Yao-Hua Song

Expression, Regulation, and Function of IGF-1, IGF-1R, and IGF-1 Binding Proteins in Blood Vessels

The vascular insulin-like growth factor (IGF)-1 system includes the IGFs, the IGF-1 receptor (IGF-1R), and multiple binding proteins. This growth factor system exerts multiple physiologic effects on the vasculature through both endocrine and autocrine/paracrine mechanisms. The effects of IGF-1 are mediated principally through the IGF-1R but are modulated by complex interactions with multiple IGF binding proteins that themselves are regulated by phosphorylation, proteolysis, polymerization, and cell or matrix association. During the last decade, a significant body of evidence has accumulated, indicating that expression of the components of the IGF system are regulated by multiple factors, including growth factors, cytokines, lipoproteins, reactive oxygen species, and hemodynamic forces. In addition, cross-talk between the IGF system and other growth factors and integrin receptors has been demonstrated. There is accumulating evidence of a role for IGF-1 in multiple vascular pathologies, including atherosclerosis, hypertension, restenosis, angiogenesis, and diabetic vascular disease. This review will discuss the regulation of expression of IGF-1, IGF-1R, and IGF binding proteins in the vasculature and summarize evidence implicating involvement of this system in vascular diseases.

Muscle-specific expression of IGF-1 blocks angiotensin II–induced skeletal muscle wasting

Advanced congestive heart failure is associated with activation of the renin-angiotensin system and skeletal muscle wasting. We previously showed that angiotensin II infusion in rats produces cachexia secondarily to increased muscle proteolysis and also decreases levels of circulating and skeletal muscle IGF-1. Here we show that angiotensin II markedly downregulates phospho-Akt and activates caspase-3 in skeletal muscle, leading to actin cleavage, an important component of muscle proteolysis, and to increased apoptosis. These changes are blocked by muscle-specific expression of IGF-1, likely via the Akt/mTOR/p70S6K signaling pathway. We also demonstrate that mRNA levels of the ubiquitin ligases atrogin-1 and muscle ring finger-1 are upregulated in angiotensin II-infused WT, but not in IGF-1-transgenic, mice. These findings strongly suggest that angiotensin II downregulation of IGF-1 in skeletal muscle is causally related to angiotensin II-induced wasting. Because the renin-angiotensin system is activated in many catabolic conditions, our findings have broad implications for understanding mechanisms of skeletal muscle wasting and provide a rationale for new therapeutic approaches.

Targeted ablation of IKK2 improves skeletal muscle strength, maintains mass, and promotes regeneration

NF-kappaB is a major pleiotropic transcription factor modulating immune, inflammatory, cell survival, and proliferative responses, yet the relevance of NF-kappaB signaling in muscle physiology and disease is less well documented. Here we show that muscle-restricted NF-kappaB inhibition in mice, through targeted deletion of the activating kinase inhibitor of NF-kappaB kinase 2 (IKK2), shifted muscle fiber distribution and improved muscle force. In response to denervation, IKK2 depletion protected against atrophy, maintaining fiber type, size, and strength, increasing protein synthesis, and decreasing protein degradation. IKK2-depleted mice with a muscle-specific transgene expressing a local Igf-1 isoform (mIgf-1) showed enhanced protection against muscle atrophy. In response to muscle damage, IKK2 depletion facilitated skeletal muscle regeneration through enhanced satellite cell activation and reduced fibrosis. Our results establish IKK2/NF-kappaB signaling as an important modulator of muscle homeostasis and suggest a combined role for IKK inhibitors and growth factors in the therapy of muscle diseases.

The role of tyrosinase in autoimmune vitiligo

Vitiligo is a common depigmenting skin disease, associated with certain autoimmune endocrinopathies, and autoantibodies to several antigens can be found in melanoma cells. We set out to identify the antigens. We examined 26 patients with vitiligo and associated endocrine disease. Of these, 18 patients (77%) and 8 immediate family members had autoantibodies specific for a 69 kDa protein in HTB-70 human melanoma cells that was not seen in control cells. The autoantibody-positive patient sera reacted with recombinant human tyrosinase expressed in Escherichia coli seen by western blots, as did antibodies raised in rabbits against hamster tyrosinase, but not to recombinant tyrosinase-related protein. Not one of 31 normal controls or 8 patients with alopecia or systemic lupus erythematosus had tyrosinase autoantibodies but a small proportion (12%) of 42 patients with autoimmune endocrine disease without a history of vitiligo had them. The results show that tyrosinase, an enzyme important in melanin formation, is a principal autoantigen of autoimmune vitiligo.

IGF-1 Reduces Inflammatory Responses, Suppresses Oxidative Stress, and Decreases Atherosclerosis Progression in ApoE-Deficient Mice

Objective—Whereas growth factors, via their ability to stimulate vascular smooth muscle cell (VSMC) proliferation and migration, have been thought to play a permissive role in atherosclerosis initiation and progression, the role of insulin-like growth factor-1 (IGF-1) is unknown. Here we report for the first time that IGF-1 infusion decreased atherosclerotic plaque progression in ApoE-deficient mice on a Western diet. Methods and Results—ApoE-null mice (8 weeks) were infused with vehicle or recombinant human IGF-1 and fed a high-fat diet for 12 weeks. Analysis of aortic sinuses revealed that IGF-1 infusion decreased atherosclerotic plaque progression and macrophage infiltration into lesions. Furthermore, IGF-1 decreased vascular expression of the proinflammatory cytokines interleukin-6 and tumor necrosis factor-α, reduced aortic superoxide formation and urinary 8-isoprostane levels, and increased aortic pAkt and eNOS expression and circulating endothelial progenitor cells, consistent with an antiinflammatory, antioxidant, and prorepair effect on the vasculature. Conclusions—Our data indicate that an increase in circulating IGF-1 reduces vascular inflammatory responses, systemic and vascular oxidant stress and decreases atherosclerotic plaque progression. These findings have major implications for the treatment of atherosclerosis.

Enhancing Repair of the Mammalian Heart

The injured mammalian heart is particularly susceptible to tissue deterioration, scarring, and loss of contractile function in response to trauma or sustained disease. We tested the ability of a locally acting insulin-like growth factor-1 isoform (mIGF-1) to recover heart functionality, expressing the transgene in the mouse myocardium to exclude endocrine effects on other tissues. supplemental mIGF-1 expression did not perturb normal cardiac growth and physiology. Restoration of cardiac function in post-infarct mIGF-1 transgenic mice was facilitated by modulation of the inflammatory response and increased antiapoptotic signaling. mIGF-1 ventricular tissue exhibited increased proliferative activity several weeks after injury. The canonical signaling pathway involving Akt, mTOR, and p70S6 kinase was not induced in mIGF-1 hearts, which instead activated alternate PDK1 and SGK1 signaling intermediates. The robust response achieved with the mIGF-1 isoform provides a mechanistic basis for clinically feasible therapeutic strategies for improving the outcome of heart disease.

Insulin-Like Growth Factor-1 Receptor Activation Inhibits Oxidized LDL-Induced Cytochrome C Release and Apoptosis via the Phosphatidylinositol 3 Kinase/Akt Signaling Pathway

Objective—We have shown previously that oxidized LDL decreases insulin-like growth factor-1 (IGF-1) and IGF-1 receptor expression in vascular smooth muscle cells and that IGF-1 and IGF-1 receptor expression are reduced in the deep intima of early atherosclerotic lesions. Because oxidized LDL is potentially important for the depletion of vascular smooth muscle cells contributing to plaque destabilization, we studied the role of IGF-1 in oxidized LDL-induced apoptosis. Methods and Results—We provide evidence that oxidized LDL-induced apoptosis is caused by decreased mitochondrial membrane potential and increased cytochrome C release in human aortic vascular smooth muscle cells. Overexpression of the IGF-1 receptor by using an adenovirus completely abrogated these effects. The antiapoptotic function of the IGF-1 receptor was associated with increased Akt kinase activity and increased expression of phosphorylated Bad. Moreover, a dominant-negative p85 phosphatidylinositol 3-kinase adenovirus blocked the capacity of the IGF-1 receptor to prevent oxidized LDL-induced apoptosis. Conclusions—Our data demonstrate that IGF-1 receptor activation inhibits oxidized LDL-induced cytochrome C release and apoptosis through the phosphatidylinositol 3-kinase/Akt signaling pathway and suggest that genetic or pharmacological activation of the IGF-1 receptor may be a useful strategy to stabilize atherosclerotic plaques.

Insulin-like growth factor I-mediated skeletal muscle hypertrophy is characterized by increased mTOR-p70S6K signaling without increased Akt phosphorylation

Background Insulin-like growth factor I (IGF-I) is an anabolic hormone that is known to induce skeletal muscle hypertrophy. However, the signaling pathways mediating IGF-Is hypertrophic effect in vivo are unknown. Method The phosphorylation of 46 proteins was investigated by Kinetworks proteomic analysis in the gastrocnemius muscle of transgenic mice overexpressing IGF-I myosin light chain/muscle specific IGF-I (MLC/mIgf-I) and wild-type littermates. Results In the hypertrophic muscle of MLC/mIgf-I mice, we observed increased phosphorylation of phosphoinositide-dependent protein kinase 1 (PDK1; 53% increase), the mammalian target of rapamycin (mTOR; 112% increase), and p70 S6 kinase (p70S6K) (254% increase) but no significant change in Akt phosphorylation (4% decrease). Furthermore, we found reduced phosphorylation of MAP kinase kinase 1 and 2 (MEK1/2) (60% decrease) and of mitogen-activated protein kinase kinases 3 and 6 (MKK3/6) (50% decrease) in muscle from transgenic mice, suggesting that the hypertrophic and mitogenic effects of IGF-I are mediated via distinct signaling pathways in skeletal muscle and that inhibition of the mitogen-activated protein (MAP) kinase pathway may be required for the IGF-I-induced hypertrophic effect. Single-fiber analysis revealed a trend toward a higher percentage of the fast twitch fibers (IIb and IIx) in the transgenic mice. Conclusion Persistent overexpression of IGF-I in mice skeletal muscle results in hypertrophy, which is likely mediated via the mTOR/p70S6K pathway, potentially via an Akt-independent signaling pathway.

The nature of autoantigens targeted in autoimmune endocrine diseases

Autoimmune endocrine diseases result from autoimmune processes involving autoreactive T cells and/or autoantibodies. Considerable progress has been made in elucidating the self- (auto-) antigens involved in these processes. These include tissue-specific membrane receptors, enzymes and secreted hormones. As discussed here by Yao-Hua Song and colleagues, diagnostic and therapeutic applications should now result from these discoveries.

The therapeutic potential of IGF-I in skeletal muscle repair

Skeletal muscle loss due to aging, motor-neuron degeneration, cancer, heart failure, and ischemia is a serious condition for which currently there is no effective treatment. Insulin-like growth factor 1 (IGF-I) plays an important role in muscle maintenance and repair. Preclinical studies have shown that IGF-I is involved in increasing muscle mass and strength, reducing degeneration, inhibiting the prolonged and excessive inflammatory process due to toxin injury, and increasing the proliferation potential of satellite cells. However, clinical trials have not been successful due to ineffective delivery methods. Choosing the appropriate isoforms or peptides and developing targeted delivery techniques can resolve this issue. Here we discuss the latest development in the field with special emphasis on novel therapeutic approaches.