Amino‐terminal deletion of heparin‐binding epidermal growth factor‐like growth factor 4−127 stimulates cell proliferation but lacks insulin‐like activity

.   Heparin‐binding epidermal growth factor‐like growth factor (HB‐EGF) Northern analysis demonstrated a novel 0.8‐kb liver‐specific HB‐EGF transcript in addition to the endogenous 2.5‐kb HB‐EGF full‐length transcript present in kidney, lung and liver tissues. Reverse transcriptase‐polymerase chain reaction screening of liver RNA suggests that the 0.8‐kb HB‐EGF transcript lacks at least a portion of the amino‐terminal EGF‐like domain. In light of these data, we have constructed a human HB‐EGF cDNA (HB‐EGF ΔN ) which lacks 373 bp, encoding the majority of the extracellular EGF‐like domain, while maintaining the ectodomain ‘shedding’ site, transmembrane and cytoplasmic domains. Objective : The goal of this study is to characterize the ability of HB‐EGF ΔN to (i) stimulate cell proliferation and (ii) determine whether down‐regulation of insulin‐like growth factor‐binding protein (IGFBP)‐3 and ‐4 mRNA is regulated by soluble, mature HB‐EGF or HB‐EGF C. Materials and methods : HB‐EGF ΔN encodes nucleotides +1–10 of exon 1 linked to nucleotides 383–627 of the carboxy‐terminal portion of exon 3 through exon 5. Results : Expression of HB‐EGF ΔN in mouse fibroblasts (MLC) resulted in 6.5‐ and 8‐kDa HB‐EGF immunoreactive proteins, stimulated tyrosine phosphorylation of p42 kDa and cell proliferation in MLC, but lacked the ability to bind EGF receptors. Finally, HB‐EGF ΔN failed to down‐regulate IGFBP‐3 and ‐4 mRNA when expressed in normal rat kidney cells. Conclusions : These findings demonstrate that amino‐terminally truncated, membrane‐bound form of HB‐EGF stimulates cell proliferation but lacks insulin‐like signalling, suggesting that insulin‐like signalling is mediated by soluble, mature HB‐EGF binding to EGF receptors.