Hypoxia is a microenvironmental stress in wounded skin, where it supports wound healing by promoting cell motility. The mechanism of the hypoxia action remained speculative. Here, we provide evidence that hypoxia promotes human dermal fibroblast (HDF) migration by inducing secretion of heat shock protein-90alpha (hsp90a) into the extracellular environment through hypoxia-inducible factor-1alpha (HIF-1a). The secreted hsp90a in turn executes hypoxia's pro-motility effect. Expression of an activated HIF-1a mimicked, whereas expression of an inactive HIF-1a or suppression of endogenous HIF-1a blocked, hypoxia-induced hsp90a secretion and HDF migration. Interestingly, the hypoxia-HIF-1 pathwayinduced hsp90a secretion required neither changes in the steady-state mRNA level nor in the promoter activity of hsp90a. Recombinant hsp90a fully duplicated the hypoxia effect on HDFs. Inhibition of extracellular hsp90a function completely blocked the hypoxia-HIF-1 pathway-stimulated HDF migration. More intriguingly, topical application of hsp90a accelerated wound healing in mice. This study has demonstrated a novel mechanism of hypoxia4HIF-14hsp90a secretion4skin cell migration4wound healing, and identified extracellular hsp90a as a potential therapeutic agent for skin wounds.

Transposons comprise large fractions of eukaryotic genomes and provide genetic reservoirs for the evolution of new cellular functions. We identified TPB2, a homolog of the piggyBac transposase gene that is required for programmed DNA deletion in Tetrahymena. TPB2 was expressed exclusively during the time of DNA excision, and its encoded protein Tpb2p was localized in DNA elimination heterochromatin structures. Notably, silencing of TPB2 by RNAi disrupts the final assembly of these heterochromatin structures and prevents DNA deletion to occur. In vitro studies revealed that Tpb2p is an endonuclease that produces double-strand breaks with four-base 5 protruding ends, similar to the ends generated during DNA deletion. These findings suggest that Tpb2p plays a key role in the assembly of specialized DNA elimination chromatin architectures and is likely responsible for the DNA cleavage step of programmed DNA deletion.

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 ␣ (TGF␣)-stimulated human keratinoytes, which contained strong motogenic, but not mitogenic, activity. Protein purification allowed us to identify the heat shock protein 90␣ (hsp90␣) as the factor fully responsible for the motogenic activity in keratinocyte secretion. TGF␣ causes rapid membrane translocation and subsequent secretion of hsp90␣ via the unconventional exosome pathway in the cells. Secreted hsp90␣ 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 hsp90␣ but is independent of the ATPase activity. Neutralizing the extracellular function of hsp90␣ blocks TGF␣-induced keratinicyte migration. Most intriguingly, unlike the effects of canonical growth factors, the hsp90␣ signaling overrides the inhibition of TGF␤, 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 hsp90␣ is potentially a new agent for wound healing.

Wounds that fail to heal in a timely manner, for example, diabetic foot ulcers, pose a health, economic, and social problem worldwide. For decades, conventional wisdom has pointed to growth factors as the main driving force of wound healing; thus, growth factors have become the center of therapeutic developments. To date, becaplermin (recombinant human PDGF-BB) is the only US FDA-approved growth factor therapy, and it shows modest efficacy, is costly, and has the potential to cause cancer in patients. Other molecules that drive wound healing have therefore been sought. In this context, it has been noticed that wounds do not heal without the participation of secreted Hsp90α. Here, we report that a 115-aa fragment of secreted Hsp90α (F-5) acts as an unconventional wound healing agent in mice. Topical application of F-5 peptide promoted acute and diabetic wound closure in mice far more effectively than did PDGF-BB. The stronger effect of F-5 was due to 3 properties not held by conventional growth factors: its ability to recruit both epidermal and dermal cells; the fact that its ability to promote dermal cell migration was not inhibited by TGF-β; and its ability to override the inhibitory effects of hyperglycemia on cell migration in diabetes. The discovery of F-5 challenges the long-standing paradigm of wound healing factors and reveals a potentially more effective and safer agent for healing acute and diabetic wounds.

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