A Novel Epidermal Growth Factor-like Molecule Containing Two Follistatin Modules Stimulates Tyrosine Phosphorylation of erbB-4 in MKN28 Gastric Cancer Cells

We have isolated a gene from stomach fibroblasts encoding novel proteins containing two follistatin modules which might bind TGF-β-related growth factors and a single epidermal growth factor (EGF)-like domain which is closely related to EGF/Neuregulin (NRG) family growth factors. Sequence analysis revealed novel cDNA clones, the protein products of which were designated tomoregulin (TR) and consisted of at least three isoforms which were distinguished by their cytoplasmic domains. The cytoplasmic domains in all isoforms were short and contained potential G-protein activating motifs. Precursors of TR (Pro-TR) are glycosylated transmembrane proteins. Two secreted soluble forms resulting from proteolytic cleavage were distinguished by the presence or absence of the EGF-like domain. The EGF-like domain of TR was highly conserved compared to EGF/NRG family growth factors with the exception of an arginine to histidine substitution at position 39 (Arg → His 39). Soluble TR stimulated erbB-4 tyrosine phosphorylation in MKN 28 gastric cancer cells, although it was weak compared to neuregulin-induced erbB-4 tyrosine phosphorylation; this suggests that TR might be a ligand for erbB-4- or erbB-4-related receptor tyrosine kinase. TR may have important roles in normal development of middle to late stages of embryos and maintenance of adult central nervous system tissues as high expression of TR mRNAs was observed in these tissues. The modular features suggest multiple roles for TR; these include functioning as a ligand for erbB- receptor, a regulator of TGF-β-related growth factor signaling by direct interaction through the follistatin modules, and a G-protein-coupled receptor.