Paclitaxel induces apoptosis via protein kinase A- and p38 mitogen-activated protein-dependent inhibition of the Na⁺/H⁺ exchanger (NHE) NHE isoform 1 in human breast cancer cells

Purpose: The molecular signal components essential to paclitaxel-dependent apoptosis in breast cancers are potential targets for combined therapy. However, the signal mechanisms underlying paclitaxel action still need to be better defined. Experimental Design: In a breast cancer cell line, pharmacological agents and transient transfection with dominant interfering and constitutive active mutants were used to identify the signal transduction module involved in the regulation of paclitaxel-induced apoptosis and to evaluate its potential as a therapeutic target. Results: In MDA-MB-435 cells, paclitaxel treatment stimulated the activity of both protein kinase A and p38, and inhibited the activity of the Na⁺H⁺ exchanger isoform 1 (NHE1) with similar IC₅₀ concentrations as for its activation of apoptosis. Activation and inhibition experiments demonstrated that protein kinase A and p38 participate sequentially upstream of the NHE1 in regulating the paclitaxel-induced apoptotic pathway. Importantly, concurrent specific inhibition of the NHE1 with paclitaxel treatment resulted in a synergistic induction of apoptosis and a reduction in the paclitaxel IC₅₀ for apoptosis. This sensitization of paclitaxel apoptotic action by specific inhibition of NHE1 was verified in breast cancer cell lines with different paclitaxel sensitivity. Conclusions: We have, for the first time, identified NHE1 as an essential component of paclitaxel-induced apoptosis in breast cancer cells and, importantly, identified that simultaneous inhibition of the NHE1 results in a synergistic potentiation of low-dose paclitaxel apoptotic action. As specific NHE1 inhibitors have finished Phase II/Phase III clinical trials for myocardial protection, there is the possibility for a rapid biological translation of this novel therapeutic strategy to a clinical setting.