Effects of temsirolimus on mTOR protein activity in NCI-H1975 lung cancer cells

Cancer is a lethal disease which occurs due to the malfunctions within the cell cycle regulation. The mutated or injured cells which are normally have to die are allowed to progress through the cell cycle with a number of accumulated mutations and thus become cancerous. Recently, the development of effective anti-cancer agents that target the cell cycle checkpoints have received much attention. mTOR is an important component of biosynthetic process that regulates eukaryotic cell survival, growth and proliferation. mTOR exists as structurally two distinct protein complexes; mTOR complex 1 (mTORC1) and complex 2 (mTORC2). mTORC1 is involved in rapamycin-sensitive control of cell growth by regulating the cell cycle checkpoints and is activated by Akt and regulation of cellular energy. Conversely, mTORC2 acts in rapamycin-insensitive control of cell growth. Inhibition of these protein complexes results in diminished cell growth and proliferation, metabolism and angiogenesis and therefore leads cell cycle block at the G1 phase. Upregulation of mTOR activity is evident in many types of cancers including NSCLC. In this respect, several compounds that selectively inhibit mTOR activity have been developed for clinical use. As an analogue of rapamycin, Temsirolimus (CCI-779), was synthesized to specifically inhibit mTOR by directly acting on mTORC1. It was reported that temsirolimus exerts antitumoral effects on renal cell carcinoma, breast cancer, glioblastoma, neuroendocrine carcinomas and mantle cell lymphoma. It is known that inhibition of mTOR protein leads to autophagic cell death. Autophagy is a process in which cellular components such as proteins and organelles are engulfed by autophagosomes, and are transmitted to the lysosomes for degradation. Autophagy can facilitate cell survival by maintaining cellular energy, homeostasis and macromolecular synthesis during cellular stress and nutrient deprivation. Many reports indicated that mild and/or slow autophagy may enhance cell survival while more severe and/or rapid autophagy would induce cell death. There are several anticancer agents which induce autophagy and thereby sensitize cells to drug treatment. Recently autophagy is considered to be an innovative therapeutic target for tumor cells which are resistant to anti-tumor drugs. Based on these literatural results, we questioned whether temsirolimus treatment could be a potential therapeutic option for NSCLC. In this study, we evaluated the cytotoxic effects of temsirolimus in NCI-H1975, a type of NSCLC, in vitro. Temsirolimus exerted a time and dose dependent cytotoxic activity against NCI-H1975 cells. Temsirolimus at 0.9 µg/ml concentration, yielded % 31.4 inhibition in cell viability at the end of 72 hours by activating caspase -3, -8 and -9 enzymes. Temsirolimus inhibited mTOR activity only at the end of 24 hours in NCI-H1975 cells.