In this clinical case-control study of patients matched for age, ethnicity, occupation, familial history of cancer, and drinking habits in northwestern China, we identified HOMA-IR levels to be a risk factor for GBC but GSD and female (16) patients, and type 2 DM had an increased risk of GBC as a recent study also discovered (17). Interestingly, we found controlling HOMA-IR or insulin levels, rather than FBG levels, can more effectively lower GBC risk relating to DM, and there was no significantly higher morbidity from GBC in IR patients with GSD or NAFLD, including females.
Our multivariate logistic regression considering HOMA-IR stratified by IR showed that DM and GBC risk had a significantly negative correlation, while after including FINS and FBG levels, the significant association between DM and GBC risk, as well as FBG levels, was absent. In contrast, the relationships between the risk of GBC and FINS, HOMA-IR were significantly positive. Moreover, HOMA-IR was the most remarkable independent risk factor for GBC in the non-IR and total patient groups. The results tended to further indicate that hyperinsulinemia or insulin resistance have key roles in increasing GBC risk, and poor blood glucose control might increase GBC risk, even in non-IR patients. On the one hand, the above results are supported by a recent study in which insulin was seen to reduce the levels of IGF-binding protein (IGFBP)-1 and IGFBP-2 in the circulation, resulting in a circulating IGF increase; and IGF and insulin might stimulate target cells to move toward malignant transformation (18). On the other hand, to our knowledge, the goal of diabetes treatment is to correct metabolic disorders, improve clinical symptoms (such as polyuria and increased eating and drinking), and delay the occurrence of diabetic complications by controlling blood sugar concentration. In China, the most common hypoglycemic drugs are insulin secretagogues and metformin. The former may induce hyperinsulinemia after long-term oral administration, especially in IR patients. The latter, metformin hydrochloride, a first-line hypoglycemic drug, induced porcelain gallbladder in C57Bl/6 mice while reducing the chance of gallstone formation (19) and may reduce blood glucose by promoting the uptake of glucose by peripheral tissues, inhibiting gluconeogenesis, delaying the absorption of glucose in the intestine, reducing hepatic glycogen output, and increasing the utilization of sugar by inducing the anaerobic fermentation of sugar in the small intestine, which leads to increased triglyceride liver synthesis. Triglyceride synthesis in cells is heavily reliant on the glycerol and fatty acids produced by glucose metabolism; glycerol is derived from the glycolysis-produced dihydroxyacetone phosphate, and fatty acids are synthesized by acetyl coenzyme A, a product of the oxidative decomposition of sugar (20). In short, a high-carbohydrate diet or sugar accumulation plays the role of a bridge (21), which supports the idea that inappropriately controlled blood glucose might promote GBC risk with hypertriglyceridemia or/and IR.
As our study showed, high TG levels are the most significant players related to GSC risk, especially in IR patients (OR=96.437). Hirasawa et al. analyzed the association between endometrial cancer and the risk of MetS in a case-control study and verified that hypertriglyceridemia was significantly frequent in endometrial cancer patients (22). Retrospective analysis by Wuermli et al. of anthropometric and laboratory parameters of 504 prostate patients and 565 age-matched patients with benign prostatic hyperplasia highlighted a relationship between hypertriglyceridemia and a greater risk of prostate cancer (23). Although few reports have clarified the mechanisms of their correlation, we propose that hypertriglyceridemia is closely associated with IR, and chronic hyperinsulinemia in the IR state could stimulate very-low-density lipoprotein (VLDL)-triglyceride synthesis (20). Reportedly, the VLDL receptor is a participant in the pathogenesis of GBC by its mitogen-activated protein kinase-based regulation of the expression of fibroblast growth factor receptor signaling pathway components (24). Moreover, it is known that hypertriglyceridemia closely relates to NAFLD, which may progress into non-alcoholic steatohepatitis, triggering the development of hepatocellular carcinoma (21). The present study demonstrated that NAFLD might not be a predictive factor of GBC risk in IR or DM patients, possibly because it is linked to the disorder of glucose metabolism.
Other authors have variously suggested that the association between cancer advancement or risk and cholesterol-related parameters is either positive, negative, or neutral (25). A recent study of Bangladeshi women revealed total cholesterol and BMI to be independent predictors of breast cancer (26), while our results showed that BMI and TC might not be independent predictors of GBC after matching for age, ethnicity, occupation, drinking habits. Although an early case report demonstrated GBC cells of hypercholesterolemia patients can secrete a substance that stimulates LDL receptor activity as result of reducing serum cholesterol (27), few research efforts have clarified the cellular mechanisms. It is known that the synthesis of a molecule of cholesterol requires 18 molecules of acetyl CoA, 36 molecules of ATP, and 16 molecules of Nicotinamide adenine dinucleotide phosphate (NADPH); most of the acetyl CoA and ATP come from the aerobic oxidation of sugar in mitochondria, and the NADPH is obtained from the pentose phosphate pathway metabolism of sugar in the cytosol (28). However, this situation can be completed difficultly in tumor tissue because its patterns of glucose metabolism is anaerobic glycolysis. In turn, LDL-c was an independent risk factor for GBC in our results, including patients stratified by IR and non-IR, potentially because blood LDL is a product of VLDL breakdown, which is the main form of endogenous triglyceride transport (20). Similarly, anthropometric and laboratory parameters in non-IR patients revealed that abnormal lipid metabolism is closely related to GBC, which might result from a disorder of fatty acid binding protein regulation (29).
Most notably, our study revealed that indexes relating to glucose metabolism (including TG, FINS, LDL-c, RBP, and HOMA-IR levels) had significant correlations with GBC in DM patients. Our multivariate analysis further demonstrated that HOMA-IR was the most significant factor predicting GBC risk (OR=5.756), while FBG levels had a negative association with GBC (OR=0.869). These outcomes prompted us to think that hyperinsulinemia, a common phenomenon in DM, directly or indirectly regulates the activity of IGF-1, contributing to the proliferation and inhibition of apoptosis in GBC cells (31). Moreover, more and more research focuses on the roles of the insulin/IGF system and dyslipidemia in cancers of females (32). Therefore, the disorder of blood glucose, not just hyperglycemia, plays a key role in carcinogenesis. Furthermore, we consider interventions employing medical nutritional therapy to treat obesity, IR, and DM to be superior to other measures, including bariatric surgery, especially as this bypass procedure might be related to remnant gastric cancer from Helicobacter pylori infections in the residual stomach (33). Instead, it is important to treat diabetes by adjusting diet and increasing aerobic exercise to control body weight and prevent IR (34-36); drugs are the second most effective way to control diabetes, and anti-IR drugs should be the first choice, as these might have further benefits of tumor prevention.
Notably, serum RBP-4 was found to be mostly positively associated with type 2 DM and obesity (37). Huang et al. demonstrated that retinoic acid 6, RBP-4’s only known specific membrane receptor, was expressed in β-cells and mediates the inhibitory effect of RBP-4 on insulin synthesis through the Janus kinase 2/STAT1/ISL-1 pathway (38), while our findings showed that serum RBP levels to have a significant negative association with GBC, including the IR patients, although the OR was low (0.922-0.964). To our knowledge, RBP-4 is the main retinol transporter in plasma, thus the serum RBP levels we studied might reflect true RBP-4 levels (39). Moreover, among the several secreted bioactive signaling molecules of adipose and liver tissue, RBP-4 has been associated with systemic insulin resistance, dyslipidemia, type 2 diabetes, and other metabolic diseases (40). A recent study revealed that RBP-4, which links obesity and cancer, is one of the pathogenic mechanisms of cancers, including prostate cancer, that is related to obesity (41), breast cancer (42), pancreatic cancer (43), and more. However, Sobotka et al. (44) found that low levels of RBP-4 at the time of renal cellar cancer diagnosis are associated with poorer prognosis after surgery, and Lorkova et al. (45) verified a decrease in RBP-4 levels in ovarian cancer patient sera by two independent methods. The observed RBP-4 decrease in our research results could advocate RBP-4 as a potential diagnostic or prognostic biomarker of tumors with high malignancy and poor prognosis, such as GBC.
Strengths and limitations
We reviewed more than 2000 GBC cases during the previous 12 years that were matched for occupation, environment, age, ethnicity, and drinking habits, adding strength to our study. However, the present study has several limitations. First, case control studies typically all have the limitations of recall bias (ability of patients to remember their exposure and habits), the uncertainties over the links between cause and effect, and sampling bias. Second, there is still no cohesive explanation for the negative correlations between HBP, SBP, DBP, and GBC in our study. Third, we chose to define IR using HOMA-IR in a large dataset from an epidemiological inquiry in China because the reference values for indirect indicators of IR diagnosis might be affected by the age, gender, and ethnicity of the study population and laboratory methods used for the determination of glucose and insulin concentrations (46). Final, there might have been errors in the family history of cancer due to the small family units in China and Chinese personality characteristics; therefore, we haven’t completed an accurate data on family history of cancer and matching, as these statistics may be biased.