Effects of hyperosmolarity on basal and insulin-stimulated muscle sugar transport

Transient activation of fission yeast AMPK is required for cell proliferation during osmotic stress

Transient activation of the cellular energy sensor AMPK during osmotic stress requires its energy-sensing subunit. Cellular ATP levels decrease during osmotic stress, which triggers energy stress, which in turn requires dynamic activation of AMPK.

The heterotrimeric kinase AMPK acts as an energy sensor to coordinate cell metabolism with environmental status in species from yeast through humans. Low intracellular ATP leads to AMPK activation through phosphorylation of the activation loop within the catalytic subunit. Other environmental stresses also activate AMPK, but it is unclear whether cellular energy status affects AMPK activation under these conditions. Fission yeast AMPK catalytic subunit Ssp2 is phosphorylated at Thr-189 by the upstream kinase Ssp1 in low-glucose conditions, similar to other systems. Here we find that hyperosmotic stress induces strong phosphorylation of Ssp2-T189 by Ssp1. Ssp2-pT189 during osmotic stress is transient and leads to transient regulation of AMPK targets, unlike sustained activation by low glucose. Cells lacking this activation mechanism fail to proliferate after hyperosmotic stress. Activation during osmotic stress requires energy sensing by AMPK heterotrimer, and osmotic stress leads to decreased intracellular ATP levels. We observed mitochondrial fission during osmotic stress, but blocking fission did not affect AMPK activation. Stress-activated kinases Sty1 and Pmk1 did not promote AMPK activation but contributed to subsequent inactivation. Our results show that osmotic stress induces transient energy stress, and AMPK activation allows cells to manage this energy stress for proliferation in new osmotic states.

Oxidative stress stimulates skeletal muscle glucose uptake through a phosphatidylinositol 3-kinase-dependent pathway

We determined the acute effects of oxidative stress on glucose uptake and intracellular signaling in skeletal muscle by incubating muscles with reactive oxygen species (ROS). Xanthine oxidase (XO) is a superoxide-generating enzyme that increases ROS. Exposure of isolated rat extensor digitorum longus (EDL) muscles to Hx/XO (Hx/XO) for 20 min resulted in a dose-dependent increase in glucose uptake. To determine whether the mechanism leading to Hx/XO-stimulated glucose uptake is associated with the production of H2O2, EDL muscles from rats were preincubated with the H2O2 scavenger catalase or the superoxide scavenger superoxide dismutase (SOD) prior to incubation with Hx/XO. Catalase treatment, but not SOD, completely inhibited the increase in Hx/XO-stimulated 2-deoxyglucose (2-DG) uptake, suggesting that H2O2 is an intermediary leading to Hx/XO-stimulated glucose uptake with incubation. Direct H2O2 also resulted in a dose-dependent increase in 2-DG uptake in isolated EDL muscles, and the maximal increase was threefold over basal levels at a concentration of 600 micromol/l H2O2. H2O2-stimulated 2-DG uptake was completely inhibited by the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin, but not the nitric oxide inhibitor NG-monomethyl-l-arginine. H2O2 stimulated the phosphorylation of Akt Ser473 (7-fold) and Thr308 (2-fold) in isolated EDL muscles. H2O2 at 600 micromol/l had no effect on ATP concentrations and did not increase the activities of either the alpha1 or alpha2 catalytic isoforms of AMP-activated protein kinase. These results demonstrate that acute exposure of muscle to ROS is a potent stimulator of skeletal muscle glucose uptake and that this occurs through a PI3K-dependent mechanism.

Characterization of the role of the AMP-activated protein kinase in the stimulation of glucose transport in skeletal muscle cells

Stimulation of AMP-activated protein kinase (AMPK) in skeletal muscle has been correlated with an increase in glucose transport. Here, we demonstrate that adenoviral-mediated expression of a constitutively active mutant of AMPK alpha leads to activation of glucose transport in a skeletal-muscle cell line, similar to that seen following treatment with 5-amino-imidazolecarboxamide (AICA) riboside, hyperosmotic stress or insulin. In contrast, expression of a dominant-negative form of AMPK blocked the stimulation of glucose transport by both AICA riboside and hyperosmotic stress, but was without effect on either insulin or phorbol-ester-stimulated transport. These results demonstrate that activation of AMPK is sufficient for stimulation of glucose uptake into muscle cells, and is a necessary component of the AICA riboside- and hyperosmotic-stress-induced pathway leading to increased glucose uptake. On the other hand, AMPK is not required in the insulin- or phorbol-ester-mediated pathways. Long-term (5 days) expression of the constitutively active AMPK mutant increased protein expression of GLUT1, GLUT4 and hexokinase II, consistent with previous reports on the chronic treatment of rats with AICA riboside. Expression of constitutively active AMPK had no detectable effect on p38 mitogen-activated protein kinase levels, although interestingly the level of protein kinase B was decreased. These results demonstrate that long-term activation of AMPK is sufficient to cause increased expression of specific proteins in muscle. Our results add further support to the hypothesis that long-term activation of AMPK is involved in the adaptive response of muscle to exercise training.