Colorectal cancer has one of the highest incidences among all forms of cancer around the world. In many cases, the cancer cannot be completely controlled by surgical intervention, so multidisciplinary treatment including chemotherapy is required. However, cancer cells often acquire drug resistance during treatment and a patient's prognosis can become very unfavorable [
1]. Acquisition of chemo-resistance not only reduces the effectiveness of drugs, but also promotes side effects and markedly reduces the patient's quality of life.
5-fluorouracil (5-FU), a fluorinated pyrimidine, is a key anti-colorectal cancer drug. 5-FU affects the synthesis and repair of DNA and RNA processing in cancer cells [
2‐
4]. 5-FU metabolic enzymes, such as thymidylate synthase (TS), orotic acid phosphoribosyltransferase (OPRT), uridine phosphorylase (UP), dihydropyrimidine dehydrogenase (DPD), and pyrimidine nucleoside phosphorylase (PyNPase), are thought to play a role in the resistance mechanism [
5‐
8].
Oligosaccharides on glycoproteins mediate a dynamic protein state, involving folding, quality control, secretion and catabolism [
9,
10]. Glycans are also related to tumor progression and metastasis as well as to immune system activity [
11], and their potential relationship to chemo-resistance has recently been examined [
12,
13]. In N-glycan biosynthesis, a precursor high-mannose type oligosaccharide, consisting of 2 N-acetylglucosamine (GlcNAc), 9 mannose (Man) and 3 glucose (Glc) residues, is synthesized on a dolichol di-phosphate and transferred to a nascent polypeptide in the endoplasmic reticulum. During protein folding, one Man and three Glc residues are removed to form an M8.1 high-mannose type N-glycan. This oligosaccharide residue functions as a tag to carry correctly folded glycoproteins to the Golgi apparatus, while misfolded proteins are recognized by the protein degradation system [
14]. In the Golgi, α-mannosidase I removes a further 3 Man residues from M8.1 to form M5.1, then N-acetylglucosaminyltransferase I attaches a GlcNAc residue to M5.1, forming the hybrid-type oligosaccharide. Next, α-mannosidase II removes two Man residues and N-acetylglucosaminyltransferase II adds another GlcNAc to form complex-type N-glycans. These glycans are modified by galactose, fucose and sialic acid residues to form a variety of oligosaccharide structures [
15]. Swainsonine, a known glycosylation inhibitor [
16,
17], inhibits α-mannosidase II activity in the N-glycan biosynthesis pathway and blocks production of complex-type oligosaccharides [
18,
19]. Swainsonine has been of great use in the study of N-glycan functions, with many important results published since its discovery [
20‐
22]. The anti-tumor activity of swainsonine has also been previously examined [
23]. Swainsonine exhibits not only cytotoxicity, but inhibits cancer cell metastasis [
24,
25], decreases the toxicity of chemotherapeutic drugs [
26,
27] and works as immunomodulator [
28,
29]. Despite its side effects, clinical studies on patients have shown that swainsonine is of some benefit as a chemotherapeutic drug [
30,
31], suggesting that it might have further applications in this field. Tunicamycin, which inhibits N-glycosylation, has been shown to enhance sensitivity to cisplatin [
32] and reduce drug-resistance in multidrug-resistant carcinoma cells [
33].
We established various gradations of 5-FU resistant cell lines from a mouse colon cancer cell line and analyzed the expression enzymes related to resistance, the effect of swainsonine and the glycoforms present in those cells.