1. Introduction
Metabolic syndrome (MS) is an intractable disease, and it may initiate from obesity and then develop into systemic disorders, such as type 2 diabetes, hyperlipidemia, arteriosclerosis, and non-alcoholic steatohepatitis (NASH) [
1,
2]. Diabetes characterized with high blood glucose level has been regarded as a risk factor for NASH-related cirrhosis and hepatocellular carcinoma (HCC) [
3,
4,
5]. Given the complexity of diabetes patients’ backgrounds and their genetical susceptibility, a question arose regarding whether a high blood glucose level could be a causal factor for liver carcinogenesis in the diabetic population. Tsumura Suzuki obese diabetic (TSOD) mice without any diet intervention spontaneously display typical phenotypes of metabolic syndrome, including severe obesity, type 2 diabetes, hyperinsulinemia, steatohepatitis, and HCC [
6,
7,
8]. TSOD mice are a multifactorial inherited diabetes model that are inbred from closed-colony ddY mice for several generations by multiplying individuals with obesity and high urinary sugar levels [
9]. The onset of pathological conditions in TSOD mice might result from multiple factors similar to those found in human diabetes. In this study, we monitored TSOD mice regarding changes in blood glucose levels, body weight, and pancreatic and liver lesions over time. Furthermore, we investigated the effect of dietary intervention with various kinds of oligosaccharide in TSOD mice in terms of lowering blood glucose levels from a young age. It has been reported that the regulation of intestinal microbiota with healthy and balanced foods could prevent the occurrence and progression of MS-related diseases [
10]. Recently, increasing reports have shown evidence that oligosaccharides improve the intestinal environment and various symptoms of metabolic syndrome [
11]. Our previous study also showed the treatment effect of fructooligosaccharides (FOS) for NASH in a diet-induced NASH model and an MSG-induced NASH model [
12,
13]. In the FOS-administered group, the level of total short-chain fatty acids (SCFAs) increased compared to the vehicle group, which indicated that FOS administration might improve intestinal microbiota. As SCFAs have been reported to prevent colonic mucosa and pancreatic beta-cells from being damaged, SCFAs have been considered as key factors for ameliorating MS and NASH [
13]. Various types of oligosaccharides are on the market; however, they lack head-to-head comparison studies regarding their efficacy in NASH. In this study, we showed the protective effect of milk-related oligosaccharides (galactooligosaccharide: GOS and lactosucrose: LS) as well as FOS on lowering blood glucose level, preventing pancreatic beta-cell damage in TSOD mice.
2. Materials and Methods
2.1. Ethics Statement
All the institutional and national guidelines for the care and use of laboratory animals were followed. This study was performed in alignment with the animal experiment guidelines specified by the Institute for Animal Reproduction (Ibaraki, Japan), (Permission number: IarAW No. 2018-N565), where the rules of guidance on animal research ethics from the International Association of Veterinary Editors’ Consensus Author Guidelines on Animal Ethics and Welfare were strictly abided by.
2.2. Animals and Experimental Design
Experiment 1: To investigate the impact of blood glucose level on the frequency of hepatic carcinogenesis at a young age in TSOD mice.
TSOD mice fed with a murine-certified diet (MF, Oriental yeast, Tokyo, Japan) developed obesity around eight weeks of age and then spontaneously developed type 2 diabetes, hyperlipidemia, and human NASH-like steatohepatitis in sequence at around six months of age. In our previous studies, liver tumors appeared in TSOD mice after eight months of age and the frequency increased over the time; for instance, over 80% of TSOD mice showed liver tumors after 12 months of age [
7]. In this study, based on blood glucose levels, male TSOD mice were divided into a hyperglycemic group (
n = 25) and a non-hyperglycemic group (
n = 24) at 12 weeks of age, and eight mice from each group were sacrificed at 32, 36, and 40 weeks of age for time-course analysis. Bodyweight was monitored for all experimental mice at 4, 5, 6, 12, 16, 24, 32, and 40 weeks of age. At each termination point, animals were first fasted for 12 h and then liver and pancreas were collected under anesthesia with carbon dioxide. After a gross necropsy was performed, the organs were fixed in 10% neutral buffered formalin, embedded in paraffin, and stained with hematoxylin and eosin (HE) for microscopy.
Experiment 2: To investigate the effect of dietary oligosaccharides on suppressing blood glucose level at a young age in TSOD mice.
Male TSOD mice showing hyperglycemia at 8 weeks of age and fed with a murine-certified diet (MF) were selected and randomly divided into 4 groups, with 4–5 mice per group; these mice were treated with FOS, GOS, or LS. Five mice without any treatment were employed as controls. Assessment started at 12 weeks of age, before the onset of MS symptoms such as obesity based on bodyweight, and it continued up to 24 weeks of age, when all mice sequentially developed the phenotypes of diabetes. Each group was continuously administered an MF with 5% oligosaccharide in drinking water ad libitum. Based on the water intake of TSOD mice per day, this amounted to approximately 4–6 mL; 0.005 μg of oligosaccharide per mouse per day was also administered. The general life signs of the animals were observed once a day during the study. All animals were weighed once a week throughout the study. The animals were fasted for 12 h before being sacrificed, and liver and pancreas were collected under anesthesia with carbon dioxide. After fixation in 10% neutral buffered formalin, representative liver slices and a maximum cut surface of pancreas were embedded in paraffin and then cut at 5 μm thickness for morphological assessment by HE staining.
2.3. Blood Glucose Level Measurement
Blood glucose levels were measured between 10 am and 12 pm at 6, 12, 16, 24, 32, and 40 weeks of age in experiment 1 and once a week in experiment 2 using a Stat Strip Express 900 device (Nova Biomedical, Waltham, MA, USA).
2.4. Urine Sugar Measurement
In experiment 2, the urine sugar level of animals was measured once a week, starting from 11 weeks and up to 24 weeks of age. Fresh urine of the animals was collected and then applied on Uro-paper IIIG (Eiken Chemical, Tokyo, Japan). The scores of urine sugar levels from 0 (−) to 5 (++++) were determined by following the instructions of the manufacturer.
2.5. Glucose Tolerance Test
In experiment 2, glucose tolerance tests were performed on the control, GOS, and LS groups at 23 weeks of age. After fasting for 17 h, fasting blood glucose levels were measured using a Stat Strip Express 900 device (Nova Biomedical, Waltham, MA, USA). Subsequently, 150 mg/mL of glucose was applied orally and blood glucose levels were measured at 60 and 120 min post-administration in examined animals.
2.6. Morphological Assessment of the Liver
In experiment 1, after fixing the liver with 10% neutral buffered formalin, whole livers were cut through at 2 mm intervals and examined grossly for tumor appearance. All sections with tumors were embedded in paraffin and thin-cut to 2 μm for microscopic examination. Routine HE staining and immunostaining using rabbit-polyclonal anti-glutamine synthetase (GS) antibody (rab73593, Abcam plc, Cambridge, UK) were performed to identify liver tumors. In experiment 2, for NASH evaluation, hepatic steatosis, lobular inflammation, ballooning, and fibrosis were scored according to NAS scoring [
14]. Since no liver tumor was found at 24 weeks of age in the previous report, we only performed histological assessments for liver tumors after 24 weeks.
2.7. Morphological Assessment of the Pancreas
In both experiment 1 and experiment 2, the destructive degree of islets of Langerhans was evaluated at three levels: no change, mild change, and severe change, based on the beta-cell expression by immunostaining of insulin (rabbit monoclonal, EP125, Epitomics, Burlingame, CA, USA). In addition, the area of islets of Langerhans was measured by image analysis software (CellSens Standard, Olympus, Tokyo, Japan).
2.8. Analysis of mRNA Expression in Liver
In experiment 2, we analyzed the expression of hepatic lipid metabolism-related genes, such as carnitine palmitoyltransferase (
Cpt-1a), fatty acid synthase (
Fasn), fatty acid transport protein 5 (
Fatp5), cluster of differentiation 36 (
CD36), carbohydrate response element binding protein (
Chrebp) and microsomal triglyceride transfer protein (
Mttp) and glycogenesis-related genes including glucose-6-phosphatase (
G6pase), phosphoenolpyruvate carboxykinase 1 (
Pepck1) and
Pepck2. Total RNA was isolated using a bead crusher (Micro Smash MS-100, TOMY, Tokyo, Japan) and a RNeasy Mini Kit (Qiagen, Hilden, Germany). cDNA was synthesized using ReverTra Ace qPCR RT Master Mix (Toyobo, Osaka, Japan) and then stored at −20 °C until we performed a StepOne real-time PCR (Applied Biosystems, Waltham, MA, USA). The sequences of the synthetic primers are listed in
Table 1. The relative ratio of each gene expression level to that of beta-actin was calculated using the 2
−ΔΔCt method and expressed as fold of levels in controls.
2.9. Analysis of Plasma SCFAs
Plasma levels of acetate, propionate, butyrate, isobutyrate, 2-methylbuthyrate, and capronate of mice were examined using UPLC-ESI-MS/MS. Reference materials were purchased from Wako Pure Chemical Co. (Tokyo, Japan). Internal standards (IS), PA- deuterium (d) 6, BA-d5, VA-d9, and CA-d11 were obtained from Sigma-Aldrich Co. (St. Louis, MO, USA) and CDN Isotopes Co. (Pointe-Claire, QC, Canada).
2.10. Statistical Analyses
Statistical difference compared among groups was analyzed by Mann–Whitney’s U test and ANOVA with Dunnett’s post hoc analysis.
4. Discussion
TSOD mice spontaneously develop obesity, type 2 diabetes, hyperlipidemia, NASH, and eventually develop a high frequency of liver tumors after 18 months of age [
7,
16]. A TSOD mouse with a genetic breeding background but no special diet intervention exhibits a characterized model mimic of human metabolic syndrome. In addition, although TSOD mice are genetically homologous, significant individual variations appear with various pathological phenotypes, with some showing hyperglycemia and some not even doing so at eight weeks of age. In this study, TSOD mice with and without hyperglycemia at 12 weeks were parallelly assessed for metabolic syndrome over the study period, and the incidence of liver tumors was also examined. It was found that TSOD mice without hyperglycemia at 12 weeks of age constantly maintained low blood glucose levels up to the end of the study, and their bodyweight was significantly lower than that of mice with hyperglycemia. In our study, we found that liver tumors occurred in about half of the hyperglycemic TSOD mice at 32 weeks of age; in contrast, no tumors were found in the non-hyperglycemic group at this age. That said, liver tumors were confirmed in the non-hyperglycemic mice with at a rate of 12.5% at 36 weeks of age and up to 50% at 40 weeks of age. These data suggested that the onset of hepatic carcinogenesis was delayed in the non-hyperglycemic TSOD mice; in other words, hyperglycemia might accelerate hepatic carcinogenetic signaling in TSOD mice due to their genetic susceptibility. These findings are well aligned with current studies showing a strong association between diabetes and hyperglycemia with carcinogenesis in various organs [
17,
18,
19].
Recently, accumulating studies had shown evidence that the disturbance of intestinal microbiota might play an important role in metabolic diseases [
20]. Various attempts have been undertaken to change intestinal flora, such as prebiotics, probiotics, and combined synbiotics [
21]. It has been reported by many studies that oligosaccharides lower blood glucose levels with different modes of action speculated among the types of oligosaccharides such as immune response [
22]. In order to focus on functional food ingredients that could be safely taken by patients in their diet, we chose to add oligosaccharides into a murine-certified diet based on their therapeutic effects on metabolic syndrome and capacity to improve intestinal bacterial flora [
12,
23,
24]. As expected, milk-derived oligosaccharides alleviated blood and urine glucose levels and restored the integrality of pancreatic islets of Langerhans in TSOD mice. These results implied that a direct or indirect beta-cell protection was exerted by the administration of milk-derived oligosaccharides. Since mild NASH-like liver pathology was found in the control group, the therapeutic window was not large enough to evaluate the efficacy of oligosaccharides against liver inflammation and fibrosis, at least not in this study. TSOD mice comprise a spontaneous model, and strong liver fibrosis could not be confirmed. We considered that the present results are insufficient for evaluating the anti-fibrotic potential of oligosaccharides. We recently developed a diet-induced NASH model showing strong liver fibrosis [
24]. An additional study using a severe liver fibrosis model is required.
As for metabolic disturbance, improvements in fatty degeneration were observed in the oligosaccharide-treated groups. To unveil the underlying metabolic actions in the liver, we examined lipid-metabolism related genes such as Cpt-1a, Fasn, Fatp5, CD36, ChREBP, Mttp, and glycogenesis-related genes including G6pase, Pepck1, Pepck2. However, all genes examined in the present study did not reveal the evidence of oligosaccharide-induced fatty liver suppression. One possibility for lacking significance between the treatment groups vs. controls in this study was considered that the degree of hepatic fatty degeneration was too mild in the control group. As mentioned above, the advantage of TSOD mice is their spontaneous development of various metabolic syndrome, however, the disadvantage is the NASH-like pathogenic phenotypes less severe than other NASH models. Therefore, these metabolic genes in the diet-induced NASH model will be further investigated.
Regarding the morphological changes in islets of Langerhans in the pancreas and their increased areas in hyperglycemic TSOD mice, it is likely that these changes reflect the compensatory swelling of beta-cells under insulin resistance; this is because of the exocrine gland’s invasion into islets and beta-cell fragmentation clustering coincidently. Moreover, the degree of islet destruction was strongly correlated with the occurrence of liver tumors. Therefore, our results propose a novel clinic perspective insofar as the risk of hepatic carcinogenesis could be predicted by the distortion degree of islets of Langerhans at a younger age. Based on the protective effects of milk-related oligosaccharides on beta-cells, we hypothesized that the potential underlying mode of action might be SCFAs produced from oligosaccharides by intestinal bacteria, but no significant changes in serum SCFA levels was observed. More sophisticated analysis on either the amount or organ-specific localization of SCFAs will be further investigated, since beta-cells have SCFAs receptors [
25,
26], and several studies suggest the effect of propionic acid and butyric acid on improving insulin sensitivity and preventing inflammation. [
27,
28,
29,
30].
A limitation of the present study is that we were unable to analyze factors related to metabolism and tissue damage, such as hormone and cytokine activities, over time. In the present study, only males were used in the experiments due to the severity of the condition; however, a new experimental design that takes into account the differences between males and females is necessary to consider the relationship between these factors regarding their response. Since female TSOD mice also develop metabolic syndrome and liver tumors, albeit mildly, we plan to conduct additional research using male and female TSOD mice in our next study.
Contrary to expectations from our previous study [
13], we were intrigued that various items were exacerbated by FOS treatment. The mice of FOS group gained weight but did not improve blood glucose levels. Oligosaccharides are not readily absorbed in the small intestine, and it is commonly believed that they cause little weight gain even when loaded with sugar [
31]. However, Liu et al. warned that short-term prebiotic administration might have adverse effects on glucose metabolism according to the results they showed that when healthy young adults were administrated FOS or GOS for 14 days, bifidobacteria increased but butyrate-producing microorganisms decreased [
32]. Taken together, changes in body weight and blood glucose levels may vary among the individuals even with the same oligosaccharide administration. Therefore, the patient-center-minded treatment with oligosaccharide for clinic purpose is pivotally necessary. In the future, when planning individualized prebiotic intervention based on gut microbiota, especially for young adults, it will be necessary to select the optimal prebiotic and determine the method of administration.
In conclusion, the dietary intake of milk-related oligosaccharides could lower blood glucose levels in TSOD mice and potentially prevent the development of diabetes and the onset of hepatocarcinogenesis in genetically susceptible individuals. Unlike strenuous exercise and dietary restrictions, milk-related oligosaccharides are pure dietary elements that can be safely administered for susceptible MS populations with good compliance.