Introduction
Colon cancer also known as colorectal cancer (CRC) or bowel cancer, is the fourth most common cancer and the third leading cause of cancer-related deaths among all cancers worldwide (Bray et al., 2018). The progression of CRC is associated with multiple genetic alterations, such as the activation of Ras, the overactivation of the phosphatidylinositol 3-kinase pathway, mutation of p53, and Wnt pathway dysregulation (Wang & Zhang, 2014). In addition, emerging data have implicated frequent activation of mTOR with the progression of most cancers, including CRC, (Francipane & Lagasse, 2013; Tian, Li, & Zhang, 2019) which represents mTOR as a compelling therapeutic target for cancer treatment.
The mammalian or mechanistic target of rapamycin (mTOR) is a serine/threonine protein kinase that regulates many cellular functions, such as protein synthesis, cell proliferation, growth, survival, metabolism, autophagy and senescence, through two distinct multiprotein complexes named mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) (Johnson, Rabinovitch, & Kaeberlein, 2013; Laplante & Sabatini, 2012). mTOR regulates normal cellular functions through integrating signals from its upstream proteins PI3K and AKT (the PI3K/AKT/mTOR pathway). Dysregulation of this pathway leds to the development of several fatal diseases, including metabolic, cardiovascular, neurological, and immunological diseases and cancers (Tsang, Qi, Liu, & Zheng, 2007). Since the mTOR pathway is dysregulated in many cancers, inhibition of mTOR represents a compelling therapeutic target for cancer treatment.
Depending on the inhibitory characteristics of mTOR complexes (mTORC1 and mTORC2), mTOR inhibitors are classified into two generations. Rapamycin and its analogs are considered first-generation mTOR inhibitors due to their ability to inhibit only mTORC1 activity (Mecca et al., 2018). Although prolonged exposure to rapamycin can also partially inhibit mTORC2, the inhibition is transient and not efficient for cancer treatment (Sarbassov et al., 2006). Therefore, efforts have been made to develop second-generation mTOR inhibitors that can efficiently inhibit both mTOR complexes. To date, numerous second-generation mTOR inhibitors have been developed to efficiently treat cancers. PP242 is an ATP-competitive second-generation inhibitor of mTOR that exerts its anticancer activity on numerous types of cancer. Previous studies have reported that PP242 can inhibit the proliferation, migration, invasiveness and stemness of glioblastoma cells (Mecca et al., 2018), induce apoptosis of acute myeloid leukemia and myeloid leukemia cells (Janes et al., 2010; Zeng et al., 2012), and suppress cell proliferation and migration of bladder and gastric cancer cells (Xing et al., 2014; Z. Zhang, Zhang, Kong, & Gong, 2016). Furthermore, PP242 also reduced the growth, proliferation and survival of colon cancer cells, and in combination with cetuximab, PP242 shows synergistic activity towards colorectal carcinoma (Blaser et al., 2012; Cheng et al., 2015). However, the underlying mechanism of the effects of PP242 on the basis of the metabolome is still not clearly understood.
Metabolomics, the comprehensive study of small molecule metabolites, appears to be a robust tool to assess diverse biological samples, including cells, biofluids and tissues, in response to biological stimuli, such as disease and drugs (Qiu et al., 2009; Zheng et al., 2017). Lipidomics, which is a subset of metabolomics, mainly contributes in the evaluation of various lipid species in complex biological samples (Murfitt et al., 2018). Combined metabolomics and lipidomics approaches have been widely applied and used assess the pathophysiological pathways related to disease progression, identify biomarkers and monitor the underlying mechanism of drug effects and toxicity (Rivera- Velez et al., 2019).
In this study, we examined the effects of the second-generation mTOR inhibitor PP242 in LS174T cell-induced colon cancer xenograft model mice in order to reveal the underlying mechanism of PP242. Comprehensive metabolomics and lipidomics profiling approaches using mouse plasma and tumor tissues were applied to investigate PP242 effects using an UHPLC-Orbitrap-MS system.