SDF-1α expression during wound healing in the aged is HIF dependent

Background: Age-related impairments in wound healing are associated with decreased neovascularization, a process that is regulated by hypoxia-responsive cytokines, including stromal cell-derived factor (SDF)-1α. Interleukin-1β is an important inflammatory cytokine involved in wound healing and is believed to regulate SDF-1α expression independent of hypoxia signaling. Thus, the authors examined the relative importance of interleukin (IL)-1β and hypoxia-inducible factor (HIF)-1α on SDF-1α expression in aged wound healing. Methods: Young and aged mice (n = 4 per group) were examined for wound healing using a murine excisional wound model. Wounds were harvested at days 0, 1, 3, 5, and 7 for histologic analysis, immunohistochemistry, enzyme-linked immunosorbent assay, and Western blot. An engineered wild-type and mutated SDF luciferase reporter construct were used to determine HIF transactivation. Results: Aged mice demonstrated significantly impaired wound healing, reduced granulation tissue, and increased epithelial gap compared with young controls. Real-time polymerase chain reaction demonstrated reduced SDF-1α levels in aged wounds that correlated with reduced CD31+ neovessels. Western blots revealed decreased HIF-1α protein in aged wounds. However, both IL-1β and macrophage infiltrate were unchanged between young and aged animals. Using the wild-type and mutated SDF luciferase reporter construct in which the hypoxia response element was deleted, only young fibroblasts were able to respond to IL-1β stimulation, and this response was abrogated by mutating the HIF-binding sites. This suggests that HIF binding is essential for SDF-1 transactivation in response to both inflammatory and hypoxic stimuli. Conclusions: SDF-1α deficiency observed during aged wound healing is attributable predominantly to decreased HIF-1α levels rather than impaired IL-1β expression.