Evidence against regulation of AMP-activated protein kinase and LKB1/STRAD/MO25 activity by creatine phosphate.

Muscle contraction results in phosphorylation and activation of the AMP-activated protein kinase (AMPK) by an AMPK kinase (AMPKK). LKB1/STRAD/MO25 (LKB1) is the major AMPKK in skeletal muscle; however, the activity of LKB1 is not increased by muscle contraction. This finding suggests that phosphorylation of AMPK by LKB1 is regulated by allosteric mechanisms. Creatine phosphate is depleted during skeletal muscle contraction to replenish ATP. Thus the concentration of creatine phosphate is an indicator of cellular energy status. A previous report found that creatine phosphate inhibits AMPK activity. The purpose of this study was to determine whether creatine phosphate would inhibit 1) phosphorylation of AMPK by LKB1 and 2) AMPK activity after phosphorylation by LKB1. We found that creatine phosphate did not inhibit phosphorylation of either recombinant or purified rat liver AMPK by LKB1. We also found that creatine phosphate did not inhibit 1) active recombinant alpha1beta1gamma1 or alpha2beta2gamma2 AMPK, 2) AMPK immunoprecipitated from rat liver extracts by either the alpha1 or alpha2 subunit, or 3) AMPK chromatographically purified from rat liver. Inhibition of skeletal muscle AMPK by creatine phosphate was greatly reduced or eliminated with increased AMPK purity. In conclusion, these results suggest that creatine phosphate is not a direct regulator of LKB1 or AMPK activity. Creatine phosphate may indirectly modulate AMPK activity by replenishing ATP at the onset of muscle contraction.

AMP-activated protein kinase kinase activity and phosphorylation of AMP-activated protein kinase in contracting muscle of sedentary and endurance-trained rats.

This study was designed to examine activity of AMP-activated protein kinase kinase (AMPKK) in muscles from nontrained and endurance-trained rats. Rats were trained 5 days/wk, 2 h/day for 8 wk at a final intensity of 32 m/min up a 15% grade with 30-s sprints at 53 m/min every 10 min. Gastrocnemius muscles were stimulated in situ in trained and nontrained rats for 5 min at frequencies of 0.4/s and 1/s. Gastrocnemius LKB1 protein, a putative component of the AMPKK complex (LKB1, STRAD, and MO25), increased approximately twofold in response to training. Phosphorylation of AMP-activated protein kinase (AMPK) determined by Western blot and AMPK activity of immunoprecipitates (both isoforms) was increased at both stimulation rates in both trained and nontrained muscles. AMPKK activity was 73% lower in resuspended polyethylene glycol precipitates of muscle extracts from the trained compared with nontrained rats. AMPKK activity did not increase in either trained or nontrained muscle in response to electrical stimulation, even though phospho-AMPK did increase. These results suggest that AMPKK is activated during electrical stimulation of both trained and nontrained muscle by mechanisms other than covalent modification.

Endurance training increases skeletal muscle LKB1 and PGC-1alpha protein abundance: effects of time and intensity.

Recent research suggests that LKB1 is the major AMP-activated protein kinase kinase (AMPKK). Peroxisome-proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) is a master coordinator of mitochondrial biogenesis. Previously we reported that skeletal muscle LKB1 protein increases with endurance training. The purpose of this study was to determine whether training-induced increases in skeletal muscle LKB1 and PGC-1alpha protein exhibit a time course and intensity-dependent response similar to that of citrate synthase. Male Sprague-Dawley rats completed endurance- and interval-training protocols. For endurance training, rats trained for 4, 11, 25, or 53 days. Interval-training rats trained identically to endurance-trained rats, except that after 25 days interval training was combined with endurance training. Time course data were collected from endurance-trained red quadriceps (RQ) after each time point. Interval training data were collected from soleus, RQ, and white quadriceps (WQ) muscle after 53 days only. Mouse protein 25 (MO25) and PGC-1alpha protein increased significantly after 4 days. Increased citrate synthase activity, increased LKB1 protein, and decreased AMPKK activity were found after 11 days. Maximal increases occurred after 4 days for hexokinase II, 25 days for MO25, and 53 days for citrate synthase, LKB1, and PGC-1alpha. In WQ, but not RQ or soleus, interval training had an additive effect to endurance training and induced significant increases in all proteins measured. These results demonstrate that LKB1 and PGC-1alpha protein abundances increase with endurance and interval training similarly to citrate synthase. The increase in LKB1 and PGC-1alpha with endurance and interval training may function to maintain the training-induced increases in mitochondrial mass.