| Understanding the process of wound healing would be invaluable in the development of therapeutic strategies to treat diseases associated with improper tissue regeneration, such as blindness induced by corneal scarring. Heparan sulfate proteoglycans (HSPG) are not normally expressed in the corneal stroma but their appearance at sites of injury suggest a role in the wound healing process. Recently, HSPG have been shown to localize to the nucleus in response to fibronectin, an extracellular matrix protein whose appearance in the corneal stroma also correlates with injury. Using primary corneal stromal fibroblasts, we demonstrate that HSPG mediate basic fibroblast growth factor (FGF-2) mitogenic activities by directing FGF-2 to the nucleus, and potentially modulate histone acetyltransferase (HAT) activity in the nucleus. FGF-2 nuclear localization was increased in cells plated on fibronectin suggesting that FGF-2 and HSPG may translocate to the nucleus in unison. Moreover, heparinase III decreased nuclear FGF-2, and heparan sulfate-deficient Chinese hamster ovary cells showed reduced nuclear FGF-2, indicating that intact heparan sulfate chains are critical for this process. Phorbol 12-myristate-13-acetate decreased nuclear levels of FGF-2 and HSPG as well as downregulated protein kinase Cα (PKCα), suggesting that PKC signaling might be involved in the mechanism of HSPG-mediated FGF-2 nuclear localization. Using isozyme-specific PKC inhibitors, we elucidated a potential mode of regulation whereby PKCα mediates FGF-2 nuclear localization while PKCα inhibits it. Lipid rafts may also be involved, as sterol-binding agents decreased nuclear FGF-2. HSPG-mediated nuclear localization does not appear to be a general property of all heparin-binding growth factors as the nuclear localization of vascular endothelial growth factor 165 was not HSPG-mediated. In addition to delivering FGF-2, HSPG itself may directly influence nuclear activities. Heparin, a molecule similar in structure to heparan sulfate, decreased HAT activity in vitro, potentially by binding to both HAT and histone substrate. Moreover, trypsin-releasable HSPG ectodomains (HSPG f) from corneal stromal fibroblasts inhibited HAT more potently than heparin, and HAT inhibition by various lyase-digested HSPG f suggests that distinct patterns within heparan sulfate sequences may provide specificity in regulating HAT. These studies illustrate novel mechanisms by which HSPG may regulate wound healing in the corneal stroma. |
