Carboxyl‐terminal truncation of leucine76 converts heparin‐binding EGF‐like growth factor from a heparin‐enhancible to a heparin‐suppressible growth factor

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Abstract
Previous studies have indicated that heparin differentially regulates heparin‐binding EGF‐like growth factor (HB‐EGF) and amphiregulin (AR) mitogenic activity. To further explore this phenomenon, these mitogens were compared under identical cell culture conditions in two different assays. The results of our present investigation demonstrated that AR‐mediated mitogenic activity in the murine AKR‐2B fibroblast‐like cell line was inhibited by heparin, while HB‐EGF activity was enhanced. However, the absolute effect of heparin appeared to be cell type specific since HB‐EGF mitogenic activity was not dramatically affected by coincubation with heparin when tested on human dermal fibroblasts. Several studies have indicated that mutation of a conserved leucine in the carboxyl‐terminal region of both EGF and transforming growth factor‐α results in decreased affinity for EGF receptors. Since this leucine is present in the analogous position of HB‐EGF, but absent in AR, we examined the effect of deleting this residue by carboxyl‐terminal truncation of HB‐EGF. Analysis of recombinant forms of HB‐EGF demonstrated that HB‐EGF can be converted to a heparin‐inhibited growth factor if the putative mature form of the protein is truncated by two residues (leucine76 and proline77) at the carboxyl terminus. Further analysis demonstrated that only leucine76 appears to be required for heparin‐dependent enhancement of HB‐EGF‐mediated mitogenic activity, indicating that this amino acid may play a pivotal role in controlling the response of HB‐EGF to heparin or related glycosaminoglycan sulfates. Our results also suggest that expression of different HB‐EGF forms in vivo could result in the production of HB‐EGFs with divergent responses to sulfated glycosaminoglycans and proteoglycans.

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