CRC has long been one of the biggest killers of human health worldwide, and some experts have even predicted that CRC may replace lung cancer as the new king of cancer. The morbidity and mortality of CRC stay at a high level, and the prevention and control situation is very severe [1]. Although it is recognized that CRC should be detected, diagnosed and treated as early as possible, unfortunately, most of the patients clinically diagnosed have already entered the advanced stage [21]. Chemotherapy is one of the few effective treatments they have, but if chemotherapy resistance ever occurs, the patients lose even this therapy means [13]. The pivotal role and unknown mechanism make more and more doctors and researchers focus on the chemotherapy resistance of CRC, hoping to make a breakthrough, reverse or eliminate chemotherapy resistance. In this era of big data, omics has become the backbone of bioinformatics analysis, playing an increasingly powerful role [22]. In this study, 174 upregulated and 58 downregulated DEGs were screened and identified from FOLFOX chemotherapy responders compared with non-responders. The GO biological enrichment analysis showed that muscle contraction was the most significantly enriched biological process, followed by muscular system processes and the regulation of muscle contraction. In the GO cellular component analysis, top of three were contractile fiber, myofibril, sarcomere. And structural constituent of muscle was the most prominently enriched molecular function in the GO analysis. In conclusion, the results of the GO biological enrichment analysis demonstrated that the composition of the muscle contraction system and its regulatory processes were related to the chemotherapy resistance of CRC. Such results were more or less surprising. However, it has been reported that the intracellular calcium concentration of drug-resistant cells is significantly higher than that of non-resistant cells, which makes drug-resistant cells more sensitive to calmodulin antagonism [23]. And calmodulin regulates muscle contraction and other processes in a calcium-dependent manner, meanwhile it is involved in the regulation of cell proliferation and cell cycle [24]. This is likely to bridge the link between muscle contraction and chemotherapy resistance in tumors. Furthermore, in the KEGG pathway analysis, the vascular smooth muscle contraction was the most enriched pathway, which also verified the above novel idea from another perspective. Both the vascular connection between the tumor and the body and the vascular components in the tumor microenvironment are closely related to the occurrence and development of tumors, of course, including the chemotherapy resistance of tumors [25, 26]. In fact, previous studies have focused more on the relationship between angiogenesis and tumor drug resistance, while ignoring the role of vasoconstriction in chemotherapy drug resistance, which may be a potential part worth studying in the future. Similarly, the GO descriptions of MCODE1 with the best-scoring were also smooth muscle contractions and muscle contractions, which fully demonstrated their close relationship with tumor drug resistance.
TPM1, TPM2 and MYH11 were top three genes among the identified hub genes. TPM1, a member of highly conserved tropomyosin family, is mainly expressed in the contractile system of striated and smooth muscles [27]. It is also an important component of the cytoskeleton in non-muscle cells [28]. There was study reporting that miR-21 can target and down-regulated TPM1, thus inducing resistance to 5-FU [29]. Shadan Ali et al. have demonstrated that the expression of miR-21 is markedly increased in drug-resistant pancreatic cancer cells, while the expression of TPM1 is observably decreased [30]. These findings suggested that TPM1 is significantly associated with chemotherapeutic resistance of tumors, which could be served as a potential biomarker, as well as target for future drug development. A member of the actin filament binding protein family, TPM2 is responsible for encoding β-tropomyosin, which is mainly distributed in type 1 chronic muscle fibers [31]. It is not only the main force of cytoskeleton, but also plays an important role in the physiological and pathological processes such as cell proliferation, migration and apoptosis [32]. Zhang J et al. have reported that comparing with normal breast cells, the expression of TPM2 was significantly down-regulated in breast cancer cells [33]. Moreover, this study also showed that down-regulation of TPM2 visibly reduced cell sensitivity to paclitaxel [33]. The above results fully indicate that TPM2 has great potential in the field of tumor drug resistance. And we will conduct further researches to explore its specific function in drug resistance of CRC in the future. Smooth muscle myosin encoded by MYH11 belongs to the myosin heavy chain family and is a contractile protein that converts chemical energy into mechanical energy through ATP hydrolysis [34]. This may be related to drug resistance caused by the efflux of chemotherapy agents. In fact, more research on MYH11 has focused on the destruction of core binding factors by CBFB/MYH11, which is an important step in the progression of acute myeloid leukemia [35]. At present, no studies have been reported on the mechanism between MYH11 and CRC or chemotherapy resistance. This is a broad field to be explored, and our future researches may fill in these gaps. Transcription factors are indispensable in gene expression. The SRF family, which ranks first among the many predicted up-regulated transcription factors, is reported to be a potential metastatic tumor molecule that regulates a variety of physiological and pathological activities including cell proliferation, angiogenesis, and epithelial mesenchymal transformation [20].
Additionally, in this study, we found an interesting phenomenon that mRNA expression of the identified hub genes was significantly decreased in CRC tissues compared with normal tissues, suggesting that they all function as tumor suppressor genes. However, survival analysis showed that poor prognosis of CRC patients was associated with high expression of hub genes, regardless of which hub genes were present. This obviously contradicts the above results. We made a bold prediction that these hub genes such as TPM1, TPM2, and MYH11 originally functioned as tumor suppressor genes in normal human body, but the roles of these hub genes would change with the onset and progression of CRC, acting as their protective umbrella or combustive improver under the influence of tumor cells. As is well-know that the microenvironment of tumor tissue is very different from the stromal environment in normal tissue. In order to facilitate their own migration and invasion, the expression of adhesion molecules in tumor cells is significantly reduced, leading lower adhesion force and easier shedding, which is the same reason for increased intercellular pressure. In most cases, tumor cells with higher expression hub genes associated with cell migration and adhesion are more likely to metastasize and invade than those with lower expression, which markedly results in reduced survival rate. Therefore, it is not surprising that high expression level of hub genes is associated with poor prognosis in CRC patients.