Transforming Growth Factor α (TGFα)-Stimulated Secretion of HSP90α: Using the Receptor LRP-1/CD91 To Promote Human Skin Cell Migration against a TGFβ-Rich Environment during Wound Healing

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Abstract
Jump-starting and subsequently maintaining epidermal and dermal cell migration are essential processes for skin wound healing. These events are often disrupted in nonhealing wounds, causing patient morbidity and even fatality. Currently available treatments are unsatisfactory. To identify novel wound-healing targets, we investigated secreted molecules from transforming growth factor α (TGFα)-stimulated human keratinoytes, which contained strong motogenic, but not mitogenic, activity. Protein purification allowed us to identify the heat shock protein 90α (hsp90α) as the factor fully responsible for the motogenic activity in keratinocyte secretion. TGFα causes rapid membrane translocation and subsequent secretion of hsp90α via the unconventional exosome pathway in the cells. Secreted hsp90α promotes both epidermal and dermal cell migration through the surface receptor LRP-1 (LDL receptor-related protein 1)/CD91. The promotility activity resides in the middle domain plus the charged sequence of hsp90α but is independent of the ATPase activity. Neutralizing the extracellular function of hsp90α blocks TGFα-induced keratinicyte migration. Most intriguingly, unlike the effects of canonical growth factors, the hsp90α signaling overrides the inhibition of TGFβ, an abundant inhibitor of dermal cell migration in skin wounds. This finding provides a long-sought answer to the question of how dermal cells migrate into the wound environment to build new connective tissues and blood vessels. Thus, secreted hsp90α is potentially a new agent for wound healing.