Prostaglandin E PGE is involved in several
Prostaglandin E2 (PGE2) is involved in several biological processes such as renal function, inflammation, angiogenesis, and tumor growth. The various biological effects of PGE2 are mediated by the so-called E-type prostanoid receptors (EP1 to EP4). Among these, the EP4 receptor has been well studied in cancer. PGE2-mediated stimulation of EP4 triggered multiple signaling pathways and led to diverse patho-physiological responses (Tonisen et al., 2017). Activation and/or high expression of EP4 were observed in several malignancies including lung cancer, and associated with growth, progression, and overall survival (Bhooshan et al., 2016; Nandi et al., 2017; Parida et al., 2016; Tveteraas et al., 2012; Zhang et al., 2017). We previously observed that natural phytochemicals from traditional medicinal plants such as solamargine inhibited the growth of NSCLC sphingosine 1 phosphate receptor through the inhibition of EP4 gene expression and its downstream targets (Chen et al., 2015; Huang et al., 2017). These results indicated an important role of EP4 in lung cancer growth and progression. However, the functional links between lncRNAs and EP4 have not been described and the detailed role of EP4 in lung cancer occurrence and progression still remains to be determined.
In the present study, we performed both in vitro and in vivo experiments to explore the potential mechanism by which XJD sensitized the effects of gefitinib in the growth inhibition of human lung cancer cells. Our results showed that XJD enhanced the inhibitory effect of gefitinib on lung cancer cell growth via extracellular signal-regulated kinases 1 and 2 (ERK1/2)-mediated suppressions of lncRNA HOTAIR and transcription factor SP1, resulting in reduced EP4 gene expression both in vitro and in vivo.
Materials and Methods
Discussion TCM compositions usually include multiple herbs and components that act in complementary fashion, have minimal toxicities, and offer therapeutic potential for patients (Tao et al., 2015). XJD is a TCM and herbal prescription that has been used for treatment of patients with advanced lung cancer and has been shown to substantially improve the quality of life and patient survival (Chai et al., 2014). We previously showed that XJD inhibited growth of NSCLC cells via AMPKα-mediated inhibition of Sp1 and DNMT1 expression both in vitro and in vivo (Zhao et al., 2016). However, due to the potential interactions among the multiple compounds and the complexes of metabolic processes that can result in various responses and outcomes, the molecular mechanisms underlying the therapeutic potential for XJD are not fully understood. In this study, cell growth inhibition and cell arrest by XJD were observed in other NSCLC cells, further demonstrating the inhibitory effects of XJD on NSCLC cells in vitro. We previously showed consistent reproducible results of batch-to-batch XJD solutions, and the tested dose ranges exhibited no toxicities in vitro or in vivo (Zhao et al., 2016). In the current study, the XJD dosages used in vitro and in vivo were determined from a series of cell-based cell growth experiments (Zhao et al., 2016) and the reported formula to calculate the dosage difference between human and mouse based on body surface area (Reagan-Shaw et al., 2008a, Reagan-Shaw et al., 2008b). Nevertheless, more experiments to determine the effective doses and toxicities in both in vitro and in vivo studies should be determined in future studies. Additionally, the relevant clinical information for potential toxicities in patients needs to be evaluated in the future for improved understanding of the active components and potential complex regulatory signaling networks, and metabolic processes of XJD. Overall, additional experiments, such as in–depth chemical fingerprints and pharmacokinetic studies are needed to characterize the main components and biological functions of XJD. We demonstrated the involvement of the ERK1/2 signaling in the inhibitory effect of XJD on NSCLC cell growth. Consistent with our findings, the activation of ERK1/2 is involved in anti-tumor effects in various cancer types (Chang et al., 2014; Kumari et al., 2017; Li et al., 2017), although conflicting findings such as the inactivation of ERK in anti-cancer mechanisms have also been shown (Cao et al., 2016; Chai et al., 2015; Zhao et al., 2017a). We previously observed that emodin, a natural anthraquinone from Rheum palmatuma, inhibited the growth of lung cancer cells through ERK-mediated regulation of transcription factors peroxisome proliferators-activated receptor gamme (PPARγ) and SP1. These changes ultimately induced insulin-like growth factor (IGF) binding protein 1 (IGFBP1) gene expression (Tang et al., 2017). The results above implied dual effects of ERK1/2 in cancer biology (Fong et al., 2017). Clearly, further work is needed to determine the mechanistic details of modulating ERK1/2 signaling on anti-tumor effects.