Heparan Sulfate Proteoglycans Control Intracellular Processing of bFGF in Vascular Smooth Muscle Cells

Abstract

Basic fibroblast growth factor (bFGF) is a potent mitogen for vascular smooth muscle cells (VSMC) and has been implicated in a number of vascular disorders. bFGF interacts with high-affinity receptors and heparan sulfate proteoglycans (HSPG) at the cell surface. HSPG have been demonstrated to enhance bFGF binding to its receptors, yet no known role for HSPG in modulating postbinding events has been identified. In the present study, we analyzed bFGF internalization, intracellular distribution, degradation, and stimulation of DNA synthesis within native and HSPG-deficient VSMC. HSPG-deficient VSMC were generated by treating cells with sodium chlorate to inhibit the sulfation of HSPG. We found that stimulation of DNA synthesis by bFGF in chlorate-treated VSMC was markedly reduced as compared with native cells, even at doses of bFGF where receptor binding was similar in the two conditions. This was not a general lack of mitogenic potential, as the addition of calf serum, or epidermal growth factor, stimulated DNA synthesis to a similar extent in native and chlorate-treated cells. Analysis of the accumulation of internalized bFGF within cytoplasmic and nuclear fractions of native and HSPG-deficient VSMC showed striking differences. At early time points (0−2 h), nearly identical amounts of bFGF were observed in the cytoplasmic fractions under both conditions, yet significant amounts of bFGF were only found in the nuclear fractions of native cells. At later time points (2−48 h), the amount of cytoplasmic bFGF was significantly greater in the native compared to HSPG-deficient cells, and nuclear deposition of bFGF began to reach similar levels under both conditions. Furthermore, the intracellular half-life of bFGF was dramatically prolonged in native compared to HSPG-deficient cells, in part, due to decreased bFGF degradation in native cells. Thus, HSPG appears to accelerate nuclear localization, increase cytoplasmic capacity, and inhibit intracellular degradation of bFGF in VSMC. Modulation of intracellular processing of bFGF by HSPG might control the biological activity of bFGF in VSMC.