Vascular Endothelial Growth Factor as an Immediate-Early Activator of Ultraviolet-Induced Skin Injury

Abstract

The negative health consequences of acute ultraviolet (UV) exposure are evident, with reports of 30,000 emergency room visits annually to treat the effects of sunburn in the United States alone. The acute effects of sunburn include erythema, edema, severe pain, and chronic overexposure to UV radiation, leading to skin cancer. Whereas the pain associated with the acute effects of sunburn may be relieved by current interventions, existing post-sunburn treatments are not capable of reversing the cumulative and long-term pathological effects of UV exposure, an unmet clinical need. Here we show that activation of the vascular endothelial growth factor (VEGF) pathway is a direct and immediate consequence of acute UV exposure, and activation of VEGF signaling is necessary for initiating the acute pathological effects of sunburn. In UV-exposed human subjects, VEGF signaling is activated within hours. Topical delivery of VEGF pathway inhibitors, targeted against the ligand VEGF-A (gold nanoparticles conjugated with anti-VEGF antibodies) and small-molecule antagonists of VEGF receptor signaling, prevent the development of erythema and edema in UV-exposed mice. These findings collectively suggest targeting VEGF signaling may reduce the subsequent inflammation and pathology associated with UV-induced skin damage, revealing a new postexposure therapeutic window to potentially inhibit the known detrimental effects of UV on human skin. It is essential to emphasize that these preclinical studies must not be construed as suggesting in any way the use of VEGF inhibitors as a sunburn treatment in humans because warranted future clinical studies and appropriate agency approval are essential in that regard.

Figure 1:. UVB radiation activates VEGF-A/VEGFR-2 signaling in endothelial cells and lymphocytes to regulate cellular functions.

UVB radiation causes increased production of VEGF-A, which binds to VEGFR-2 expressed on the surface of endothelial cells and immune cells. VEGF-A, a member of a family of proteins including VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E, and PlGF, binds VEGFR-2 and various co-receptors to activate downstream signal transduction pathways that regulate survival, migration, and permeability in endothelial cells. In lymphocytes, VEGF/VEGFR-2 interactions stimulate T-cell activation and a shift towards a Th1 phenotype along with upregulation of IL-6 and TNF-alpha in PBMCs. VEGF-A also interacts with VEGFR-1 and VEGFR-3 to promote vasculogenesis and lymphangiogenesis, respectively. VEGF-A, vascular endothelial growth factor A; VEGFR-2, vascular endothelial growth factor receptor 2; PlGF, placental growth factor; Th1, Type 1 T helper; IL-6, interleukin 6; TNF-alpha, tumor necrosis factor alpha; PBMCs, peripheral blood mononuclear cells.

Figure 2:. UV exposure activates VEGF signaling in human subjects.

Skin tissue specimens collected from human subjects (n=4) at 5 minutes, 1 hour, 5 hours, and 24 hours post-exposure to 2.5 MED solar-simulated UV light were immunostained using antibodies that detect total VEGFR-2 protein (left column) and VEGFR-2 phosphorylated at its Y1175 residue (p-VEGFR-2; right column). Representative images are shown. Scale bar indicates 20 μm. VEGF, vascular endothelial growth factor; VEGFR-2, vascular endothelial growth factor receptor 2; MED, minimal erythema dose.

Figure 3:. Topical inhibition of VEGF signaling reduces UV-induced skin damage in mice.

(a) Quantitative PCR was used to measure induction of VEGF transcript in mouse skin at 48 hours post-exposure to various indicated doses of UVB (n=3 mice). (b, c) Ears of mice (2 separate experiments, n=4 mice per group) were exposed to a single 1.0 MED UVB exposure of 0.144 J/cm² after treatment with 50 μg VEGF-neutralizing antibody 2C3 or negative control PBS by intraperitoneal injection 24 hours prior. Representative images are shown (b). Extent of ear edema was determined by measuring ear thickness 48 hours post radiation. (c). (d) Ears of mice (n=5 mice per group) were exposed once to 1.0 MED of UVB radiation at 0.144 J/cm². Indicated topical treatments (25 μg per mouse; 12.5 μg per ear) or control vehicle (Vanicream) were applied two hours post-radiation. Extent of ear edema was determined by measuring the change in ear thickness 48 hours post-radiation. (e, f) Topical administration of GNP-VEGF antibody (25 ug per mouse; 12.5 μg per ear) or small molecule inhibitor of VEGF signaling, sorafenib (25 ug per mouse; 12.5 μg per ear) reduces edema (2 separate experiments, n=5 mice per group). All treatments were applied 2 hours following 1.0 MED UVB exposure of 0.144 J/cm2 UVB. Ear thickness was measured 2 days after UVB exposure. Quantitative PCR was used to measure induction of VEGF transcript in mouse ear skin following UV exposure and treatment in a subset of mice (f). VEGF, vascular endothelial growth factor; MED, minimal erythema dose; PBS, phosphate buffered saline; GNP, gold nanoparticles.