PUMA and BIM Are Required for Oncogene Inactivation–Induced Apoptosis

Combining drugs that inhibit antiapoptotic proteins with oncogenic kinase inhibitors may overcome drug resistance in cancer therapy. Using Addiction Against Cancer Oncogene-addicted cancer cells depend on a particular oncogenic protein for survival and die when the oncogenic protein is inactivated. For example, lung cancers with an abnormally active form of the epidermal growth factor receptor (EGFR) and breast cancers with amplification of the gene encoding the human epidermal growth factor receptor 2 (HER2) regress when exposed to drugs called tyrosine kinase inhibitors that block the activity of these receptors. Bean et al. analyzed breast and lung cancer cell lines treated with tyrosine kinase inhibitors and mice in which EGFRs were inactivated by genetic ablation. They found that two signaling pathways, the phosphoinositide 3-kinase (PI3K)–AKT and mitogen-activated or extracellular signal–regulated protein kinase kinase (MEK)–extracellular signal–regulated kinase (ERK) pathways, were inactivated, resulting in an increase in the abundance of PUMA and BIM, two proteins that promote cell death. Furthermore, cancer cell lines resistant to tyrosine kinase inhibitors were killed by the combination of a PI3K inhibitor and a drug that inhibits antiapoptotic proteins. Thus, pharmacologically enhancing the death pathway may overcome or prevent resistance to tyrosine kinase inhibitors. The clinical efficacy of tyrosine kinase inhibitors supports the dependence of distinct subsets of cancers on specific driver mutations for survival, a phenomenon called “oncogene addiction. ” We demonstrate that PUMA and BIM are the key apoptotic effectors of tyrosine kinase inhibitors in breast cancers with amplification of the gene encoding human epidermal growth factor receptor 2 (HER2) and lung cancers with epidermal growth factor receptor (EGFR) mutants. The BH3 domain containing proteins BIM and PUMA can directly activate the proapoptotic proteins BAX and BAK to permeabilize mitochondria, leading to caspase activation and apoptosis. We delineated the signal transduction pathways leading to the induction of BIM and PUMA by tyrosine kinase inhibitors. Inhibition of the mitogen-activated or extracellular signal–regulated protein kinase kinase (MEK)–extracellular signal–regulated kinase (ERK) pathway caused increased abundance of BIM, whereas antagonizing the phosphoinositide 3-kinase (PI3K)–AKT pathway triggered nuclear translocation of the FOXO transcription factors, which directly activated the PUMA promoter. In a mouse breast tumor model, the abundance of PUMA and BIM was increased after inactivation of HER2. Moreover, deficiency of Bim or Puma impaired caspase activation and reduced tumor regression caused by inactivation of HER2. Similarly, deficiency of Puma impeded the regression of EGFRL858R-driven mouse lung tumors upon inactivation of the EGFR-activating mutant. Overall, our study identified PUMA and BIM as the sentinels that interconnect kinase signaling networks and the mitochondrion-dependent apoptotic program, which offers therapeutic insights for designing novel cell death mechanism–based anticancer strategies.