The effects of exogenous heregulin on tyrosine phosphorylation of ErbB2, ErbB3 and ErbB4 in both the cytosolic and nuclear fractions of colon cancer cell lines. After heregulin treatment, the cells were lysed and immunoprecipitated with an antibody against phosphotyrosine (PY-20). Immunoprecipitates were analyzed by Western blotting with anti-phosphorylated ErbB receptor antibodies. A, pErbB2 and pErbB3 were detected in both the nuclear and cytosolic fractions of both DLD-1 and Caco-2 cells after heregulin stimulation. B, ErbB4 phosphorylation increased with exogenous heregulin stimulation in only the cytosolic fractions of HCT 116 cells.