Berberine-induced activation of 5'-adenosine monophosphate-activated protein kinase and glucose transport in rat skeletal muscles.

Berberine (BBR) is the main alkaloid of Coptis chinensis, which has been used as a folk medicine to treat diabetes mellitus in Asian countries. We explored the possibility that 5'-adenosine monophosphate-activated protein kinase (AMPK) is involved in metabolic enhancement by BBR in skeletal muscle, the important tissue for glucose metabolism. Isolated rat epitrochlearis and soleus muscles were incubated in a buffer containing BBR, and activation of AMPK and related events were examined. In response to BBR treatment, the Thr(172) phosphorylation of the catalytic α-subunit of AMPK, an essential step for full kinase activation, increased in a dose- and time-dependent manner. Ser(79) phosphorylation of acetyl-coenzyme A carboxylase, an intracellular substrate of AMPK, increased correspondingly. Analysis of isoform-specific AMPK activity revealed that BBR activated both the α1 and α2 isoforms of the catalytic subunit. This increase in enzyme activity was associated with an increased rate of 3-O-methyl-d-glucose transport in the absence of insulin and with phosphorylation of AS160, a signaling intermediary leading to glucose transporter 4 translocation. The intracellular energy status estimated from the phosphocreatine concentration was decreased by BBR. These results suggest that BBR acutely stimulates both AMPKα1 and AMPKα2 and insulin-independent glucose transport in skeletal muscle with a reduction of the intracellular energy status.

Leucine modulates contraction- and insulin-stimulated glucose transport and upstream signaling events in rat skeletal muscle.

Leucine has profound effects on glucose metabolism in muscle; however, the effects of leucine on glucose transport in muscle have not been well documented. We investigated the effects of leucine on contraction- and insulin-stimulated glucose transport in isolated rat epitrochlearis muscle in vitro. In the absence of insulin, tetanic contraction increased 3-O-methyl-D-glucose (3-MG) transport and Thr(172) phosphorylation of the catalytic alpha-subunit of 5'-AMP-activated protein kinase (AMPK), a signaling intermediary leading to insulin-independent glucose transport. Leucine (2 mM, 30 min) significantly enhanced contraction-stimulated 3-MG transport and AMPK phosphorylation, accompanied by increased phosphorylation of p70 S6 kinase (p70S6K) Thr(389). The stimulatory effects of leucine on 3-MG transport and AMPK phosphorylation were canceled by STO-609 blockade of Ca(2+)/calmodulin-dependent protein kinase kinase (CaMKK) or rapamycin blockade of p70S6K. On the other hand, leucine blunted insulin-stimulated 3-MG transport and reduced insulin-stimulated Akt Thr(473) phosphorylation. Leucine increased insulin-stimulated p70S6K Thr(389) phosphorylation and enhanced the inhibitory phosphorylation of the insulin receptor substrate 1 (IRS1) Ser(636/639). Furthermore, the effects of leucine on insulin-stimulated 3-MG transport and IRS phosphorylation were abolished by rapamycin. These results indicate that leucine activates contraction-stimulated glucose transport and inhibits insulin-stimulated glucose transport in skeletal muscle by activating mammalian target of rapamycin (mTOR)/p70S6K signaling. Enhanced increases in contraction-stimulated AMPK Thr(172) phosphorylation and insulin-stimulated IRS1 Ser(636/639) phosphorylation might be responsible for these opposing effects of leucine, respectively.

Caffeine acutely activates 5'adenosine monophosphate-activated protein kinase and increases insulin-independent glucose transport in rat skeletal muscles.

Caffeine (1,3,7-trimethylxanthine) has been implicated in the regulation of glucose and lipid metabolism including actions such as insulin-independent glucose transport, glucose transporter 4 expression, and fatty acid utilization in skeletal muscle. These effects are similar to the exercise-induced and 5'adenosine monophosphate-activated protein kinase (AMPK)-mediated metabolic changes in skeletal muscle, suggesting that caffeine is involved in the regulation of muscle metabolism through AMPK activation. We explored whether caffeine acts on skeletal muscle to stimulate AMPK. Incubation of rat epitrochlearis and soleus muscles with Krebs buffer containing caffeine (> or =3 mmol/L, > or =15 minutes) increased the phosphorylation of AMPKalpha Thr(172), an essential step for full kinase activation, and acetyl-coenzyme A carboxylase Ser(79), a downstream target of AMPK, in dose- and time-dependent manners. Analysis of isoform-specific AMPK activity revealed that both AMPKalpha1 and alpha2 activities increased significantly. This enzyme activation was associated with a reduction in phosphocreatine content and an increased rate of 3-O-methyl-d-glucose transport activity in the absence of insulin. These results suggest that caffeine has similar actions to exercise by acutely stimulating skeletal muscle AMPK activity and insulin-independent glucose transport with a reduction of the intracellular energy status.

Morus alba leaf extract stimulates 5'-AMP-activated protein kinase in isolated rat skeletal muscle.

AIM OF THE STUDY: Morus alba (mulberry) leaf is a natural therapeutic agent that has been shown to have an antidiabetic effect. We explored the possibility that 5'-AMP-activated protein kinase (AMPK) is involved in metabolic enhancement by the Morus alba leaf. MATERIALS AND METHODS: Isolated rat epitrochlearis muscle was incubated in a buffer containing Morus alba leaf hot water extract (MLE) and the AMPK activation and related events were examined. RESULTS: In response to MLE treatment, the Thr(172) phosphorylation of the catalytic alpha subunit of AMPK, an essential step for full kinase activation increased in a dose- and time-dependent manner. Ser(79) phosphorylation of acetyl CoA carboxylase, an intracellular substrate of AMPK, increased similarly. Analysis of isoform-specific AMPK activity revealed that MLE activated both the alpha1 and alpha2 isoforms of the catalytic subunit. This increase in enzyme activity was associated with an increased rate of 3-O-methyl-D-glucose transport in the absence of insulin and with phosphorylation of AS160, a signaling intermediary leading to glucose transporter 4 translocation. The intracellular energy status, estimated from the ATP and phosphocreatine concentrations, was not affected by MLE. CONCLUSION: MLE stimulates skeletal muscle AMPK activity acutely without changing the intracellular energy status.