Exercise and Ischemia-Activated Pathways in Limb Muscle Angiogenesis and Vascular Regeneration

Exercise has a profound effect on cardiovascular disease, particularly through vascular remodeling and regeneration. Peripheral artery disease (PAD) is one such cardiovascular condition that benefits from regular exercise or rehabilitative physical therapy in terms of slowing the progression of disease and delaying amputations. Various rodent pre-clinical studies using models of PAD and exercise have shed light on molecular pathways of vascular regeneration. Here, I review key exercise-activated signaling pathways (nuclear receptors, kinases, and hypoxia inducible factors) in the skeletal muscle that drive paracrine regenerative angiogenesis. The rationale for highlighting the skeletal muscle is that it is the largest organ recruited during exercise. During exercise, skeletal muscle releases several myokines, including angiogenic factors and cytokines that drive tissue vascular regeneration via activation of endothelial cells, as well as by recruiting immune and endothelial progenitor cells. Some of these core exercise-activated pathways can be extrapolated to vascular regeneration in other organs. I also highlight future areas of exercise research (including metabolomics, single cell transcriptomics, and extracellular vesicle biology) to advance our understanding of how exercise induces vascular regeneration at the molecular level, and propose the idea of "exercise-mimicking" therapeutics for vascular recovery.

Nuclear receptors as potential therapeutic targets in peripheral arterial disease and related myopathy

Peripheral arterial disease (PAD) is a prevalent cardiovascular complication of limb vascular insufficiency, causing ischemic injury, mitochondrial metabolic damage and functional impairment in the skeletal muscle, and ultimately leading to immobility and mortality. While potential therapies have been mostly focussed on revascularization, none of the currently available pharmacological treatments are fully effective in PAD, often leading to amputations, particularly in chronic metabolic diseases. One major limitation of focussed angiogenesis and revascularization as a therapeutic strategy is a limited effect on metabolic restoration and muscle regeneration in the affected limb. Therefore, additional preclinical investigations are needed to discover novel treatment options for PAD preferably targeting multiple aspects of muscle recovery. In this review, we propose nuclear receptors expressed in the skeletal muscle as potential candidates for ischemic muscle repair in PAD. We review classic steroid and orphan receptors that have been reported to be involved in the regulation of paracrine muscle angiogenesis, oxidative metabolism, mitochondrial biogenesis and muscle regeneration, and discuss how these receptors could be critical for recovery from ischemic muscle damage. Furthermore, we identify existing gaps in our understanding of nuclear receptor signalling in the skeletal muscle and propose future areas of research that could be instrumental in exploring nuclear receptors as therapeutic candidates for treating PAD.

Therapeutic Effects of Fenofibrate Nano-Emulsion Eye Drops on Retinal Vascular Leakage and Neovascularization

Macular edema caused by retinal vascular leakage and ocular neovascularization are the leading causes of severe vision loss in diabetic retinopathy (DR) and age-related macular degeneration (AMD) patients. Oral administration of fenofibrate, a PPARα agonist, has shown therapeutic effects on macular edema and retinal neovascularization in diabetic patients. To improve the drug delivery to the retina and its efficacy, we have developed a nano-emulsion-based fenofibrate eye drop formulation that delivered significantly higher amounts of the drug to the retina compared to the systemic administration, as measured by liquid chromatography-mass spectrometer (LC-MS). The fenofibrate eye drop decreased leukocytes adherent to retinal vasculature and attenuated overexpression of multiple inflammatory factors in the retina of very low-density lipoprotein receptor knockout (Vldlr^-/-) mice, a model manifesting AMD phenotypes, and streptozotocin-induced diabetic rats. The fenofibrate eye drop also reduced retinal vascular leakage in these models. The laser-induced choroidal neovascularization was also alleviated by the fenofibrate eye drop. There were no detectable ocular toxicities associated with the fenofibrate eye drop treatment. These findings suggest that fenofibrate can be delivered efficiently to the retina through topical administration of the nano-emulsion eye drop, which has therapeutic potential for macular edema and neovascularization.

Estrogen-related receptor α is involved in angiogenesis and skeletal muscle revascularization in hindlimb ischemia

Skeletal muscle ischemia is a major consequence of peripheral arterial disease (PAD) or critical limb ischemia (CLI). Although therapeutic options for resolving muscle ischemia in PAD/CLI are limited, the issue is compounded by poor understanding of the mechanisms driving muscle vascularization. We found that nuclear receptor estrogen-related receptor alpha (ERRα) expression is induced in murine skeletal muscle by hindlimb ischemia (HLI), and in cultured myotubes by hypoxia, suggesting a potential role for ERRα in ischemic response. To test this, we generated skeletal muscle-specific ERRα transgenic (TG) mice. In these mice, ERRα drives myofiber type switch from glycolytic type IIB to oxidative type IIA/IIX myofibers, which are typically associated with more vascular supply in muscle. Indeed, RNA sequencing and functional enrichment analysis of TG muscle revealed that "paracrine angiogenesis" is the top-ranked transcriptional program activated by ERRα in the skeletal muscle. Immunohistochemistry and angiography showed that ERRα overexpression increases baseline capillarity, arterioles and non-leaky blood vessel formation in the skeletal muscles. Moreover, ERRα overexpression facilitates ischemic neo-angiogenesis and perfusion recovery in hindlimb musculature of mice subjected to HLI. Therefore, ERRα is a hypoxia inducible nuclear receptor that is involved in skeletal muscle angiogenesis and could be potentially targeted for treating PAD/CLI.

PPARs and Angiogenesis-Implications in Pathology

Peroxisome proliferator-activated receptors (PPARs) belong to the family of ligand-activated nuclear receptors. The PPAR family consists of three subtypes encoded by three separate genes: PPARα (NR1C1), PPARβ/δ (NR1C2), and PPARγ (NR1C3). PPARs are critical regulators of metabolism and exhibit tissue and cell type-specific expression patterns and functions. Specific PPAR ligands have been proposed as potential therapies for a variety of diseases such as metabolic syndrome, cancer, neurogenerative disorders, diabetes, cardiovascular diseases, endometriosis, and retinopathies. In this review, we focus on the knowledge of PPAR function in angiogenesis, a complex process that plays important roles in numerous pathological conditions for which therapeutic use of PPAR modulation has been suggested.

Potent and PPARα-independent anti-proliferative action of the hypolipidemic drug fenofibrate in VEGF-dependent angiosarcomas in vitro

Angiosarcomas are highly aggressive tumors of endothelial origin, which carry a poor prognosis. Fenofibrate is a hypolipidemic drug, which acts by activating the transcription factor PPARα. It has also been widely reported to have ‘anti-cancer’ activity. The current study investigated its effect in a murine VEGF-dependent angiosarcoma cell-line, MS1 VEGF. The study utilised assays to monitor cell proliferation and viability, apoptosis, cell cycle progression, mitochondrial membrane potential, changes in protein expression, and changes in miRNA expression using microarrays. Fenofibrate showed potent anti-proliferative action in MS1 VEGF angiosarcoma cells, without inducing apoptosis. It enriched cells in G2/M cell cycle phase and hyperpolarised mitochondria. Other PPARα activators failed to mimic fenofibrate action. Inhibitors of PPARα and NFκB failed to reverse the inhibitory effect of fenofibrate and their combination with fenofibrate was cytotoxic. Fenofibrate downregulated the expression of key VEGF-effector proteins, including Akt, ERK, Bcl-2 and survivin, and a chemical inhibitor screen discovered relevance of these proteins to cell proliferation. A miRNA microarray revealed that fenofibrate differentially regulated cellular miRNAs with known roles in cancer and angiogenesis. The data raise the possibility that fenofibrate could be useful in angiosarcoma therapy, especially considering its well-established clinical safety and tolerability profile.