Sensitization of apoptotically-resistant breast carcinoma cells to TNF and TRAIL by inhibition of p38 mitogen-activated protein kinase signaling.

The mitogen-activated protein kinase (MAPK) cascade is a critical component in the regulation of cell survival and proliferation decisions. In breast carcinoma cells, activation of the p38-MAPK member of this family occurs in response to pro-inflammatory cytokines and cellular stress. The involvement of p38-MAPK in the activation of the transcription factor, NF-kappaB, suggests a potential role and mechanism for regulation of cell survival and drug resistance. Generation of the resistant MCF-7 variant (MCF-7TN-R) was achieved by prolonged exposure of MCF-7N cells to increasing concentrations of TNF. Differences in MAPK activation and function in the MCF-7 cell variants were determined. The role of the p38-MAPK pathway in regulation of resistance was determined using pharmacological (SB 203580) or molecular [Dominant Inhibitory (DI)-p38] inhibition. The effect of p38 inhibition on NF-kappaB transcriptional activation was analyzed. As compared to the sensitive MCF-7N parent cell line, the MCF-7TN-R cell line displayed significant resistance to TNF- and TRAIL-induced cell death. Analysis of the expression and phosphorylation of members of the MAPK family revealed an increased basal activation of p38 in the MCF-7TN-R variant. The p38-mediated phosphorylation and transcriptional activity were suppressed by pharmacologic inhibition with SB 230580. Treatment of MCF-7TN-R cells with SB partially restored sensitivity to TNF-induced cell death. In addition, use of a DI-p38 construct with or without the addition to TNF induced cell death, thus restoring TNF-sensitivity to these cells. The ability of p38 inhibition to restore apoptotic sensitivity was correlated with suppression of the TNF-induced cell survival pathway, NF-kappaB. The increased activation of p38-MAPK in MCF-7TN-R cells demonstrates that this signaling pathway through activation of NF-kappaB is an important route for control of resistance to cell death in breast carcinoma. Molecular and pharmacological inhibition of p38-MAPK signaling may represent a mechanism for sensitizing cancer cells to chemotherapeutic regimens and restoration of apoptotic signaling.