A Role for Focal Adhesion Kinase in the Stimulation of Glucose Transport in Cardiomyocytes

Differential intracellular management of fatty acids impacts on metabolic stress-stimulated glucose uptake in cardiomyocytes

Stimulation of glucose uptake in response to ischemic metabolic stress is important for cardiomyocyte function and survival. Chronic exposure of cardiomyocytes to fatty acids (FA) impairs the stimulation of glucose uptake, whereas induction of lipid droplets (LD) is associated with preserved glucose uptake. However, the mechanisms by which LD induction prevents glucose uptake impairment remain elusive. We induced LD with either tetradecanoyl phorbol acetate (TPA) or 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR). Triacylglycerol biosynthesis enzymes were inhibited in cardiomyocytes exposed to FA ± LD inducers, either upstream (glycerol-3-phosphate acyltransferases; GPAT) or downstream (diacylglycerol acyltransferases; DGAT) of the diacylglycerol step. Although both inhibitions reduced LD formation in cardiomyocytes treated with FA and LD inducers, only DGAT inhibition impaired metabolic stress-stimulated glucose uptake. DGAT inhibition in FA plus TPA-treated cardiomyocytes reduced triacylglycerol but not diacylglycerol content, thus increasing the diacylglycerol/triacylglycerol ratio. In cardiomyocytes exposed to FA alone, GPAT inhibition reduced diacylglycerol but not triacylglycerol, thus decreasing the diacylglycerol/triacylglycerol ratio, prevented PKCδ activation and improved metabolic stress-stimulated glucose uptake. Changes in AMP-activated Protein Kinase activity failed to explain variations in metabolic stress-stimulated glucose uptake. Thus, LD formation regulates metabolic stress-stimulated glucose uptake in a manner best reflected by the diacylglycerol/triacylglycerol ratio.

Chronic AICAR treatment prevents metabolic changes in cardiomyocytes exposed to free fatty acids

The stimulation of glucose transport by metabolic stress is an important determinant of myocardial susceptibility to ischemia and reperfusion injury. Stimulation of glucose transport is markedly impaired in cardiomyocytes chronically exposed to excess free fatty acids (FFA), as occurs in vivo in type 2 diabetes. To determine whether chronic low-grade activation of AMP-activated kinase (AMPK) improves substrate metabolism in cardiomyocytes exposed to FFA, isolated cultured cardiomyocytes were exposed for 7 days to FFA ± the AMPK agonist 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR). Glucose transport and glycolysis were then measured during acute metabolic stress provoked by oligomycin. Chronic treatment with AICAR improved basal and oligomycin-stimulated glucose transport in FFA-exposed but not in control cardiomyocytes. Similarly, basal and oligomycin-stimulated glycolysis was reduced in FFA-exposed cardiomyocytes but restored by chronic AICAR treatment. Conversely, fatty acid oxidation was increased in FFA-exposed cardiomyocytes and reduced by chronic AICAR treatment. Chronic AICAR treatment induced in FFA-exposed cardiomyocytes the biogenesis of numerous lipid droplets. Curiously, whereas acute treatment of cardiomyocytes with AICAR increased phosphorylation of the AMPKα subunit on T¹⁷², a classical marker of AMPK activation, chronic AICAR treatment almost completely obliterated T¹⁷² phosphorylation. However, phosphorylation of the AMPK target protein raptor on S⁷⁹² was reduced in FFA-exposed cardiomyocytes but restored by AICAR treatment. In conclusion, chronic AICAR treatment induces a metabolic shift in FFA-exposed cardiomyocytes, characterized by improved glucose transport and glycolysis and redirection of fatty acids towards neutral storage. Such metabolic changes in vivo could protect the hearts of patients with type 2 diabetes against ischemia-reperfusion injury.

Sugar or fat: The metabolic choice of the trained heart

Mammals respond to muscular exercise by increasing cardiac output to meet the increased demand for oxygen in the working muscles and it is well-established that regular bouts of exercise results in myocardial remodeling. Depending on exercise type, intensity and duration, these cardiac adaptations lead to changes in the energetic substrates required to sustain cardiac contractility. In contrast to the failing heart, fatty acids are the preferred substrate in the trained heart, though glucose metabolism is also enhanced to support oxidative phosphorylation. The participation of AMPK/eNOS and PPARα/PGC-1α pathways in the regulation of cardiac metabolism is well known but other players also contribute including sirtuins and integrins-mediated outside-in activation of FAK and other kinases. These regulatory players act by up-regulating fatty acid uptake, transport to mitochondria and oxidation, and glucose uptake via GLUT4. This exercise-induced increase in mitochondria metabolic flexibility is important to sustain the energetic demand associated with cardiomyocyte hypertrophy and hyperplasia promoted by IGF-1 and neuregulin-1-induced PI3K/Akt signaling. So, the timeless advice of Hippocrates "walking is the best medicine" seems to be justified by the promotion of mitochondrial health and, consequently, the beneficial metabolic remodeling of the heart.

FAK tyrosine phosphorylation is regulated by AMPK and controls metabolism in human skeletal muscle

AIMS/HYPOTHESIS

Insulin-mediated signals and AMP-activated protein kinase (AMPK)-mediated signals are activated in response to physiological conditions that represent energy abundance and shortage, respectively. Focal adhesion kinase (FAK) is implicated in insulin signalling and cancer progression in various non-muscle cell types and plays a regulatory role during skeletal muscle differentiation. The role of FAK in skeletal muscle in relation to insulin stimulation or AMPK activation is unknown. We examined the effects of insulin or AMPK activation on FAK phosphorylation in human skeletal muscle and the direct role of FAK on glucose and lipid metabolism. We hypothesised that insulin treatment and AMPK activation would have opposing effects on FAK phosphorylation and that gene silencing of FAK would alter metabolism.

METHODS

Human muscle was treated with insulin or the AMPK-activating compound 5-aminoimadazole-4-carboxamide ribonucleotide (AICAR) to determine FAK phosphorylation and glucose transport. Primary human skeletal muscle cells were used to study the effects of insulin or AICAR treatment on FAK signalling during serum starvation, as well as to determine the metabolic consequences of silencing the FAK gene, PTK2.

RESULTS

AMPK activation reduced tyrosine phosphorylation of FAK in skeletal muscle. AICAR reduced p-FAK^Y397 in isolated human skeletal muscle and cultured myotubes. Insulin stimulation did not alter FAK phosphorylation. Serum starvation increased AMPK activation, as demonstrated by increased p-ACC^S222, concomitant with reduced p-FAK^Y397. FAK signalling was reduced owing to serum starvation and AICAR treatment as demonstrated by reduced p-paxillin^Y118. Silencing PTK2 in primary human skeletal muscle cells increased palmitate oxidation and reduced glycogen synthesis.

CONCLUSIONS/INTERPRETATION

AMPK regulates FAK signalling in skeletal muscle. Moreover, siRNA-mediated FAK knockdown enhances lipid oxidation while impairing glycogen synthesis in skeletal muscle. Further exploration of the interaction between AMPK and FAK may lead to novel therapeutic strategies for diabetes and other chronic conditions associated with an altered metabolic homeostasis.

Pleiotropic Effects of Chronic Phorbol Ester Treatment to Improve Glucose Transport in Insulin-Resistant Cardiomyocytes

Stimulation of glucose transport is an important determinant of myocardial susceptibility to ischemia and reperfusion. Stimulation of glucose transport is markedly impaired in cardiomyocytes exposed to free fatty acids (FFA). Deactivation of the Focal Adhesion Kinase (FAK) by FFA contributes to glucose transport impairment, and could be corrected by chronic treatment with the phorbol ester TPA. However, TPA must have effects in addition to FAK reactivation to restore stimulated glucose transport. Chronic treatment with TPA improved basal and stimulated glucose transport in FFA-exposed, but not in control cardiomyocytes. Chronic FFA exposure induced the activation of PKCδ and PKCϵ. TPA markedly downregulated the expression of PKCα, PKCδ, and PKCϵ, suggesting that PKCδ or PKCϵ activation could contribute to inhibition of glucose transport by FFA. Rottlerin, a specific PKCδ inhibitor, improved glucose transport in FFA-exposed cardiomyocytes; and PKCδ was reduced in the particulate fraction of FFA + TPA-exposed cardiomyocytes. TPA also activated Protein Kinase D 1(PKD1) in FFA-exposed cardiomyocytes, as assessed by autophosphorylation of PKD1 on Y916. Pharmaceutical inhibition of PKD1 only partially prevented the improvement of glucose transport by TPA. Chronic TPA treatment also increased basal and stimulated glycolysis and favored accumulation of lipid droplets in FFA-exposed cardiomyocytes. In conclusion, basal and stimulated glucose transport in cardiomyocytes is reduced by chronic FFA exposure, but restored by concomitant treatment with a phorbol ester. The mechanism of action of phorbol esters may involve downregulation of PKCδ, activation of PKD1 and a general switch from fatty acid to glucose metabolism. J. Cell. Biochem. 9999: 4716-4727, 2017. © 2017 Wiley Periodicals, Inc.