Heteronemin and Tetrac Induce Anti-proliferation by Blocking EGFR- mediated Signaling in Colorectal Cancer Cells

The mutations in KRAS can be found in 30%~40% of colorectal cancer (CRC) patients. Anti-epidermal growth factor receptor (EGFR) monoclonal antibodies are effective only in CRC patients with KRAS wild-type (WT) status. To discover new therapeutic strategies urgently need for colorectal cancer patients. Sponge extract, heteronemin, has been exerted antitumor effect against several cancer types but not in colorectal cancers. Thyroid hormone analogue, Tetraiodothyroacetic acid (tetrac) has been shown to suppress Ras-mutant cancer cell growth. This study intended to explore the combined inhibitory effect of heteronemin and tetrac, and also its molecular mechanism(s) of action in HT-29 cells (KRAS WT cells) and HCT-116 cells (KRAS MT cells). Cell viability was investigated using a CyQUANT® cell proliferation assay. Protein expressions levels and the percentages of a cell population in the different phases of the cell cycle were respectively determined by Western blotting and flow cytometry. Gene expression profiles were detected by NanoString technology as well as RT-qPCR. Heteronemin in combination with tetrac suppressed the growth of mutant and wild-type KRAS CRC cells. Its combination also caused sub-G1 and S phases cell cycle arrest in both cell lines. In KRAS MT cells, heteronemin combined tetrac downregulated genes involved in cancer progression such as EGFR and decreased cell motility by Nanostring technology. The combination treatment exhibited the inhibition of EGFR signaling through the reduction of phosphorylated ERK1/2 protein expressions in both CRC cell lines. Besides, inactivation of phosphorylated and total PI3K protein were induced by combined treatment in KRAS WT cells. Furthermore, PD-L1 protein levels were decreased after treated KRAS WT cells with tetrac, whereas its expression was downregulated by heteronemin combined tetrac in KRAS MT cells. Hence, tetrac enhances the antitumor effect of heteronemin, and its combination might be a potential candidate for the treatment of CRC to overcome KRAS mutation-acquired resistance to anti-EGFR therapy.