Epidemiological and experimental data implicate branched chain amino acids (BCAAs) in the development of insulin resistance, but the mechanisms underlying this link remain unclear.1–3 Insulin resistance in skeletal muscle stems from excess accumulation of lipid species4, a process that requires blood-borne lipids to first traverse the blood vessel wall. Little is known, however, of how this trans-endothelial transport occurs or is regulated. Here, we leverage PGC-1α, a transcriptional coactivator that regulates broad programs of FA consumption, to identify 3-hydroxy-isobutyrate (3-HIB), a catabolic intermediate of the BCAA valine, as a novel paracrine regulator of trans-endothelial fatty acids (FA) transport. 3-HIB is secreted from muscle cells, activates endothelial FA transport, stimulates muscle FA uptake in vivo, and promotes muscle lipid accumulation and insulin resistance in animals. Conversely, inhibiting the synthesis of 3-HIB in muscle cells blocks the promotion of endothelial FA uptake. 3-HIB levels are elevated in muscle from db/db mice and from subjects with diabetes. These data thus unveil a novel mechanism that regulates trans-endothelial flux of FAs, revealing 3-HIB as a new bioactive signaling metabolite that links the regulation of FA flux to BCAA catabolism and provides a mechanistic explanation for how increased BCAA catabolic flux can cause diabetes.

Vascular smooth muscle cells (VSMCs) 3 are highly plastic cells that undergo phenotype modulation in response to physiological and pathological cues (1). In response to vascular injury or growth factors, such as platelet-derived growth factor (PDGF) (2), VSMCs dedifferentiate and adopt a highly migratory, proliferative phenotype known as a "synthetic" phenotype that is required for vascular injury repair or during angiogenesis (1). However, prolonged or deregulated dedifferentiation can cause occlusion of the vasculature and contributes to development of vascular proliferative disorders, such as atherosclerosis, restenosis following angioplasty, as well as both systemic and pulmonary hypertension (1). Unlike PDGF, the TGF-␤ family of growth factors, including TGF-␤ and BMP4, promote a less migratory and proliferative phenotype known as the "contractile" phenotype (3). Contractile VSMC phenotype is characterized by alterations in the gene expression profile of VSMC. In particular, high expression of VSMC-specific genes, such as smooth muscle ␣-actin (SMA), calponin1 (CNN), and SM22␣ (SM22) are associated with the contractile VSMC phenotype. Transcription of contractile genes is regulated by SRF through a DNA sequence motif known as the CArG box (CC(A/T) 6 GG), which is present in the promoters of VSMC-specific genes (1). A coactivator of SRF, Myocd, interacts with SRF and activates VSMC expression of contractile genes (4 -6). Similarly, the Myocd-related transcription factor (MRTF) family of proteins, MRTF-A and MRTF-B, are also involved in the transcriptional regulation of contractile gene markers as coactivators of SRF (7,8). Myocd is constitutively localized to the nucleus and its activity is regulated primarily at the level of expression. Conversely, MRTFs are sequestered in the cytoplasm through interaction with monomeric G-actin (9, 10). In response to BMP4 or other stimuli, Rho signaling promotes actin polymerization and MRTF translocation into the nucleus where MRTFs associate with SRF (3), resulting in the activation of contractile gene transcription. Unlike BMP4, TGF-␤ does not activate Rho signal-* This work was supported, in whole or in part, by National Institutes of Health 3 The abbreviations used are: VSMC, vascular smooth muscle cell; TGF-{␤}, transforming growth factor-{␤}; PDGF, platelet-derived growth factor; PAI-1, plasminogen activator inhibitor-1; PDCD4, programed cell death 4; BMP, bone morphogenetic protein; MRTF, Myocd-related transcription factor; PASMC, pulmonary artery smooth muscle cell; ESC, embryonic stem cell; SRF, serum response factor; Myocd, myocardin; miRNA, microRNA; pri-miRNA; primary miRNA; SBE, Smad-binding element; rAoSMC, rat aortic SMC; qRT, quantitative reverse transcriptase.

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.