VEGF-A: the Prototypical Angiogenic Growth Factor

VEGF-A, discovered by Senger et al. in 1983 [53], is the most critical promoter of blood vessel growth, during both normal human development and abnormal pathological processes [40]. VEGF-A stimulates the growth and movement of endothelial cells and inhibits their apoptosis [32, 54, 141, 142]. Its expression can be induced in a wide variety of cell types and tissues [143], including muscle fibers [144, 145], myofibroblasts [146], endothelial cells [119, 147], and vascular smooth muscle cells [132]. The VEGF-A gene generates several isoforms by alternative mRNA splicing, each with slightly different effects on the vascular system [32, 127, 148]. VEGF-A expression requires tight control during embryogenesis [42, 43] and in postnatal life [149, 150].

VEGF-A plays an essential role not only in angiogenesis but also in vasculogenesis (the embryonic de novo formation of blood vessels) [151] and embryonic hematopoiesis [41], for which it remains important in adult life by protecting the hematopoietic stem cells from apoptosis [171, 172]. VEGF-A also increases vascular permeability and vasodilation [53, 152], effects linked to pathological conditions in which VEGF-A is overexpressed [40], though their physiological relevance remains uncertain.

The Pathogenic Roles of VEGF-A

Because blood vessels penetrate virtually all tissues, VEGF-A is integral to many pathological processes [56]. VEGF-A enables tumor growth (Fig. 4A) and metastasis [153, 154]. VEGF-A also drives pathological angiogenesis in neovascular eye diseases, such as diabetic retinopathy [155, 156], and inflammatory diseases, such as rheumatoid arthritis or osteoarthritis [157, 158]. In atherosclerosis, rather than stimulating beneficial neovessel formation, VEGF-A seems to contribute to plaque buildup by promoting foam cell formation [159, 160]. However, unregulated, pathological VEGF-A expression produces irregular, leaky, dysfunctional vessels (Fig. 4B) [161], and anti-VEGF-A therapies have improved the therapeutic outcomes in several neovascular diseases, in some cases even dramatically [110, 161]. Conversely, VEGF-A doping (as well as pro-angiogenic therapy for heart diseases) has the potential to trigger or exacerbate the same diseases in which antiangiogenic therapy is beneficial.

Fig. 4: A potential consequence of angiogenic doping is to promote the growth of otherwise clinically dormant micro-tumors. A The original concept of tumor blood vessel dependency [162] was supported by experimental evidence, including the discovery of VEGF-A [53, 54] and the angiogenic switch [163, 164]. B Tumors also demonstrate that VEGF-A alone does not create a functional, hierarchical network. VEGF-A expression by this mouse ovarian tumor resulted in higher capillary density, but did not produce a hierarchical network optimized for oxygen delivery (see also [24, 25]); image CC-licensed from [165]. VEGF vascular endothelial growth factor

The Role of VEGF-A in Increasing Sports Performance

There is ample evidence from both animal and human studies for the role of VEGF-A in the vascular response to endurance training (reviewed by [166, 167]). In a study by Richardson et al., the amount of VEGF-A mRNA in skeletal muscle increased more than tenfold in untrained human subjects within 1 h after endurance exercise, while the response in trained subjects was less pronounced. Muscle capillarization increased on average by 18% after 8 weeks of training, but the capillary density remained largely the same as the muscle fiber area increased to a similar degree [102]. However, angiogenesis is not governed by VEGF-A alone, but rather by a dynamic balance of pro- and antiangiogenic factors. While VEGF-A is the primary direct driver, some of the capillary response persisted even when VEGFR-2 was blocked in a rat exercise model [168], indicating that other pro-angiogenic players contribute. For instance, in both human and animal studies, endurance exercise upregulated Ang-2 and downregulated Ang-1 [168–171], destabilizing the vessel and thereby permitting endothelial cells to proliferate and migrate in response to VEGF-A. For a review that discusses non-VEGF angiogenic factors, see [172].

Key physical stimuli that upregulate VEGF-A in skeletal muscle include mechanical forces, such as increased hemodynamic shear stress on the capillary endothelium from elevated blood flow, and passive stretch and compression of the vascular network during muscle contractions [173, 174]. Both VEGFR-2 and VEGFR-3 are part of mechanosensing complexes [175, 176]. The same forces also upregulate endothelial nitric oxide synthase (eNOS), a key enzyme whose product, nitric oxide (NO), is itself a potent stimulator of VEGF-A expression [166, 177]. The relative hypoxia experienced by muscle fibers during intense exercise activates HIF-1α [169, 178]. This process is further modulated by the transcriptional coactivator PGC-1α, which coordinates mitochondrial biogenesis with angiogenesis [179]. However, the association between HIF-1α and VEGF-A mRNA expression in endurance-trained skeletal muscle is surprisingly weak [180], and HIF-1α knockout (KO)-mice show no significant difference in VEGF-A mRNA response to acute exercise compared with wild-type mice. Even more surprisingly, HIF-1α KO mice exhibit greater basal capillarization than wild-type littermates [181]. Lindholm and Rundqvist [182] proposed that while acute exercise activates HIF-1, long-term endurance training blunts the HIF-1α response by inducing its negative regulators, most notably the PHD enzymes and factor inhibiting HIF-1 (FIH), and a similar blunting has also been seen in the heart muscle of mice as a response to prolonged training [183]. Hence, the attention has shifted to HIF-1-independent pathways. For example, metabolic byproducts of exercise, such as adenosine and lactate, have been identified as important chemical signals that promote VEGF-A expression and secretion [184–186].

Beyond gene expression, the local bioavailability of VEGF-A protein has been shown to be critical in several human studies. VEGF-A levels in the abluminal, muscle interstitium increased several-fold during an exercise bout [187, 188] (see also Sect. 5.5 for why blood VEGF-A levels are not relevant). This VEGF-A release is mediated, at least in part, by adenosine acting on A2B receptors on muscle cells [184]. During endurance training, while the acute exercise-induced increase in VEGF mRNA is attenuated, the basal protein level of VEGF-A within the muscle tissue is often elevated. This adaptation ensures that a readily available pool of VEGF-A is stored and can be rapidly secreted in response to subsequent exercise bouts, thereby sustaining the angiogenic drive [170, 189].

Direct evidence for an endurance performance-enhancing effect of isolated increased vascular density is lacking, as all interventions (training, HIF-1α inhibitors, etc.) have effects beyond increasing vascular density. Nevertheless, a plethora of indirect evidence points to the importance of neovascularization for improving endurance performance. Both untrained and trained human individuals respond to endurance exercise training with increases in VEGF-A levels [170, 190] and, subsequently, in vascular density [167, 191]. In line with the importance of peripheral flow for endurance performance is the observation that the size and blood flow capacity of major vessels vary with the muscle groups experiencing the highest activity (e.g., the subclavian arteries in tennis players and the femoral arteries in cyclists) [192, 193]. Furthermore, polymorphisms in the VEGFA gene appear to be among the genetic determinants of human endurance performance, acting through VEGFA gene expression and maximal oxygen consumption [194, 195].