CaMKII and Kalirin, a Rac1-GEF, regulate Akt phosphorylation involved in contraction-induced glucose uptake in skeletal muscle cells

A comprehensive view of muscle glucose uptake: regulation by insulin, contractile activity, and exercise

Skeletal muscle is the main site of glucose deposition in the body during meals and the major glucose utilizer during physical activity. Although in both instances the supply of glucose from the circulation to the muscle is of paramount importance, in most conditions the rate-limiting step in glucose uptake, storage, and utilization is the transport of glucose across the muscle cell membrane. This step is dependent upon the translocation of the insulin- and contraction-responsive glucose transporter GLUT4 from intracellular storage sites to the sarcolemma and T tubules. Here, we first analyze how glucose can traverse the capillary wall into the muscle interstitial space. We then review the molecular processes that regulate GLUT4 translocation in response to insulin and muscle contractions and the methodologies utilized to unravel them. We further discuss how physical activity and inactivity, respectively, lead to increased and decreased insulin action in muscle and touch upon sex differences in glucose metabolism. Although many key processes regulating glucose uptake in muscle are known, the advent of newer and bioinformatics tools has revealed further molecular signaling processes reaching a staggering level of complexity. Much of this molecular mapping has emerged from cellular and animal studies and more recently from application of a variety of -omics in human tissues. In the future, it will be imperative to validate the translatability of results drawn from experimental systems to human physiology.

CASK promotes prostate cancer progression via kinase-dependent activation of AKT

Until now, the role of calcium/calmodulin-dependent serine protein kinase (CASK) in prostate cancer (PCa) progression remains unknown. In this study, we investigated the roles of CASK in PCa progression, cell migration, and invasion. We found that CASK is up-regulated in PCa tissues of patients. Lentivirus-based CASK silencing does not affect cell growth or serum-free-induced cell death in PC3 and LNCaP cells, regardless of the presence or absence of TGF-β. CASK silencing decreases cell migration and invasion, either in the absence or presence of TGF-β stimulation. Immunoblotting data indicate that CASK silencing does not alter TGF-β-induced Smad2/3 and ERK phosphorylation but reduces TGF-β-induced AKT phosphorylation. To understand the roles of AKT and CaMK-like activity of CASK in cellular responses in PCa cells, we treated cells with AKT inhibitor and specific kinase inhibitors of CASK (NR162) and CaMKII (KN-93). We found that these agents can inhibit cell invasion and migration. In addition, NR162 and KN-93 also reduce TGF-β-induced AKT phosphorylation. Moreover, the co-immunoprecipitation data indicate the association between CASK and AKT. In HEK293 cells overexpressing system, we further found that CASK can enhance AKT S473 phosphorylation. The tumorigenic effect of CASK is confirmed in the xenograft mouse system. In summary, CASK is a promoter of PCa progression and can enhance PCa cell migration and invasion via kinase-dependent AKT activation independent of TGF-β-induced Smad2/3 and ERK signaling.

N-cadherin-triggered myosin II inactivation provides tumor cells with a mechanical cell competition advantage and chemotherapy resistance

The concept that mechanical cell competition may contribute to tumor cell expansion has been widely discussed. However, whether this process could occur during natural tumor progression, as well as its underlying mechanisms and clinical implications, remains largely unknown. In this study, we observed that self-seeded tumor cell lines of human oral cancer, SCC9- and SCC25-seeded cells, exhibited a mechanical competitive advantage, outcompeted neighboring cells, and became "winner" cells. Mechanical compression-induced calcium influx activates myosin II in "loser" cells, leading to apoptotic nuclear breakdown and subsequent clearance. N-cadherin/Rac1/PAK1/myosin light-chain kinase (MLCK)-controlled myosin II inactivation endows cells with resistance to mechanical stress and superior cellular flexibility, thus providing a cell competition advantage to self-seeded cells. The activation of the N-cadherin/Rac1/PAK1/MLCK/myosin II signaling axis is associated with drug resistance. Together, these results suggest that N-cadherin/Rac1/PAK1/MLCK signaling-induced myosin II inactivation enables tumor cells to acquire resistance to mechanical stress and a competitive advantage. Our study also provides insights into drug resistance from a stress-sensitivity perspective.

Non-canonical Ca2+- Akt signaling pathway mediates the antiproteolytic effects induced by oxytocin receptor stimulation in skeletal muscle

Although it has been previously demonstrated that oxytocin (OXT) receptor stimulation can control skeletal muscle mass in vivo, the intracellular mechanisms that mediate this effect are still poorly understood. Thus, rat oxidative skeletal muscles were isolated and incubated with OXT or WAY-267,464, a non-peptide selective OXT receptor (OXTR) agonist, in the presence or absence of atosiban (ATB), an OXTR antagonist, and overall proteolysis was evaluated. The results indicated that both OXT and WAY-267,464 suppressed muscle proteolysis, and this effect was blocked by the addition of ATB. Furthermore, the WAY-induced anti-catabolic action on protein metabolism did not involve the coupling between OXTR and Gαi since it was insensitive to pertussis toxin (PTX). The decrease in overall proteolysis induced by WAY was probably due to the inhibition of the autophagic/lysosomal system, as estimated by the decrease in LC3 (an autophagic/lysosomal marker), and was accompanied by an increase in the content of Ca2+-dependent protein kinase (PKC)-phosphorylated substrates, pSer473-Akt, and pSer256-FoxO1. Most of these effects were blocked by the inhibition of inositol triphosphate receptors (IP3R), which mediate Ca2+ release from the sarcoplasmic reticulum to the cytoplasm, and triciribine, an Akt inhibitor. Taken together, these findings indicate that the stimulation of OXTR directly induces skeletal muscle protein-sparing effects through a Gαq/IP3R/Ca2+-dependent pathway and crosstalk with Akt/FoxO1 signaling, which consequently decreases the expression of genes related to atrophy, such as LC3, as well as muscle proteolysis.

Kalirin mediates Rac1 activation downstream of calcium/calmodulin-dependent protein kinase II to stimulate glucose uptake during muscle contraction

In this study, we investigated the role of calcium/calmodulin-dependent protein kinase II (CaMKII) in contraction-stimulated glucose uptake in skeletal muscle. C2C12 myotubes were contracted by electrical pulse stimulation (EPS), and treadmill running was used to exercise mice. The activities of CaMKII, the small G protein Rac1, and the Rac1 effector kinase PAK1 were elevated in muscle by running exercise or EPS, while they were lowered by the CaMKII inhibitor KN-93 and/or small interfering RNA (siRNA)-mediated knockdown. EPS induced the mRNA and protein expression of the Rac1-GEF Kalirin in a CaMKII-dependent manner. EPS-induced Rac1 activation was lowered by the Kalirin inhibitor ITX3 or siRNA-mediated Kalirin knockdown. KN-93, ITX3, and siRNA-mediated Kalirin knockdown reduced EPS-induced glucose uptake. These findings define a CaMKII-Kalirin-Rac1 signaling pathway that contributes to contraction-stimulated glucose uptake in skeletal muscle myotubes and tissue.