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/jc.87.1.226
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发表时间：
2002-01-01
影响因子：
5.8
通讯作者：
Olefsky, JM
中科院分区：
文献类型：
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作者：
Kruszynska, YT;Worrall, DS;Olefsky, JM
血浆非酯化脂肪酸(NEFA)水平升高导致骨骼肌胰岛素抵抗的机制尚不清楚。NEFA引起的PI3K激活缺陷，在胰岛素刺激葡萄糖运输中起着关键作用。我们试图检测升高的血浆NEFA(类似于1 mmol/L)对肌肉PI3K活性、胰岛素受体底物(IRS)-1(对PI3K激活至关重要)以及Akt的影响，Akt位于PI3K下游，通过PI3K依赖的丝氨酸和苏氨酸上的磷酸化激活。10名正常人[年龄37+/-9岁(平均+/-SD)；体重指数，25.2+/-3.8 kg/m(2)]接受了两次5h的高胰岛素正血糖钳夹(80 mU/m(2).min)，钳夹的基础和末梢的股外侧肌活检术。在一项研究中，静脉滴注20%Intralipid(1ml/min)和肝素(900U/h)，并提前2.5h，血浆NEFAs升高。用Western blotting对骨骼肌蛋白质水平进行定量。升高的血浆NEFA使全身胰岛素刺激的葡萄糖处置减少了24%(42.1+/-4.0vs 54.8+/-3.6mumol/kg.min；P<；0.001)。两项研究的基础肌IRS-1相同。胰岛素受体-1水平在对照葡萄糖钳夹中降低了40%(P<；0.005)，但在Intralipid研究期间没有变化。总酪氨酸磷酸化IRS-1在对照钳制期间增加了29%(P<；0.05)，但在Intralipid研究期间仅增加了18%(NS)。无论是基础状态还是葡萄糖钳夹状态，PI3K和Akt的p85α亚基总水平均不受血浆NEFA水平的影响。胰岛素诱导的IRS-1相关的PI3K活性的增加被升高的NEFA所减弱，因此Intralipid钳夹末端的活性比对照组低35%(P<；0.05)。PI3K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。葡萄糖钳夹组P-丝氨酸和P-Thr Akt水平显著升高。尽管Intralipid研究中PI3K减少，但没有发现Akt磷酸化缺陷。综上所述，NEFA诱导的胰岛素抵抗与IRS-1酪氨酸磷酸化和IRS-1相关的PI3K激活受损有关。IRS-1水平的下调也受到了损害。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PI3K活性受损的结果。这反过来可能通过Akt非依赖性途径导致葡萄糖转运受损，因为Akt的磷酸化不受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 (similar to1 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).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 mumol/kg.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.