Nuclear localization of basic fibroblast growth factor is mediated by heparan sulfate proteoglycans through protein kinase C signaling

Abstract

Understanding the process of wound healing will provide valuable insight for the development of new strategies to treat diseases associated with improper regeneration, such as blindness induced by corneal scarring. Heparan sulfate proteoglycans (HSPG) are not normally expressed in the corneal stroma, but their presence at sites of injury suggests their involvement in the wound healing response. Primary cultured corneal stromal fibroblasts constitutively express HSPG and represent an injured phenotype. Recently, nuclear localization of HSPG was shown to increase in corneal stromal fibroblasts plated on fibronectin (FN), an extracellular matrix protein whose appearance in the corneal stroma correlates with injury. One possible role for the nuclear localization of HSPG is to function as a shuttle for the nuclear transport of heparin‐binding growth factors, such as basic fibroblast growth factor (FGF‐2). Once in the nucleus, these growth factors might directly modulate cellular activities. To investigate this hypothesis, cells were treated with ¹²⁵I‐labelled FGF‐2 under various conditions and fractionated. Our results show that nuclear localization of FGF‐2 was increased in cells plated on FN compared to those on collagen type I (CO). Interestingly, FGF‐2‐stimulated proliferation was increased in cells plated on FN compared to CO and this effect was absent in the presence of heparinase III. Furthermore, pre‐treatment with heparinase III decreased nuclear FGF‐2, and CHO cells defective in the ability to properly synthesize heparan sulfate chains showed reduced nuclear FGF‐2 indicating that the heparan sulfate chains of HSPG are critical for this process. HSPG signaling, particularly through the cytoplasmic tails of syndecans, was investigated as a potential mechanism for the nuclear localization of FGF‐2. Treatment with phorbol 12‐myristate‐13‐acetate (PMA), under conditions that caused downregulation of protein kinase Cα (PKCα), decreased nuclear FGF‐2. Using pharmacological inhibitors of specific PKC isozymes, we elucidated a potential mode of regulation whereby PKCα mediates the nuclear localization of FGF‐2 and PKCδ inhibits it. Our studies suggest a novel mechanism in which FGF‐2 translocates to the nucleus in response to injury. J. Cell. Biochem. 88: 1214–1225, 2003.