Fatty acid-induced insulin resistance: decreased muscle PI3K activation but unchanged Akt phosphorylation.
Fatty acid-induced insulin resistance: decreased muscle PI3K activation but unchanged Akt phosphorylation.
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DOI：
10.1210/jcem.87.1.8187
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发表时间：
2002
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影响因子：
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通讯作者：
Y. Kruszynska;D. S. Worrall;Jachelle M. Ofrecio;J. Frias;G. Macaraeg;J. Olefsky
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文献类型：
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作者：
Y. Kruszynska;D. S. Worrall;Jachelle M. Ofrecio;J. Frias;G. Macaraeg;J. Olefsky
血浆非酯化脂肪酸（NEFA）水平升高诱导骨骼肌胰岛素抵抗的机制尚不清楚。NEFA诱导的PI 3 K激活缺陷在胰岛素刺激葡萄糖转运中起关键作用。我们试图检查升高的血浆NEFA（约1 mmol/L）对肌肉PI 3 K活性、胰岛素受体底物（IRS）-1（对PI 3 K活化很重要）和Akt（位于PI 3 K下游，通过丝氨酸和苏氨酸磷酸化以PI 3 K依赖性方式活化）的影响。10名正常男性[年龄37 ± 9岁（平均值± SD）;体重指数25.2 ± 3.8 kg/m2]接受了两次5小时高胰岛素（80 mU/m2 x min）正葡萄糖钳夹术，钳夹术结束时和钳夹术基底部对股外侧肌进行活检。在一项研究中，通过在整个过程中和之前2.5小时输注20% Intralidid（1 ml/min）和肝素（900 U/h），血浆NEFA增加。通过蛋白质印迹法定量测定骨骼肌蛋白水平。血浆NEFA升高使全身胰岛素刺激的葡萄糖处置减少24%（42.1 +/- 4.0 vs. 54.8 +/- 3.6 micromol/kg x min; P < 0.001）。基底肌IRS-1在两项研究中是相同的。IRS-1水平在对照葡萄糖钳夹中下降了40%（P < 0.005），但在Intradherid研究中没有变化。总酪氨酸磷酸化IRS-1在对照钳夹期间增加了29%（P < 0.05），但在Intramidid研究期间仅增加了18%（NS）。在基础状态或葡萄糖钳夹期间，PI 3 K和Akt的p85 α亚基的总水平不受血浆NEFA水平的影响。胰岛素诱导的IRS-1相关PI 3 K活性的增加被升高的NEFA所削弱，因此在含Intraperoid的钳夹末端的活性比对照钳夹低35%（P < 0.05）。PI 3 K活性降低的百分比与NEFA升高引起的胰岛素刺激的葡萄糖消失率降低相关（r = 0.70; P < 0.05）。基础P-ser-和P-thr-Akt水平非常低，不受NEFA水平的影响。葡萄糖钳夹导致P-ser和P-thr Akt水平显著增加。总之，NEFA诱导的胰岛素抵抗与IRS-1酪氨酸磷酸化和IRS-1相关的PI 3 K活化的损伤有关。IRS-1水平的下调也受到损害。NEFA诱导的肌肉葡萄糖摄取缺陷似乎是胰岛素信号通路缺陷导致PI 3 K活化受损的结果。这反过来又可能导致葡萄糖通过Akt非依赖性途径转运受损，因为Akt磷酸化不受NEFA水平升高的影响。
The mechanisms by which elevated plasma nonesterified fatty acid (NEFA) levels induce skeletal muscle insulin resistance remain unclear. A NEFA-induced defect in the activation of PI3K, which plays a key role in insulin's stimulation of glucose transport, has been invoked. We sought to examine the effects of elevated plasma NEFA (approximately 1 mmol/liter) on muscle PI3K activity, insulin receptor substrate (IRS)-1 (important for activation of PI3K), and Akt, which is downstream of PI3K and activated by phosphorylation on serine and threonine in a PI3K-dependent manner. Ten normal men [age, 37 +/- 9 yr (mean +/- SD); body mass index, 25.2 +/- 3.8 kg/m(2)] underwent two 5-h hyperinsulinemic (80 mU/m(2) x min) euglycemic clamps with basal and end of clamp biopsies of the vastus lateralis muscle. Plasma NEFAs were increased in one study by infusion of 20% Intralipid (1 ml/min) and heparin (900 U/h) throughout and for 2.5 h beforehand. Skeletal muscle protein levels were quantified by Western blotting. Elevated plasma NEFA reduced whole-body insulin-stimulated glucose disposal by 24% (42.1 +/- 4.0 vs. 54.8 +/- 3.6 micromol/kg x min; P < 0.001). Basal muscle IRS-1 was the same in the two studies. IRS-1 levels decreased by 40% in the control glucose clamps (P < 0.005), but did not change during the Intralipid study. Total tyrosine phosphorylated IRS-1 increased by 29% during the control clamps (P < 0.05), but by only 18% (NS) during the Intralipid studies. Total levels of p85alpha subunit of PI3K and Akt were not influenced by plasma NEFA levels either in the basal state or during the glucose clamps. The insulin-induced increase in IRS-1-associated PI3K activity was impaired by elevated NEFA, so that activity at the end of the clamps with Intralipid was 35% lower than in the control clamps (P < 0.05). The percentage reduction in PI3K activation correlated with the reduction in insulin-stimulated glucose disappearance rate that was induced by elevated NEFA (r = 0.70; P < 0.05). Basal P-ser- and P-thr-Akt levels were very low and unaffected by NEFA levels. The glucose clamps resulted in a marked increase in P-ser and P-thr Akt levels. Despite the decrease in PI3K in the Intralipid study, no defect in Akt phosphorylation was found. In summary, NEFA-induced insulin resistance is associated with an impairment of IRS-1 tyrosine phosphorylation and IRS-1-associated PI3K activation. Down-regulation of IRS-1 levels is also impaired. The NEFA-induced defect in muscle glucose uptake appears to be a consequence of a defect in the insulin-signaling pathway leading to impaired PI3K activation. This in turn may lead to impaired glucose transport through an Akt-independent pathway because Akt phosphorylation was unaffected by elevated NEFA levels.