New Directions in Therapeutic Angiogenesis and Arteriogenesis in Peripheral Arterial Disease

Abstract

The prevalence of peripheral arterial disease (PAD) in the United States exceeds 10 million people, and PAD is a significant cause of morbidity and mortality across the globe. PAD is typically caused by atherosclerotic obstructions in the large arteries to the leg(s). The most common clinical consequences of PAD include pain on walking (claudication), impaired functional capacity, pain at rest, and loss of tissue integrity in the distal limbs that may lead to lower extremity amputation. Patients with PAD also have higher than expected rates of myocardial infarction, stroke, and cardiovascular death. Despite advances in surgical and endovascular procedures, revascularization procedures may be suboptimal in relieving symptoms, and some patients with PAD cannot be treated because of comorbid conditions. In some cases, relieving obstructive disease in the large conduit arteries does not assure complete limb salvage because of severe microvascular disease. Despite several decades of investigational efforts, medical therapies to improve perfusion to the distal limb are of limited benefit. Whereas recent studies of anticoagulant (eg, rivaroxaban) and intensive lipid lowering (such as PCSK9 [proprotein convertase subtilisin/kexin type 9] inhibitors) have reduced major cardiovascular and limb events in PAD populations, chronic ischemia of the limb remains largely resistant to medical therapy. Experimental approaches to improve limb outcomes have included the administration of angiogenic cytokines (either as recombinant protein or as gene therapy) as well as cell therapy. Although early angiogenesis and cell therapy studies were promising, these studies lacked sufficient control groups and larger randomized clinical trials have yet to achieve significant benefit. This review will focus on what has been learned to advance medical revascularization for PAD and how that information might lead to novel approaches for therapeutic angiogenesis and arteriogenesis for PAD.

Keywords: cytokines; metabolism; microvessels; permeability; stem cells.

Figure 1.. Processes to restore perfusion in peripheral arterial disease (PAD).

In angiogenesis, new microvasculature is derived from the sprouting of preexisting capillaries under the influence of VEGF (vascular endothelial growth factor), FGF (fibroblast growth factor), and other angiogenic cytokines. Arteriogenesis is the positive remodeling of preexisting collaterals when blood flow is redirected through these channels. The positive remodeling of these channels is due, in part, to the mobilization of monocytes by G-CSF (granulocyte-colony stimulating factor) and vascular expression of monocyte chemotactic protein, the ligand for the monocyte CCR2 (C-C chemokine receptor type 2). Circulating angiogenic cells (CACs) are also mobilized by G-CSF to participate in adult vasculogenesis. These cells express CXCR-4 (C-X-C chemokine receptor type 4), the receptor for SDF-1 (stromal-derived factor 1) that is expressed by the ischemic tissue. Most CACs are of hematopoietic origin and contribute to restoration of perfusion by secreting angiogenic cytokines. Rare CACs are of endothelial lineage and can incorporate into the newly forming vessels. Transdifferentiation of fibroblasts to endothelial cells may also contribute to angiogenesis. In the setting of inflammatory signaling, increased DNA accessibility permits fluidity of cell fate, and environmental influences such as hypoxia activate transcription factors determining endothelial lineage.

Figure 2.. Damage-associated molecular patterns (DAMPs) generated by hypoxic injury stimulate PRRs (pattern recognition receptors) which activate signaling pathways that alter transcription or posttranslational modifications of epigenetic enzymes to promote DNA accessibility.

In addition, a glycolytic shift supplies metabolites to the nucleus for histone modifications that further facilitate an open chromatin configuration. Together with environmental influences that favor an endothelial phenotype (eg, VEGF [vascular endothelial growth factor]), fibroblasts undergo a transdifferentiation to angiogenic cells that generate vascular growth factors and incorporate into the microvasculature.