The tyrosine kinase activity of the C‐erbB‐2 gene product (p185) is required for growth inhibition by anti‐p185 antibodies but not for the cytotoxicity of an anti‐p185‐ricin‐a chain immunotoxin

Our previous studies have demonstrated that 7 of 10 lgG antibodies against distinct epitopes on the extracellular domain of the c‐erbB‐2 gene product (p185) inhibit the anchorage‐independent growth of SKBr3 human breast‐cancer cells that overexpress this transmembrane tyrosine kinase growth‐factor receptor. Two of 7 growth‐inhibitory antibodies also block the binding and function of the gp30 and p75 c‐erbB‐2 ligands. In this report we have studied phosphorylation of p185 and different intracellular substrates after binding of antibodies that do or do not inhibit tumor‐cell growth. A correlation has been found between antibodies that inhibit growth and the intensity of tyrosine phosphorylation of p185. At late intervals, serine phosphorylation of at least 3 intracellular substrates is increased preferentially by growth‐inhibitory antibodies. To test the importance of p185 kinase activity more critically, NIH3T3 cells were transfected with an expression vector containing the full‐length human c‐erbB‐2 gene (cell line 17313), c‐erbB‐2 with deletion of the kinase region from codons 751–979 (cell line 9309) or c‐erbB‐2 with deletion of most of the intracellular domain from codons 684–1255 (cell line 9310). Unconjugated antibodies inhibited anchorage‐independent growth of 17313 cells as well as SKBr3 cells, but did not inhibit growth of either 9309 or 9310 cells. In contrast, the cytotoxic effect of anti‐p185‐ricin A chain (RTA) conjugates was comparable for 17313, 9309 and 9310. The tyrosine‐kinase activity of p185 is required for growth inhibition mediated by unconjugated anti‐p185 antibodies, but not for the cytotoxic activity of anti‐p185‐RTA immunotoxins.