In the present study, we examinated the in vivo effects of a VEGF165 mimetic, named QK, modeled on the region of the VEGF protein responsible for binding to and activating the VEGFRs that are known to trigger angiogenesis. We previously showed that QK can bind to the VEGFRs, initiate VEGF-induced signaling cascades and stimulate angiogenesis in vitro . This is the first report to show that this peptide is able to recapitulate the in vivo responses of VEGF.
Angiogenesis is known to be a process of new blood vessel formation from a pre-existing endothelial structure. It is tuned by proangiogenic and antiangiogenic factors, and the shift from this equilibrium may lead to pathological angiogenesis [18, 19]. Indeed, deregulation of angiogenesis is involved in several conditions including cancer, ischemic, and inflammatory diseases (atherosclerosis, rheumatoid arthritis, or age-related macular degeneration). Therefore, the research for drugs able to regulate angiogenesis constitutes a pivotal research field. In particular, occlusive vascular disease remains an important cause for death and morbidity in industrialized society [1, 20], despite efforts to design new and efficient treatment strategies [19, 21].
Unfortunately, numerous reports indicate that in laboratory animals over-expression of VEGF may lead to metabolic dysfunction, formation of leaky vessels and transient edema [1, 22]. Indeed, VEGF actions include the induction of endothelial cells proliferation and migration; it is also known as a vascular permeability factor, based on its ability to induce vascular leakage and vasodilatation in a dose dependent fashion as a result of endothelial cell-derived nitric oxide [12, 23].
In humans, various clinical trials were designed to verify new vessel growth by exogenous administration of proangiogenic factors in patients with refractory ischemic symptoms. Albeit initial small open-labeled trials yielded promising results, subsequent larger double-blind randomized placebo-controlled clinical trials have failed to show much clinical benefit [19, 24, 25]. These largely disappointing results may in part be explained by suboptimal delivery of genetic material to target cells or tissue. Moreover, although adenoviral vectors provide high levels of gene transfer and expression, there are well known virus-related adverse effects, such as the induction of immune and inflammatory response [6, 21, 26]. Recently, several side effects have been reported for VEGF administration in human subjects [1, 8, 25] such as increase in atherosclerotic plaques, lymphatic edema or uncontrolled neoangiogenesis leading to the development of functionally abnormal blood vessels, so to preclude its use in a large share of ischemic population [21, 27].
A hopeful alternative could be to use angiogenic stimulators of smaller size, such as peptides, with a well-characterized biologic mechanism of action. Indeed, recent reports revealed a specific antagonistic relationship between VEGF and other vascular growth factors, such as the placental growth factor (PlGF), the basic fibroblast growth factor (bFGF) and the platelet-derived growth factor (PDGF), with a dichotomous role for VEGF and VEGFRs [28–30]. So, the function of VEGF is far more intricate: it can also negatively regulate angiogenesis and tumorigenesis, by impeding the function of the PDGF receptor on pericytes, leading to a loss of pericyte coverage of blood vessels . Moreover, several studies demonstrated a more efficacious action obtained with a specific stimulation of VEGFRs [32, 33] if compared to VEGF overexpression [22, 34]. These findings suggest that the multifaceted array of the biological responses linked to VEGF may be ascribable to its proneness to dimerize or interact with other molecules . Thus, because of lower molecular and biological complexity, peptides that ensure only the needed interaction with specific receptors could be candidate lead compounds for a safer proangiogenic drug, also to avoid adverse effects.
We show that the VEGF mimetic QK is able to increase neoangiogenesis and collateral flow in WKY rats. Our findings evidence the proangiogenic properties of this small peptide, suggesting that also in vivo QK resembles the full VEGF protein. Thus, a single peptide, that would not be expected to dimerize, is still able to induce VEGF specific angiogenic responses. Clearly, further studies are needed to fully understand this mechanism, that appears of intriguing interest. Anyway, these data open to new fields of investigation on the mechanisms of activation of VEGFRs, also to clarify complex angiogenesis pathways, with strong clinical implications for treatment of pathophysiological conditions such as chronic ischemia.