Currently, there is no information about the clinical significance of CERS genes in CRC progression. High expression levels of ceramide have been reported in patients with ovarian, lung and colorectal cancer (1, 15) (16). In the present study, we comprehensively analyzed genes that contribute to ceramide synthesis and metabolism using colorectal cancer tissue. This study revealed two things. First, significant differences in gene expression between normal tissue and colorectal cancer tissue were observed for ACER3, CERS2, and CERS6. Second, we compared the expression of genes involved in ceramide synthesis and metabolism for KRAS mutations in CRC. It was revealed that CERS4 expression was significantly lower in KRAS mutant CRC tissues compared with wild-type KRAS CRC tissues.
Several relationships between gene expression related to ceramide synthesis and metabolism and cancer sites have been reported. Zhang, X.et al. reported that the expression level of CERS2 was decreased in highly metastatic ovarian cancer cells (1). In ovarian cancer, Sheng et al. reported an association of high expression of CERS2 with an unfavorable prognosis in patients (17) (18).
Some reports have suggested that overexpressed CERS6 promoted cancer invasion in lung cancer. They showed that overexpressed CERS6 in lung cancer synthesizes a bioactive lipid called C16 ceramide, which activates the intracellular protein kinase, RAC1 complex. As a result, morphogenesis called lamellipodia, which is essential for cell migration, occurs on the cell surface, and cancer cells metastasize (2, 16).
In the present study, expression of CERS4 was reduced in KRAS mutant colorectal cancer. There are two possible pathways for this result. The first is that the Wnt pathway signal is involved in the regulation of CERS4 or the Wnt pathway may be involved in the KRAS mutation in colorectal cancer. Peters et al. used mice to show that CERS4 is highly expressed in the epidermis of adult mice and is localized in defined populations within the interfollicular epidermis and hair follicle sebaceous unit. They reported that decreased bone morphogenetic protein signaling in CERS4-/-mice may promote Wnt / β-catenin signaling and strongly stimulate the activation of hair follicle stem cells (17).
The other possibility is that there is a relationship between KRAS mutations and the NF-kB (NF kappa B) pathway. NF-κB (NF kappa B) is a protein complex that acts as a transcription factor. Five types of proteins (NF-κB family) are known in mammals: p50, p52, p65 (RelA), c-Rel, and RelB (18) (19). It has been reported that p65 activity, which is one of the NF-KB family, was higher in tumors with KRAS mutation (50.8%) than in tumors with the wild-type KRAS gene (30.6%) (P = 0.012) (20). In addition, there is a report that NF-KB activation is involved in KRAS mutant colorectal cancer, with NF-kB activity being higher than it is in wild-type KRAS colorectal cancer (21). High expression of CERS4 was observed in liver cancer tissues, and it has been reported that the nuclear factor (NF)-κB signaling pathway was affected after knockdown of CERS4 in liver cancer cells (22). Thus, even in KRAS mutant colorectal cancer, CERS4 expression may be reduced due to the influence of the NF-kB pathway.
It has been reported that overexpression of CERS4 and CERS6 in colon cancer cells induced the production of short-chain ceramides (C16: 0, C18: 0 and C20: 0 ceramides), weakened cell proliferation and promoted apoptosis. (23). This is consistent with the poor prognosis of G12V and G12C mutation colorectal cancers we reported compared to wild-type KRAS colorectal cancers (12) (13). The present finding that CERS4 functions as an important regulator of KRAS mutation, may suggest its potential use as a marker for CRC and may also guide the development of new drugs for CRC.
Sphingosine 1-phosphate (S1P) is a blood-borne lipid mediator implicated in the regulation of vascular and immune systems. Blood flow and circulating S1P activate endothelial S1P1 to stabilize blood vessels in development and homeostasis (24). Ceramides are apoptosis-inducing sphingolipids and precursors of other bioactive sphingolipids such as S1P. When ceramide is downregulated, its downstream S1P is also downregulated. This may be why low-CERS4 cancers had more venous invasion.
This study’s limitation was that it included patients from just a single institution. Our study findings need further review and validation in more CRC patients.