IGF binding proteins and their functions

Abstract

The insulin‐like growth factor (IGF) binding proteins (IGFBPs) have several functions, including transporting the IGFs in the circulation, mediating IGF transport out of the vascular compartment, localizing the IGFs to specific cell types, and modulating both IGF binding to receptors and growth‐promoting actions. The functions of IGFBPs appear to be altered by posttranslational modifications. IGFBP‐3, ‐4, ‐5, and ‐6 have been shown to be glycosylated. Likewise all the IGFBPs have a complex disulfide bond structure that is required for maintenance of normal IGF binding. IGFBP‐2, ‐3, ‐4, and ‐5 are proteolytically cleaved, and specific proteases have been characterized for IGFBP‐3, ‐4, and ‐5. Interestingly, attachment of IGF‐I or II to IGFBP‐4 results in enhancement of proteolysis, whereas attachment of either growth factor to IGFBP‐5 results in inhibition of proteolytic cleavage. Cleavage of IGFBP‐3 results in the appearance of a 31 kDa fragment that is 50‐fold reduced in its affinity for the IGF‐I or IGF‐II. In spite of the reduction in its affinity, this fragment is capable of potentiating the effect of IGF‐I on cell growth responses; therefore, proteolysis may be a specific mechanism that alters IGFBP modulation of IGF actions. Other processes that result in a reduction in IGF binding protein affinity are associated with potentiation of cellular responses to IGF‐I and ‐II. Specifically, the binding of IGFBP‐3 to cell surfaces is associated with its ability to enhance IGF action and with a ten‐ to 12‐fold reduction in its affinity for IGF‐I and IGF‐II. Likewise, binding of IGFBP‐5 to extracellular matrix (ECM) results in an eightfold reduction in its affinity and a 60% increase in cell growth in response to IGF‐I. Another post‐translational modification that modifies IGFBP activity is phosphorylation. IGFBP‐1, ‐2, ‐3, and ‐5 have been shown to be phosphorylated. Phosphorylation of IGFBP‐1 results in a sixfold enhancement in its affinity for IGF‐I and ‐II. Following this enhancement of IGFBP‐1 affinity, this binding protein loses its capacity to potentiate IGF‐I growth‐promoting activity. Future studies using site‐directed mutagenesis to modify these proteins should enable us to determine the effect of these posttranslational modifications on the ability of IGFBPs to modulate IGF biologic activity.