Effect of EGF-induced HDAC6 activation on corneal epithelial wound healing.

The reversible acetylation targeting to both histone and nonhistone proteins plays important roles in posttranslational modifications of proteins. The balance between acetylation and de-acetylation is maintained by histone acetyltransferases (HATs) and histone deacetylases (HDACs). Most HATs and HDACs are localized inside nuclei and regulate the acetylation of nuclear proteins.1 In the nuclei, histone acetylation generally induces chromatin relaxation and gene transcription, whereas nonhistone acetylation functions widely, depending on its targeting proteins.2,3 Interestingly, HDAC6 is one of the exceptional members in the HDAC family for its unique cytosol localization and enzymatic activity for deacetylation of several important cytosol proteins, including α-tubulin, Hsp90, and cortactin.4–6 HDAC6 involves both microtubule- and microfilament-dependent cell motilities by regulating the acetylation level of these targeted proteins. Most evidence of the defective cell mobility found in HDAC6 knockout mice indicates that HDAC6 activity is essential for cell motility.7,8 Cell migration is involved in significant physiological and pathologic activities, such as wound healing and cancer metastasis. It has been shown that HDAC6 activity is associated with cancer invasion,1,9,10 thus representing an attractive target for cancer chemotherapy.11 However, accelerated cell migration is favored for wound healing and tissue repair. For example, corneal wound healing requires proper activities of cell migration that are essential for successful reepithelialization.12 In the corneal epithelial wound-healing process, epithelial migration is initiated by microtubule and microfilament reorganization and is continued by microtubules forming distinct weblike patterns while circumferential microfilament bundles form lamellipodia, filopodia, and prominent stress.12–14 A recent study shows that HDAC inhibitors block the migration of corneal stromal cells; however, there are no reports indicating the role of HDAC6 in regulating cell migration through altering the acetylation of cytoskeleton proteins in corneal epithelial wound healing.15 Based on the evidence that acetylation of cytoskeleton-related proteins, such as α-tubulin and cortactin, is closely associated with cell migration, we hypothesized that corneal epithelial wound healing involves the regulation of acetylation modifications. In the present study, we were interested in determining the role of HDAC6 activation and acetylation modification of cytoskeleton proteins in corneal epithelial wound healing. It is likely that HDAC6 is involved in growth factor-induced cell migration because HDAC6 overexpression could promote serum-induced cell migration,6 and PDGF-induced Rac-1 activation is absent in HDAC6 knockout mice.8 We found that EGF stimulation significantly enhanced the deacetylase activity of HDAC6, presenting a novel mechanism and detailing how growth factors regulate HDAC6 function. In addition, we demonstrated significant evidence for the first time to support the notion that corneal epithelial wound healing was dampened by the inhibitor of deacetylases and by knocking-down HDAC6. We also found that there was an enhanced activity of HDAC6 and a reduced level of acetylated α-tubulin in response to EGF stimulation in corneal epithelial cells. Thus, our study results contribute to a theory responsible for initiating cell migration in corneal wound healing.