development of insulin resistance and type 2 diabetes. However, mechanisms responsible for smoking-induced insulin resistance are unclear. In this study, we found smokers were less insulin sensitive compared with controls, which increased after either 1 or 2 weeks of smoking cessation. Improvements in insulin sensitivity after smoking cessation occurred with normalization of IRS-1(ser636) phosphorylation. In muscle cell culture, nicotine exposure significantly increased IRS-1(ser636) phosphorylation and decreased insulin sensitivity, recapitulating the phenotype of smoking-induced insulin resistance in humans. The two pathways known to stimulate IRS-1(ser636) phosphorylation (p44/42 mitogen-activated protein kinase [MAPK] and mammalian target of rapamycin [mTOR]) were both stimulated by nicotine in culture. Inhibition of mTOR, but not p44/42 MAPK, during nicotine exposure prevented IRS-1(ser636) phosphorylation and normalized insulin sensitivity. These data indicate nicotine induces insulin resistance in skeletal muscle by activating mTOR. Therapeutic agents designed to oppose skeletal muscle mTOR activation may prevent insulin resistance in humans who are unable to stop smoking or are chronically exposed to secondhand smoke.

FIG. 1.

Time-course breath CO (A) and urinary cotinine (B) concentration at baseline and during smoking cessation. Values are means ± SEM.

FIG. 2.

Insulin sensitivity measured using the Bergman minimal model in nonsmokers and smokers. Data for the change in each individual are shown. Values are means ± SEM. §Significantly different than nonsmokers, P < 0.05; *significantly different after smoking cessation, P < 0.05. Si, insulin sensitivity index.

FIG. 3.

Palmitate Ra (A), oxidation (B), and glucose Ra (C) before and after smoking cessation intervention in control subjects and cigarette smokers. Values are means ± SEM. §Significantly different than control subjects, P < 0.05.

FIG. 4.

IMTG concentration (A), FSR (B), and saturation (C) and intramuscular DAG concentration (D) and saturation (E) in nonsmokers and smokers. Values are means ± SEM. §Significantly different than nonsmokers, P < 0.05.

FIG. 5.

IRS-1 serine⁶³⁶ phosphorylation before and after smoking cessation in control subjects and cigarette smokers. Values are means ± SEM. §Significantly different than control subjects, P < 0.05; *significantly different after smoking cessation, P < 0.05.

FIG. 6.

Effects of nicotine on cell signaling and insulin sensitivity in L6 skeletal muscle myotubes. A: Serine phosphorylation of IRS-1 in response to nicotine exposure at 200 μmol. B: Stimulation of p44/p42 MAPK over time with continuous nicotine exposure at 200 μmol with and without MEK inhibitor UO126, the upstream kinase of p44/p42 activation. C: Stimulation of p70s6k over time with continuous nicotine exposure at 200 μmol with and without the mTOR inhibitor rapamycin. D: Effects of nicotine at 200 μmol on IRS-1^ser636 phosphorylation in L6 muscle cell culture with and without inhibitors against p44/p42 MAPK and mTOR. Values are mean ± SEM. §Significantly different than control, P < 0.05; ¥significantly different than time 0, P < 0.05; #significantly different than nicotine, P < 0.05.

FIG. 6.

Effects of nicotine on cell signaling and insulin sensitivity in L6 skeletal muscle myotubes. A: Serine phosphorylation of IRS-1 in response to nicotine exposure at 200 μmol. B: Stimulation of p44/p42 MAPK over time with continuous nicotine exposure at 200 μmol with and without MEK inhibitor UO126, the upstream kinase of p44/p42 activation. C: Stimulation of p70s6k over time with continuous nicotine exposure at 200 μmol with and without the mTOR inhibitor rapamycin. D: Effects of nicotine at 200 μmol on IRS-1^ser636 phosphorylation in L6 muscle cell culture with and without inhibitors against p44/p42 MAPK and mTOR. Values are mean ± SEM. §Significantly different than control, P < 0.05; ¥significantly different than time 0, P < 0.05; #significantly different than nicotine, P < 0.05.

FIG. 7.

Effects of nicotine, with and without inhibitors against p44/p42 MAPK and mTOR, on insulin-stimulated glucose uptake in L6 muscle cell culture. Values are mean ± SEM. §Significantly different than control, P < 0.05; #significantly different than nicotine, P < 0.05.