Bax deficiency mediated drug resistance can be reversed by endoplasmic reticulum stress induced death signaling

Abstract

Tumors often acquire drug resistance due to functional loss of pro apoptotic gene Bax, a critical and essential component of cell death rendering them insensitive to most anti-tumor agents. Compounds that can induce Bax independent apoptotic cell death are expected to overcome such drug resistance. We have employed a live cell based screening platform to identify potential compounds that can induce programmed cell death in Bax deficiency. Release of cytochrome C from mitochondria into the cytosol is a decisive initial event required for the caspase dependent cell death. We have engineered both wild type and Bax knock out colon cancer cells stably expressing cytochrome C with EGFP fusion protein to identify compounds that can trigger cytochrome C release in both cells with equal efficiency. In the fluorescent translocation assay, most of the drugs tested failed to induce cytochrome C release in Bax deficient cells validating the sensitivity of the assay. This study identified five lead compounds such as thapsigargin, tunicamycine, MG132, kaempferol and camptothecin that could induce cytochrome C release in both wild type and Bax deficient cells with equal potency. All the positive hits induced ER stress signaling as evidenced by up-regulation of Grp78. Studies with a Bak deficient cells indicate that Bak deficiency confers protection to cells from ER stress through autophagy. Further studies revealed that ER stress inducing agents are capable of triggering classical mitochondrial apoptotic cell death through the conformational activation of Bak, substantiating the potential of this pathway in designing drugs against Bax deficiency mediated drug resistance.