PDK4 Inhibits Cardiac Pyruvate Oxidation in Late Pregnancy.

RATIONALE: Pregnancy profoundly alters maternal physiology. The heart hypertrophies during pregnancy, but its metabolic adaptations, are not well understood. OBJECTIVE: To determine the mechanisms underlying cardiac substrate use during pregnancy. METHODS AND RESULTS: We use here 13C glucose, 13C lactate, and 13C fatty acid tracing analyses to show that hearts in late pregnant mice increase fatty acid uptake and oxidation into the tricarboxylic acid cycle, while reducing glucose and lactate oxidation. Mitochondrial quantity, morphology, and function do not seem altered. Insulin signaling seems intact, and the abundance and localization of the major fatty acid and glucose transporters, CD36 (cluster of differentiation 36) and GLUT4 (glucose transporter type 4), are also unchanged. Rather, we find that the pregnancy hormone progesterone induces PDK4 (pyruvate dehydrogenase kinase 4) in cardiomyocytes and that elevated PDK4 levels in late pregnancy lead to inhibition of PDH (pyruvate dehydrogenase) and pyruvate flux into the tricarboxylic acid cycle. Blocking PDK4 reverses the metabolic changes seen in hearts in late pregnancy. CONCLUSIONS: Taken together, these data indicate that the hormonal environment of late pregnancy promotes metabolic remodeling in the heart at the level of PDH, rather than at the level of insulin signaling.

PGC-1α induces SPP1 to activate macrophages and orchestrate functional angiogenesis in skeletal muscle.

RATIONALE: Mechanisms of angiogenesis in skeletal muscle remain poorly understood. Efforts to induce physiological angiogenesis hold promise for the treatment of diabetic microvascular disease and peripheral artery disease but are hindered by the complexity of physiological angiogenesis and by the poor angiogenic response of aged and patients with diabetes mellitus. To date, the best therapy for diabetic vascular disease remains exercise, often a challenging option for patients with leg pain. Peroxisome proliferation activator receptor-γ coactivator-1α (PGC-1α), a powerful regulator of metabolism, mediates exercise-induced angiogenesis in skeletal muscle. OBJECTIVE: To test whether, and how, PGC-1α can induce functional angiogenesis in adult skeletal muscle. METHODS AND RESULTS: Here, we show that muscle PGC-1α robustly induces functional angiogenesis in adult, aged, and diabetic mice. The process involves the orchestration of numerous cell types and leads to patent, nonleaky, properly organized, and functional nascent vessels. These findings contrast sharply with the disorganized vasculature elicited by induction of vascular endothelial growth factor alone. Bioinformatic analyses revealed that PGC-1α induces the secretion of secreted phosphoprotein 1 and the recruitment of macrophages. Secreted phosphoprotein 1 stimulates macrophages to secrete monocyte chemoattractant protein-1, which then activates adjacent endothelial cells, pericytes, and smooth muscle cells. In contrast, induction of PGC-1α in secreted phosphoprotein 1(-/-) mice leads to immature capillarization and blunted arteriolarization. Finally, adenoviral delivery of PGC-1α into skeletal muscle of either young or old and diabetic mice improved the recovery of blood flow in the murine hindlimb ischemia model of peripheral artery disease. CONCLUSIONS: PGC-1α drives functional angiogenesis in skeletal muscle and likely recapitulates the complex physiological angiogenesis elicited by exercise.

Molecular basis for antagonism between PDGF and the TGFbeta family of signalling pathways by control of miR-24 expression.

Modulation of the vascular smooth-muscle-cell (vSMC) phenotype from a quiescent 'contractile' phenotype to a proliferative 'synthetic' phenotype has been implicated in vascular injury repair, as well as pathogenesis of vascular proliferative diseases. Both bone morphogenetic protein (BMP) and transforming growth factor-beta (TGFbeta)-signalling pathways promote a contractile phenotype, while the platelet-derived growth factor-BB (PDGF-BB)-signalling pathway promotes a switch to the synthetic phenotype. Here we show that PDGF-BB induces microRNA-24 (miR-24), which in turn leads to downregulation of Tribbles-like protein-3 (Trb3). Repression of Trb3 coincides with reduced expression of Smad proteins and decrease in BMP and TGFbeta signalling, promoting a synthetic phenotype in vSMCs. Inhibition of miR-24 by antisense oligonuclotides abrogates the downregulation of Trb3 as well as pro-synthetic activity of the PDGF-signalling pathway. Thus, this study provides a molecular basis for the antagonism between the PDGF and TGFbeta pathways, and its effect on the control of the vSMC phenotype.

down-regulation of Kruppel-like factor-4 (KLF4) by microRNA-143/145 is critical for modulation of vascular smooth muscle cell phenotype by transforming growth factor-beta and bone morphogenetic protein 4.

In the postnatal vasculature, fully differentiated and quiescent vascular smooth muscle cells (VSMCs) in a "contractile" phenotype are required for the normal regulation of vascular tone. The transforming growth factor-ß (TGF-ß) superfamily of growth factors (TGF-ßs and bone morphogenetic proteins (BMPs)) are potent inducers of contractile phenotype and mediate (i) induction of contractile genes, and (ii) inhibition of VSMC growth and migration. Transcription of contractile genes is positively regulated by a regulatory DNA element called a CArG box. The CArG box is activated by the binding of serum response factor and its coactivators, myocardin (Myocd) or Myocd-related transcription factors (MRTFs). Krüppel-like factor-4 (KLF4) is known to inhibit activation of the CArG box. However, the potential role of KLF4 in the contractile activities of TGF-ß or BMP has not been explored. Here, we demonstrate that TGF-ß and BMP4 rapidly down-regulate KLF4 through induction of microRNA-143 (miR-143) and miR-145, which leads to a reduction of KLF4 transcripts and decreased KLF4 protein expression. Inhibition of miR-145 prevents down-regulation of KLF4 and activation of contractile genes by TGF-ß or BMP4, suggesting that modulation of KLF4 is a prerequisite for induction of contractile genes by TGF-ß and BMP4. Interestingly, both TGF-ß and BMP4 activate transcription of the miR-143/145 gene cluster through the CArG box, however, TGF-ß mediates this effect through induction of Myocd expression, whereas BMP4 utilizes nuclear translocation of MRTF-A. Thus, this study sheds light on both the similarities and the differences of TGF-ß and BMP4 signaling in the regulation of KLF4 and contractile genes.

A novel regulatory mechanism of the bone morphogenetic protein (BMP) signaling pathway involving the carboxyl-terminal tail domain of BMP type II receptor.

Bone morphogenetic protein (BMP) signaling regulates many different biological processes, including cell growth, differentiation, and embryogenesis. BMPs bind to heterogeneous complexes of transmembrane serine/threonine (Ser/Thr) kinase receptors known as the BMP type I and II receptors (BMPRI and BMPRII). BMPRII phosphorylates and activates the BMPRI kinase, which in turn activates the Smad proteins. The cytoplasmic region of BMPRII contains a "tail" domain (BMPRII-TD) with no enzymatic activity or known regulatory function. The discovery of mutations associated with idiopathic pulmonary artery hypertension mapping to BMPRII-TD underscores its importance. Here, we report that Tribbles-like protein 3 (Trb3) is a novel BMPRII-TD-interacting protein. Upon BMP stimulation, Trb3 dissociates from BMPRII-TD and triggers degradation of Smad ubiquitin regulatory factor 1 (Smurf1), which results in the stabilization of BMP receptor-regulated Smads and potentiation of the Smad pathway. Downregulation of Trb3 inhibits BMP-mediated cellular responses, including osteoblast differentiation of C2C12 cells and maintenance of the smooth muscle phenotype of pulmonary artery smooth muscle cells. Thus, Trb3 is a critical component of a novel mechanism for regulation of the BMP pathway by BMPRII.

miR-29b contributes to multiple types of muscle atrophy.

A number of microRNAs (miRNAs, miRs) have been shown to play a role in skeletal muscle atrophy, but their role is not completely understood. Here we show that miR-29b promotes skeletal muscle atrophy in response to different atrophic stimuli in cells and in mouse models. miR-29b promotes atrophy of myotubes differentiated from C2C12 or primary myoblasts, and conversely, its inhibition attenuates atrophy induced by dexamethasone (Dex), TNF-α and H2O2 treatment. Targeting of IGF-1 and PI3K(p85α) by miR-29b is required for induction of muscle atrophy. In vivo, miR-29b overexpression is sufficient to promote muscle atrophy while inhibition of miR-29b attenuates atrophy induced by denervation and immobilization. These data suggest that miR-29b contributes to multiple types of muscle atrophy via targeting of IGF-1 and PI3K(p85α), and that suppression of miR-29b may represent a therapeutic approach for muscle atrophy induced by different stimuli.

DDiT4L promotes autophagy and inhibits pathological cardiac hypertrophy in response to stress.

Physiological cardiac hypertrophy, in response to stimuli such as exercise, is considered adaptive and beneficial. In contrast, pathological cardiac hypertrophy that arises in response to pathological stimuli such as unrestrained high blood pressure and oxidative or metabolic stress is maladaptive and may precede heart failure. We found that the transcript encoding DNA damage-inducible transcript 4-like (DDiT4L) was expressed in murine models of pathological cardiac hypertrophy but not in those of physiological cardiac hypertrophy. In cardiomyocytes, DDiT4L localized to early endosomes and promoted stress-induced autophagy through a process involving mechanistic target of rapamycin complex 1 (mTORC1). Exposing cardiomyocytes to various types of pathological stress increased the abundance of DDiT4L, which inhibited mTORC1 but activated mTORC2 signaling. Mice with conditional cardiac-specific overexpression of DDiT4L had mild systolic dysfunction, increased baseline autophagy, reduced mTORC1 activity, and increased mTORC2 activity, all of which were reversed by suppression of transgene expression. Genetic suppression of autophagy also reversed cardiac dysfunction in these mice. Our data showed that DDiT4L may be an important transducer of pathological stress to autophagy through mTOR signaling in the heart and that DDiT4L could be therapeutically targeted in cardiovascular diseases in which autophagy and mTOR signaling play a major role.

Heme oxygenase and carbon monoxide protect from muscle dystrophy.

BACKGROUND: Duchenne muscle dystrophy (DMD) is one of the most common lethal genetic diseases of children worldwide and is 100% fatal. Steroids, the only therapy currently available, are marred by poor efficacy and a high side-effect profile. New therapeutic approaches are urgently needed. METHODS: Here, we leverage PGC-1α, a powerful transcriptional coactivator known to protect against dystrophy in the mdx murine model of DMD, to search for novel mechanisms of protection against dystrophy. RESULTS: We identify heme oxygenase-1 (HO-1) as a potential novel target for the treatment of DMD. Expression of HO-1 is blunted in the muscles from the mdx murine model of DMD, and further reduction of HO-1 by genetic haploinsufficiency worsens muscle damage in mdx mice. Conversely, induction of HO-1 pharmacologically protects against muscle damage. Mechanistically, HO-1 degrades heme into biliverdin, releasing in the process ferrous iron and carbon monoxide (CO). We show that exposure to a safe low dose of CO protects against muscle damage in mdx mice, as does pharmacological treatment with CO-releasing molecules. CONCLUSIONS: These data identify HO-1 and CO as novel therapeutic agents for the treatment of DMD. Safety profiles and clinical testing of inhaled CO already exist, underscoring the translational potential of these observations.

A branched-chain amino acid metabolite drives vascular fatty acid transport and causes insulin resistance.

Epidemiological and experimental data implicate branched-chain amino acids (BCAAs) in the development of insulin resistance, but the mechanisms that underlie this link remain unclear. Insulin resistance in skeletal muscle stems from the excess accumulation of lipid species, a process that requires blood-borne lipids to initially traverse the blood vessel wall. How this trans-endothelial transport occurs and how it is regulated are not well understood. Here we leveraged PPARGC1a (also known as PGC-1α; encoded by Ppargc1a), a transcriptional coactivator that regulates broad programs of fatty acid consumption, to identify 3-hydroxyisobutyrate (3-HIB), a catabolic intermediate of the BCAA valine, as a new paracrine regulator of trans-endothelial fatty acid transport. We found that 3-HIB is secreted from muscle cells, activates endothelial fatty acid transport, stimulates muscle fatty acid uptake in vivo and promotes lipid accumulation in muscle, leading to insulin resistance in mice. Conversely, inhibiting the synthesis of 3-HIB in muscle cells blocks the ability of PGC-1α to promote endothelial fatty acid uptake. 3-HIB levels are elevated in muscle from db/db mice with diabetes and from human subjects with diabetes, as compared to those without diabetes. These data unveil a mechanism in which the metabolite 3-HIB, by regulating the trans-endothelial flux of fatty acids, links the regulation of fatty acid flux to BCAA catabolism, providing a mechanistic explanation for how increased BCAA catabolic flux can cause diabetes.

The many roles of PGC-1α in muscle--recent developments.

Skeletal muscle is the largest organ in the body and contributes to innumerable aspects of organismal biology. Muscle dysfunction engenders numerous diseases, including diabetes, cachexia, and sarcopenia. At the same time, skeletal muscle is also the main engine of exercise, one of the most efficacious interventions for prevention and treatment of a wide variety of diseases. The transcriptional coactivator PGC-1α has emerged as a key driver of metabolic programming in skeletal muscle, both in health and in disease. We review here the many aspects of PGC-1α function in skeletal muscle, with a focus on recent developments.

Post-natal induction of PGC-1α protects against severe muscle dystrophy independently of utrophin.

BACKGROUND: Duchenne muscle dystrophy (DMD) afflicts 1 million boys in the US and has few effective treatments. Constitutive transgenic expression of the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator (PGC)-1α improves skeletal muscle function in the murine "mdx" model of DMD, but how this occurs, or whether it can occur post-natally, is not known. The leading mechanistic hypotheses for the benefits conferred by PGC-1α include the induction of utrophin, a dystrophin homolog, and/or induction and stabilization of the neuromuscular junction. METHODS: The effects of transgenic overexpression of PGC-1ß, a homolog of PGC-1α in mdx mice was examined using different assays of skeletal muscle structure and function. To formally test the hypothesis that PGC-1α confers benefit in mdx mice by induction of utrophin and stabilization of neuromuscular junction, PGC-1α transgenic animals were crossed with the dystrophin utrophin double knock out (mdx/utrn-/-) mice, a more severe dystrophic model. Finally, we also examined the effect of post-natal induction of skeletal muscle-specific PGC-1α overexpression on muscle structure and function in mdx mice. RESULTS: We show here that PGC-1ß does not induce utrophin or other neuromuscular genes when transgenically expressed in mouse skeletal muscle. Surprisingly, however, PGC-1ß transgenesis protects as efficaciously as PGC-1α against muscle degeneration in dystrophin-deficient (mdx) mice, suggesting that alternate mechanisms of protection exist. When PGC-1α is overexpressed in mdx/utrn-/- mice, we find that PGC-1α dramatically ameliorates muscle damage even in the absence of utrophin. Finally, we also used inducible skeletal muscle-specific PGC-1α overexpression to show that PGC-1α can protect against dystrophy even if activated post-natally, a more plausible therapeutic option. CONCLUSIONS: These data demonstrate that PGC-1α can improve muscle dystrophy post-natally, highlighting its therapeutic potential. The data also show that PGC-1α is equally protective in the more severely affected mdx/utrn-/- mice, which more closely recapitulates the aggressive progression of muscle damage seen in DMD patients. The data also identify PGC-1ß as a novel potential target, equally efficacious in protecting against muscle dystrophy. Finally, the data also show that PGC-1α and PGC-1ß protect against dystrophy independently of utrophin or of induction of the neuromuscular junction, indicating the existence of other mechanisms.

The amiloride derivative phenamil attenuates pulmonary vascular remodeling by activating NFAT and the bone morphogenetic protein signaling pathway.

Pulmonary artery hypertension (PAH) is characterized by elevated pulmonary artery resistance and increased medial thickness due to deregulation of vascular remodeling. Inactivating mutations of the BMPRII gene, which encodes a receptor for bone morphogenetic proteins (BMPs), are identified in ∼60% of familial PAH (FPAH) and ∼30% of idiopathic PAH (IPAH) patients. It has been hypothesized that constitutive reduction in BMP signal by BMPRII mutations may cause abnormal vascular remodeling by promoting dedifferentiation of vascular smooth muscle cells (vSMCs). Here, we demonstrate that infusion of the amiloride analog phenamil during chronic-hypoxia treatment in rat attenuates development of PAH and vascular remodeling. Phenamil induces Tribbles homolog 3 (Trb3), a positive modulator of the BMP pathway that acts by stabilizing the Smad family signal transducers. Through induction of Trb3, phenamil promotes the differentiated, contractile vSMC phenotype characterized by elevated expression of contractile genes and reduced cell growth and migration. Phenamil activates the Trb3 gene transcription via activation of the calcium-calcineurin-nuclear factor of activated T cell (NFAT) pathway. These results indicate that constitutive elevation of Trb3 by phenamil is a potential therapy for IPAH and FPAH.