Colorectal cancer (CRC) is recognized as the third most common malignant tumor due to its high incidence rate in the world. In 2020, there were 1.93 million new cases diagnosed as CRC and 0.94 million deaths of colorectal cancer worldwide [20]. In recent years, although great progress has been made in the diagnosis and treatment of CRC, metastasis of cancer cells still poses a big challenge for CRC patients. Cetuximab (CET), a monoclonal antibody targeting epidermal growth factor receptor (EGFR), significantly improves the prognosis of patients with advanced CRC [3]. However, the influences of multi-factors such as genetic mutation (RAS, BRAF) contribute to CET resistance for metastatic CRC patients [16]. To further prevent CET resistance in metastatic CRC patients, therefore, the molecular mechanisms of CET resistance in CRC need to be investigated.

Growing evidence has proposed that tumor-associated macrophages (TAMs) in tumor microenvironment play an indispensable role in the regulation of CRC progression [23], [33]. M2 macrophages in tumor microenvironment effectively promoted the capabilities of tumor cell proliferation, metastasis and even exerted drug resistance of tumor [4], [7], [10]. For instance, Lan et.al found that M2 macrophage polarization in TAMs prompted oxaliplatin (OX) resistance in CRC via m6 A modification of TRAF5 which was affected by METTL3 medication [10]. Forkhead box O1 (FOXO1), a well-known transcription factor, is extensively investigated in tumor area [5]. It was reported that FOXO1 was a promotive regulator for M2 macrophages polarization in TAMs in esophageal squamous cell carcinoma (ESCC) [22]. Of note, in ovarian cancers, FOXO1 silencing could attenuate paclitaxel resistance in paclitaxel-resistant cell line [22]. However, whether FOXO1 mediates M2 macrophages polarization to affect CET resistance to CRC remains largely unknown.

Long noncoding RNA (LncRNA) with more than 200 nts in length is a member of non-coding RNAs. At present, lncRNAs were reported to be implicated to the progression of CRC through affecting cell proliferation, metastasis and drug resistance [6], [19]. For instance, CAF-derived exosome carrying lncRNA H19 promoted chemoresistance (oxaliplatin) of CRC [19]; lncRNA CRNDE targeting miR-181a-5p/Wnt/β-catenin axis facilitated 5-fluorouracil (5-Fu) resistance to CRC[6]; suggesting lncRNAs are intimately correlated with drug resistance in CRC. LncRNA HCG18 functioned as a promotor for development of multi-tumors such as osteosarcoma, renal carcinoma and CRC [12], [29], [32]. Xu et.al. put forward that HCG18 suppressed CD8+ T cells to promote CET resistance to CRC through regulating miR-20b-5p/PD-L1 axis [27]. At present, the mechanism by which HCG18 drives drug resistance to CRC is still insufficient.

In this study, CRC cells with CET treatment and a co-culture system of CRC cells and M0 macrophage were employed to investigate how HCG18 targeting miR-365a-3p/FOXO1/colony stimulating factor 1 (CSF-1) axis mediates M2 macrophage polarization to promote CET resistance to CRC. Our findings provide a novel regulatory mechanism of CET resistance involving in M2 macrophage polarization, which will assist in-depth understanding of CET resistance to CRC.