Insulin-like growth factor-1 (IGF-1) is a key growth factor that regulates both anabolic and catabolic pathways in skeletal muscle. IGF-1 increases skeletal muscle protein synthesis via PI3K/Akt/mTOR and PI3K/Akt/GSK3β pathways. PI3K/Akt can also inhibit FoxOs and suppress transcription of E3 ubiquitin ligases that regulate ubiquitin proteasome system (UPS)-mediated protein degradation. Autophagy is likely inhibited by IGF-1 via mTOR and FoxO signaling, although the contribution of autophagy regulation in IGF-1-mediated inhibition of skeletal muscle atrophy remains to be determined. Evidence has suggested that IGF-1/Akt can inhibit muscle atrophy-inducing cytokine and myostatin signaling via inhibition of the NF-κΒ and Smad pathways, respectively. Several miRNAs have been found to regulate IGF-1 signaling in skeletal muscle, and these miRs are likely regulated in different pathological conditions and contribute to the development of muscle atrophy. IGF-1 also potentiates skeletal muscle regeneration via activation of skeletal muscle stem (satellite) cells, which may contribute to muscle hypertrophy and/or inhibit atrophy. Importantly, IGF-1 levels and IGF-1R downstream signaling are suppressed in many chronic disease conditions and likely result in muscle atrophy via the combined effects of altered protein synthesis, UPS activity, autophagy, and muscle regeneration.

Background-Cytotoxic oxidized LDL (oxLDL) has been shown to promote apoptosis in cultured vascular smooth muscle cells (VSMCs). We investigated the localization of oxLDL and its association with apoptosis and the expression of apoptosis-related proteins in early and advanced atherosclerotic lesions. Methods and Results-Atherosclerotic plaques (nϭ23) from patients undergoing aortic, carotid, or femoral arterial surgery were studied. In early lesions, oxLDL was located predominantly in the superficial intima and in the media just beneath the internal elastic lamina. Medial VSMCs staining positive for oxLDL showed expression of BAX, a proapoptotic protein of the BCL-2 family. Apoptosis, as detected by DNA in situ terminal deoxynucleotidyl transferase end-labeling (TUNEL), was not present in these early lesions. In advanced plaques, areas of the intima positive for oxLDL showed lower ␣-smooth muscle actin immunoreactivity (PϽ0.01) and higher BAX immunoreactivity (PϽ0.05). Furthermore, these areas showed an increased number of apoptotic VSMCs (PϽ0.01). Western blot analysis revealed that oxLDL increases BAX expression in cultured human coronary VSMCs. Conclusions-We conclude that in early atherosclerotic lesions, oxLDL-positive VSMCs express BAX, which increases the susceptibility of these cells to undergo apoptosis. This could be important in our understanding of the transition of early lesions into advanced atherosclerotic plaques, which are characterized by regions of cell death. In advanced plaques, oxLDL-positive areas of the intima show higher BAX immunoreactivity and TUNEL-positive VSMCs, and this may contribute to plaque instability and rupture.

Sarcopenia and cachexia are muscle wasting syndromes associated with aging and with many chronic diseases such as congestive heart failure (CHF), diabetes, cancer, chronic obstructive pulmonary disease and chronic kidney disease (CKD). While mechanisms are complex these conditions are often accompanied by elevated angiotensin II (Ang II). Patients with advanced CHF or CKD often have increased Ang II levels and cachexia, and angiotensin-converting enzyme (ACE) inhibitor treatment improves weight loss. We found that Ang II infusion in rodents leads to skeletal muscle wasting. Ang II increases cytokines and circulating hormones such as tumor necrosis factor-α, interleukin-6, serum amyloid-A and glucocorticoids, which regulate muscle protein synthesis and degradation. Ang II-induced muscle wasting is caused by alterations in insulin-like growth factor-1 signaling, enhanced muscle protein breakdown via the ubiquitin-proteasome system, and decreased appetite resulting from downregulation of hypothalamic orexigenic neuropeptides such as Npy and orexin. Ang II also inhibits 5′ AMP-activated protein kinase (AMPK) activity and disrupts normal energy balance via activation of AMPK phosphatase PP2Cα. Furthermore, Ang II inhibits skeletal muscle stem (satellite) cell proliferation, leading to lowered muscle regenerative capacity. Distinct satellite cell angiotensin receptor subtypes have different effects on different stages of differentiation and are critical for regulation of muscle regeneration. These data suggest that the renin-angiotensin system (RAS) plays a critical role in mechanisms underlying cachexia in chronic disease states, and is a promising target for the treatment of muscle atrophy in patients with diseases such as CHF and CKD.

US Department of Veterans Affairs Cooperative Studies Program.