Iressa induces cytostasis and augments Fas‐mediated apoptosis in acinic cell adenocarcinoma overexpressing HER2/neu

Understanding the role of signal transduction in regulating pathways responsible for cell growth, survival and apoptosis is critical for cancer therapy. We developed and characterized a HER2/neu and Fas overexpressing cell line (BNT.888 ACA2) from a salivary gland adenocarcinoma that arose in a HER2/neu transgenic mouse. We evaluated the effects of Iressa on signal transduction networks downstream of the activated HER2 and the impact on proliferation, cell cycle and apoptosis. Iressa treatment diminished phosphorylation of the HER2/neu and EGFR. Phosphorylation of STAT‐3 also decreased and mitogenic signaling through the MAPK pathways was greatly reduced. Cyclin D1 levels decreased, and cells were arrested in G0 and failed to enter S‐phase because of hypophosphorylation of Rb and to traverse the G2M checkpoint because of degradation of cyclin B1. Cytostasis occurred within 48 hr at 250–500 nM Iressa. Levels of proapoptotic factors (bim and bax) increased and levels of antiapoptotic factors (bcl‐2 and bcl‐xL) decreased in a dose‐dependent manner. Higher doses of Iressa diminished phosphorylation of Akt slightly, but failed to induce apoptosis. Fas antibody was a potent agonist of apoptosis. Pretreatment with Iressa (1 μM, 24 hr) greatly enhanced Fas‐mediated apoptosis as determined by Annexin V binding, cleavage of caspase‐3 and PARP. Augmentation of apoptosis was associated with increased Fas expression and membrane localization. Iressa pretreatment increased bid activation, cleavage of caspases ‐3, ‐9 and ‐12 and stress signaling via c Jun. These data showing that Iressa induces cytostasis and primes the extrinsic (Fas) and intrinsic (mitochondrial and endoplasmic reticulum) apoptotic pathways should lead to the development of novel therapeutic targets and strategies. © 2006 Wiley‐Liss, Inc.