Hypoxia Inhibits the Peroxisome Proliferator-activated Receptor α/ Retinoid X Receptor Gene Regulatory Pathway in Cardiac Myocytes

Abstract

Hypoxia triggers a cascade of cellular energy metabolic responses including a decrease in mitochondrial oxidative flux. To characterize gene regulatory mechanisms by which mitochondrial fatty acid oxidative capacity is diminished in response to hypoxia, cardiac myocytes in culture were exposed to long-chain fatty acids (LCFA) under normoxic or hypoxic conditions. Hypoxia prevented the known LCFA-induced accumulation of mRNA encoding muscle carnitine palmitoyltransferase I (M-CPT I), an enzyme that catalyzes the rate-limiting step in mitochondrial fatty acid oxidation (FAO). Under hypoxic conditions, myocytes exhibited significant accumulation of intracellular neutral lipid consistent with reduced CPT I activity and diminished FAO capacity. Transient transfection experiments demonstrated that the hypoxia-mediated blunting of M-CPT I gene expression occurs at the transcriptional level, is localized to an LCFA/peroxisome proliferatoractivated receptor ␣ (PPAR␣)/retinoid X receptor (RXR) response element within the M-CPT I gene promoter, and is PPAR␣-dependent. DNA-protein binding studies demonstrated that exposure to hypoxia reduces PPAR␣/RXR binding activity. Immunoblotting studies demonstrated that whereas hypoxia had no effect on nuclear levels of PPAR␣ protein, nuclear and cellular RXR␣ levels were reduced. Hypoxia also diminished the 9-cis-retinoic acidmediated activation of a reporter containing an RXR homodimer response element. These results demonstrate that hypoxia deactivates PPAR␣ by reducing the availability of its obligate partner RXR.Energy metabolic responses have evolved to permit preservation of cellular function in hypoxic environments. One critical adaptive response involves the suppression of overall cellular energy consumption and improved efficiency of oxygen utilization during ATP production. Under hypoxic conditions, decreased oxygen consumption is achieved in part by increasing cellular glycolytic capacity while down-regulating mitochondrial fatty acid ␤-oxidation (FAO) 1 (1-3). The mechanisms responsible for the transcriptional activation of glycolytic enzyme gene expression in response to hypoxia have recently been characterized. Several key genes involved in cellular glucose utilization pathways are transcriptionally activated by the hypoxia-inducible factor-1␣ (HIF-1␣) (4 -6). Hypoxia also triggers additional transcriptional control mechanisms involving the Sp factor family (7) as well as regulatory effects at the post-transcriptional level (8).Despite recent progress in characterizing the regulation of cellular glucose utilization in response to hypoxia, little is known about the mechanisms involved in hypoxia-mediated suppression of mitochondrial flux. This response is particularly relevant for tissues such as the mammalian heart which relies largely on mitochondrial FAO for energy (9) yet exhibits a dynamic plasticity related to energy substrate preferences. For example, during the transition from the fetal to postnatal period, the predominant myocardial energy source switches from g...

p38 Mitogen-activated Protein Kinase Activates Peroxisome Proliferator-activated Receptor α

“…DISCUSSION PPAR␣, a lipid-activated transcription factor, plays a critical role in the control of cellular energy metabolism in a variety of physiologic and pathologic states. Evidence has emerged that PPAR␣ activity is modulated in heart and liver during diverse stress responses, including fasting (1), cardiac hypertrophy (4), and cellular hypoxia (32). This implies that upstream signaling events activated by cellular stressors are linked to changes in PPAR␣ activity, which in turn regulates mitochondrial energy metabolism.…”

“…Myocytes-To determine whether PPAR␣ is a target for SAPK-mediated phosphorylation in cardiac myocytes, 32 P labeling of adenoviral-expressed, epitope-tagged PPAR␣ in primary cultures of neonatal rat cardiac myocytes was performed under serum-free conditions. Immunoprecipitation of 32 P-labeled FLAG-PPAR␣ demonstrated that PPAR␣ exists as a phosphoprotein under basal culture conditions in cardiac myocytes (Fig.…”

PPAR-α effects on the heart and other vascular tissues

“…The rationale in this case is that glucose utilization requires less oxygen consumption for energy generation than does FAO and does not worsen acidosis in the damaged myocardium the way FAO can. Severe ischemia itself turns off PPAR-␣-regulated gene expression as a source of ATP production from fatty acids (39,65). Glucose transporter GLUT4-deficient mice develop profound myocardial dysfunction during ischemia (89), suggesting the ability to switch to glucose utilization after ischemia is critical (56).…”

Transcriptional Activation by Hypoxia and Low-Density Lipoprotein Loading in Cultured Vascular Smooth Muscle Cells

“…Incubation with oxidized LDL or beta verylow-density lipoprotein can accelerate lipid accumulation in vascular SMC 2) ; however, even high concentrations of native LDL, such as in the order of 5 mg/mL, are not sufficient to induce lipid deposition in vascular SMCs 3) . Arterial wall cells are exposed to various types of stimulants 4) , from growth factors and cytokines to achieved in part by an increase in cellular glycolytic capacity with concomitant down-regulation of mitochondrial fatty acid oxidation 6) . These metabolic changes are thought to be transcriptionally activated by hypoxiainducible factor 1-alpha (HIF-1 ) in a wide variety of cell types and by hypoxia-inducible factor 2 /EPAS1 in vascular endothelial cells 7,8) .…”