Trastuzumab and lapatinib modulation of HER2 tyrosine/threonine phosphorylation and cell signaling.

Cell surface transmembrane signaling receptors EGFR, HER3, and HER4 are activated by ligand-binding-mediated dimerization and phosphorylation. In contrast, HER2 amplification promotes signaling by increasing homo/heterodimerization and ligand binding. Trastuzumab or lapatinib therapy of HER2 amplicon-positive breast cancer cells induces growth inhibition and intracellular growth pathway signaling modulation. The mechanism(s) by which trastuzumab, an IgG1 humanized antibody, induces modification of cell signaling upon binding to an extracellular determinant on a ligand-less "receptor" membrane protein remains unexplained. Using immune detection methodology comprised of antibodies detecting three distinct domains of HER and five tyrosine/threonine phosphorylation sites, the effects of trastuzumab and lapatinib were defined during steady state growth inhibition. Here, we show that lapatinib markedly reduces HER2 tyrosine phosphorylation, while in contrast, no change in tyrosine phosphate levels is detected during trastuzumab-mediated cell growth inhibition. As trastuzumab treatment does not change either the steady state HER2 protein levels or HER2 mRNA, these findings argue against an antibody-dependent alteration in internalization kinetics. We further show a sequenced relationship between lapatinib-induced blockage of phosphorylation (6-8 h) and induction of delayed cell death (5-6 days), while trastuzumab-treated cells showed no evidence of cell death up to 9 days. Taken together, these results demonstrate that inhibition of HER2 phosphorylation by lapatinib is sufficient to induce apoptosis while trastuzumab binding to the extracellular HER2 domain may function by sterically modulating the detection of phosphate moieties by cytoplasmic signal transducers. This investigation also detected a 20 kD protein, which is down-regulated by lapatinib, further demonstrating the complexity of this signal transduction system.