Summary
Cultured human umbilical vein endothelial cells (HUVEC) were used as a model to study
transendothelial IGF-I transport, and its deposition into the extracellular matrix
(ECM). Specific binding of 125I-IGF-I to HUVEC monolayers was demonstrated, which was inhibited by aIR-3, a specific
antibody directed against the IGF-I receptor. ECM-associated 125I-IGF-I was approximately 10% of cell-bound IGF-I at 22 °C, and increased 4.5-fold
at 37 °C, indicating that endothelial metabolism is required for the transport. However,
neither monensin and cytochalasin B, both of which block endocytosis, nor aIR-3 did
inhibit transport of 125I-IGF-I into the ECM. In order to characterize IGF-I binding to the subendothelial
ECM, HUVEC were removed nonenzymatically by treatment with Triton X-100 and ammonia.
Specific, saturable binding of 125I-IGF-I to the isolated ECM was observed, which was protease-sensitive. Antibodies
directed against vitronectin inhibited IGF-I binding to the matrix by 35%, while antibodies
directed against other ECM proteins had no significant influence on IGF-I binding.
Using radioimmunoassays the IGF binding protein-2 was detected in the ECM, while IGFBP-1
and IGFBP-3 were below the detection limits. In order to evaluate functional aspects
of IGF-I binding to the matrix, HUVEC were incubated under serum-free conditions in
the absence and presence of IGF-I. Under serum-free conditions 48% of cells rounded
up and started to detach after 2 hours incubation, while only 23% of the cells started
to detach in the presence of IGF-I. These data indicate that IGF-I is transported
via a paracelluar route across endothelial cells, and becomes bound to the subendothelial
ECM. Vitronection seems to be involved in binding of IGF-I to the ECM. ECM-associated
IGF-I might play a role in endothelial cell survival and stability.
Key words:
Insulin-like growth factor-I - Endothelial cells - Extracelluar matrix - Vitronectin
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Dr. J. Grulich-Henn
University Children's Hospital Heidelberg
Im Neuenheimer Feld 150
D-69120 Heidelberg
Germany
Phone: + 49-6221-562311
Email: Juergen_Grulich-Henn@med.uni-heidelberg.de