In vivo involvement of heparan sulfate proteoglycan in the bioavailability, internalization, and catabolism of exogenous basic fibroblast growth factor

The in vivo bioavailability of exogenous fibroblast growth factor 2 (FGF2) was studied after i.v. injection of uniformly C-14-labeled FGF2 into young rats. C-14-FGF2 was rapidly accumulated in almost all solid organs within 5 min. After 30 min, more than 65% of FGF2 was retained in liver, 4.5% in kidneys, 1.2% in spleen, 0.15% in adrenal glands, and trace amounts in bone marrow, eyes, lungs,and heart. Suborgan distribution of C-14-FGF2 showed that for kidneys and adrenal glands, the labeling was mainly concentrated in the cortical zone. Incubation of organ sections with 2 M NaCl or heparin eluted all the radioactivity, indicating that labeling was due to FGF2-heparan sulfate proteoglycan (HSPG) interactions. Electrophoretic analysis show only native C-14-FGF2 in the blood and extracellular matrix; however, FGF2 is continuously catabolized in solid organs, indicating that all participate in the clearance of FGF2 by cellular internalization and subsequent catabolism. All FGF2 catabolic fragments bound heparin, demonstrating the preservation of their HSPG-binding site during the in vivo intracellular catabolism of FGF2. Analysis of the high-affinity receptors of FGF2 (FGFR-1 and FGFR-3) and the mitogen-activated protein kinase did not show any increase in either FGFR tyrosine phosphorylation or in mitogen-activated protein kinase activation. This study shows for the first time that exogenous FGF2 is cleaved by HSPG cellular internalization and catabolism without inducing the activation of FGFRs within at least five organs in vivo, which strongly suggests that the HSPG-dependent internalization and catabolism pathway may control the in vivo bioavailability of FGF2.