Nerve growth factor (
NGF) is known to exert a mitogenic effect on human
breast cancer cells through proto-TrkA activation.
Reverse transcriptase-PCR analysis of proto-TrkA expression in human
breast carcinoma specimens and cell lines revealed trkA transcript in 12 of 14 human
breast carcinoma specimens and in all of four cell lines tested. While cytofluorimetric and Western blot analysis indicated proto-TrkA expression in three of the four cell lines,
NGF stimulated growth in only two of the three positive cell lines. Inhibition of
NGF-induced MAPK activation by an antibody directed against the extracellular domain of TrkA but not by an inhibitor of TrkA phosphorylation demonstrated the requirement of
NGF binding but not of proto-TrkA
kinase activity for MAPK activation, suggesting the recruitment of another
kinase for transmission of the mitogenic signaling. Indeed,
NGF induced
tyrosine phosphorylation and stimulated
kinase activity of p185(HER2), a
kinase receptor of the HER family. A TrkA phosphorylation inhibitor did not affect this activation. Moreover, the two receptors were coprecipitated by
antibodies directed against proto-TrkA and p185(HER2). Down-modulation of p185(HER2) expression in a
breast carcinoma line transfected with a construct containing an anti-p185(HER2) antibody sequence and expressing proto-TrkA impaired
NGF-induced MAPK activation and proliferation. Together these data show that in cells expressing low levels of TrkA such as
breast carcinoma cells,
NGF must recruit other overexpressed receptors such as p185(HER2) in order to generate a biological signal that can induce
breast cancer cell growth.