AMPK and insulin action--responses to ageing and high fat diet.

The 5'-AMP-activated protein kinase (AMPK) is considered "a metabolic master-switch" in skeletal muscle reducing ATP- consuming processes whilst stimulating ATP regeneration. Within recent years, AMPK has also been proposed as a potential target to attenuate insulin resistance, although the exact role of AMPK is not well understood. Here we hypothesized that mice lacking α2AMPK activity in muscle would be more susceptible to develop insulin resistance associated with ageing alone or in combination with high fat diet. Young (∼4 month) or old (∼18 month) wild type and muscle specific α2AMPK kinase-dead mice on chow diet as well as old mice on 17 weeks of high fat diet were studied for whole body glucose homeostasis (OGTT, ITT and HOMA-IR), insulin signaling and insulin-stimulated glucose uptake in muscle. We demonstrate that high fat diet in old mice results in impaired glucose homeostasis and insulin stimulated glucose uptake in both the soleus and extensor digitorum longus muscle, coinciding with reduced insulin signaling at the level of Akt (pSer473 and pThr308), TBC1D1 (pThr590) and TBC1D4 (pThr642). In contrast to our hypothesis, the impact of ageing and high fat diet on insulin action was not worsened in mice lacking functional α2AMPK in muscle. It is concluded that α2AMPK deficiency in mouse skeletal muscle does not cause muscle insulin resistance in young and old mice and does not exacerbate obesity-induced insulin resistance in old mice suggesting that decreased α2AMPK activity does not increase susceptibility for insulin resistance in skeletal muscle

Body composition and metabolic characterization.

<p>Body composition, VO2 and RER were determined in young and old AMPK KD mice and WT littermates on chow diet (CHO) or in old mice after 17 weeks of high fat diet (FA).</p>‡<p>Main effect of fasting vs. fed conditions, p<0.001.</p>#<p>Main effect of diet, p<0.01.</p>$<p>Main effect of age, p<0.005.</p>†<p>Main effect of genotype, p<0.05. Values are means ± SE. n = 7–16.</p

Adipose triglyceride lipase plays a key role in the supply of the working muscle with fatty acids

FAs are mobilized from triglyceride (TG) stores during exercise to supply the working muscle with energy. Mice deficient for adipose triglyceride lipase (ATGL-ko) exhibit defective lipolysis and accumulate TG in adipose tissue and muscle, suggesting that ATGL deficiency affects energy availability and substrate utilization in working muscle. In this study, we investigated the effect of moderate treadmill exercise on blood energy metabolites and liver glycogen stores in mice lacking ATGL. Because ATGL-ko mice exhibit massive accumulation of TG in the heart and cardiomyopathy, we also investigated a mouse model lacking ATGL in all tissues except cardiac muscle (ATGL-ko/CM). In contrast to ATGL-ko mice, these mice did not accumulate TG in the heart and had normal life expectancy. Exercise experiments revealed that ATGL-ko and ATGL-ko/CM mice are unable to increase circulating FA levels during exercise. The reduced availability of FA for energy conversion led to rapid depletion of liver glycogen stores and hypoglycemia. Together, our studies suggest that ATGL-ko mice cannot adjust circulating FA levels to the increased energy requirements of the working muscle, resulting in an increased use of carbohydrates for energy conversion. Thus, ATGL activity is required for proper energy supply of the skeletal muscle during exercise

Akt Thr308 phosphorylation.

<p>Basal (0 µU/ml) and insulin (500 µU/ml) stimulated Akt Thr308 phosphorylation measured by Western blot analyses in m. Soleus (SOL) and m. Extensor Digitorum Longus (EDL). Measurements were made in young and old AMPK KD mice and WT littermates on chow diet (CHO) or in old mice after 17 weeks of high fat diet (FA). *: Main effect of insulin, p<0.001. ‡: Interaction between diet and insulin action, p<0.001. (‡): Trend towards interaction between diet and insulin action, p = 0.06. $: Interaction between genotype and insulin action, p<0.05. Values are means ± SE. n = 11–15.</p

TBC1D1 Thr590 phosphorylation.

<p>Basal (0 µU/ml) and insulin (500 µU/ml) stimulated TBC1D1 Thr590 phosphorylation measured by Western blot analyses in m. Soleus (SOL) and m. Extensor Digitorum Longus (EDL). Measurements were made in young and old AMPK KD mice and WT littermates on chow diet (CHO) or in old mice after 17 weeks of high fat diet (FA). *: Main effect of insulin, p<0.001. #: Main effect of diet p<0.001. †: Main effect of genotype, p<0.05. Values are means ± SE. n = 11–15.</p

Akt Ser473 phosphorylation.

<p>Basal (0 µU/ml) and insulin (500 µU/ml) stimulated Akt Ser473 phosphorylation measured by Western blot analyses in m. Soleus (SOL) and m. Extensor Digitorum Longus (EDL). Measurements were made in young and old AMPK KD mice and WT littermates on chow diet (CHO) or in old mice after 17 weeks of high fat diet (FA). *: Main effect of insulin, p<0.001. ‡: Interaction between diet and insulin action, p<0.001. †: Main effect of genotype in either CHO fed or Old groups, p<0.05. Values are means ± SE. n = 11–15.</p