The cDNAs of a putative growth factor-bound (Grb) 7 signal transduction molecule and Grb7V novel splice variant were isolated from an invasive human esophageal carcinoma. Although both Grb7 isoforms share homology with the Mig-10 cell migration gene, the Grb7V isoform lacks 88 base pairs in the C terminus; the resultant frame shift leads to substitution of an SH2 domain with a short hydrophobic sequence. The wild-type Grb7 protein, but not the Grb7V isoform, is rapidly tyrosyl phosphorylated in response to EGF stimulation in esophageal carcinoma cells. Analysis of human esophageal tumor tissues and regional lymph nodes with metastases revealed that Grb7V was expressed in 40% of Grb7-positive esophageal carcinomas. More importantly, Grb7V expression was enhanced after metastatic spread to lymph nodes as compared to the original tumor tissues. Finally, transfection of an antisense Grb7 RNA expression construct lowered endogenous Grb7 protein levels and suppressed the invasive phenotype exhibited by esophageal carcinoma cells. These findings suggest that Grb7 isoforms are involved in cell invasion and metastatic progression of human esophageal carcinomas.
S Tanaka, M Mori, T Akiyoshi, Y Tanaka, K Mafune, J R Wands, K Sugimachi