1. Introduction
Metabolic dysfunction-associated steatotic liver disease (MASLD), previously termed nonalcoholic fatty liver disease (NAFLD), is defined as the presence of hepatic steatosis in conjunction with at least one cardiometabolic risk factor (obesity, hypertension, type 2 diabetes, dyslipidemia) [
1]. Approximately 1/4 of people with hepatic steatosis progress to metabolic dysfunction-associated steatohepatitis (MASH), which is characterized by hepatocellular ballooning and lobular necroinflammation, and an increased risk of fibrosis, cirrhosis, hepatic decompensation, hepatocellular carcinoma, and all-cause mortality [
2]. The global burden of MASLD is increasing at an alarming rate, with global prevalence reaching up to 32.4% [
3,
4].
A retrospective analysis of patients with MASLD showed that cardiovascular disease (CVD) (38.3%), followed by non-liver malignancy (18.7%) and complications of liver cirrhosis (7.8%), were the three most common causes of death in MASLD patients [
5], suggesting that CVD was the most crucial determinant of mortality in MASLD patients. Considering that the development of CVD determines the prognosis of MASLD patients, the therapeutic interventions for MASLD should improve coronary risk factors such as obesity, diabetes and dyslipidemia, in addition to an improving in liver function [
6]. The features of metabolic syndrome are not only highly prevalent in patients with MASLD but also components of metabolic syndrome also increase the risk of developing MASLD [
7]. The established conditions for developing MASLD include obesity, type 2 diabetes, hypertension and dyslipidemia such as high TG and low HDL-C levels [
7]. Obesity, especially, visceral obesity is the most common and well-documented risk factor for MASLD. In the meta-analysis, the pooled odds ratio (OR) for the formation of MASLD in waist circumference and body mass index (BMI) were 2.34 (95%CI, 1.83 to 3.00) and 2.85 (95%CI, 1.60 to 5.08), respectively [
8].
Sodium-glucose cotransporter 2 inhibitors (SGLT2is), are reversible inhibitor of SGLT2, leading to reduction of renal glucose reabsorption and decrease of plasma glucose, in an insulin-independent manner [
9,
10,
11]. Such property of SGLT2is is beneficial to reduce visceral fat as well as body weight [
11]. Recent meta-analysis provided evidence that SGLT2is significantly reduced visceral adipose tissue, subcutaneous adipose tissue, and ectopic liver fat, in addition to body weight, especially in young type 2 diabetic patients with MASLD and high BMI [
12]. However, the evidence on effect of SGLT2is, especially the long-term effects, on the progression of MASLD in Asian population is very limited.
Here, we investigated the long-term (> 3 years) effect of SGLT2is on glucose/lipid metabolism and the markers for hepatic steatosis and hepatic fibrosis in Japanese patients with type 2 diabetes.
4. Discussion
Insulin resistance observed in patients with type 2 diabetes and/or obesity enhances the expression and activity of hormone sensitive lipase (HSL) which catalyzes the hydrolysis of TG into free fatty acids (FFA) in adipose tissue [
18]. Insulin resistance induces an excess TG storage within the skeletal muscle by reducing FA oxidation [
19]. Serum FFA increase due to increased release from the adipose tissue and decreased FA oxidation in the skeletal muscle. An increased amount of FFA enters the liver, leading to overproduction of TG-rich lipoproteins such as very-low-density lipoprotein (VLDL). In insulin resistance, an increased FFA entry to liver, increases in components of VLDL such as apo B100 and apo CIII, and an increase in microsomal TG transfer protein (MTP) which is involved in VLDL assembly may elevate hepatic production of VLDL [
20,
21,
22]. Insulin resistance increases expression of sterol regulatory element binding protein 1c (SREBP-1c), which induces FA synthesis [
23]. FA accumulation and resulting over-production of VLDL which are induced by insulin resistance may be the features of MASLD. In short, FA accumulation and over-production of VLDL may induce inflammation and oxidative stress which develop MASLD [
6].
In intravascular space, insulin resistance reduces the activity of lipoprotein lipase (LPL) which is the rate-limiting enzyme for the catabolism of TG-rich lipoproteins such as VLDL [
24,
25]. The formation of HDL is related to the catabolism of TG-rich lipoproteins by LPL [
26]. Therefore, reduced LPL activity increases VLDL and reduces HDL. Therefore, serum high TG and low HDL-C are commonly observed in patients with type 2 diabetes, obesity and metabolic syndrome. Insulin resistance reduces LDL receptor expression [
27], which can explain the increase in LDL-C in patients with type 2 diabetes and/or obesity. Niemann-Pick C1-like 1 (NPC1L1) plays a pivotal role in intestinal cholesterol absorption. Diabetes increases the expression of NPC1L1 [
28], which may also contribute to an increase in TG-rich lipoproteins and LDL-C in type 2 diabetic patients.
The evidence on effects of SGLT2is on serum lipids is very limited, and we found four meta-analyses. The meta-analysis including a total of 28 randomized controlled trials (RCTs) showed that SGLT2is significantly increased serum LDL-C levels (mean difference [MD], 0.09 mmol/L; 95% confidence interval (CI), 0.03 to 0.16), p = 0.0046] and HDL-C levels (MD, 0.08 mmol/L; 95% CI, 0.06 to 0.11; p < 0.0001). Neutral effect of SGLT2is on TC (MD, 0.08 mmol/L; 95% CI, -0.08 to 0.24; p = 0.3150) and TG (MD, -0.03 mmol/L; 95% CI, -0.23, 0.16; p = 0.7382) [
29]. The meta-analysis including 60 RCTs showed that SGLT2is treatment increased TC by 0.09 mmol/L (95% CI, 0.06 to 0.13), LDL-C by 0.08 mmol/L (95% CI, 0.05 to 0.10), and HDL-C by 0.06 mmol/L (95% CI, 0.05 to 0.07), while it reduced TG by 0.10 mmol/L (95% CI, 0.06 to 0.14) [
30]. In Asian compared to non-Asian populations, a slightly larger increase in HDL-C and a decrease in TG were observed, but with similar results for TC and LDL-C. The meta-analysis including a total of 57 RCTs demonstrated that SGLT2is significantly increased HDL-C by 0.07 mg/dL (95% CI, 0.06 to 0.08, P < 0.00001), LDL-C by 0.11 mg/dL, (95% CI, 0.09 to 0.13 mg/dL, p < 0.00001) and TC by 0.10 mg/dL (95% CI, 0.06 to 0.15, p < 0.0001) [
31]. SGLT2is significantly reduced TG by -0.10 mg/dL (95% CI, -0.13 to -0.06, p < 0.00001). The meta-analysis of 48 RCTs revealed that SGLT2is had a significant increase on TC by 0.09 mmol/L (95 % CI, 0.05 to 0.13; p < 0.0001), LDL-C by 0.10 mmol/L (95 % CI, 0.07 to 0.12; p < 0.00001), HDL-C by 0.06 mmol/L (95 % CI, 0.05 to 0.08; p < 0.00001). Additionally, SGLT2is administration showed a significant decrease in TG by -0.10 mmol/L (95 % CI, -0.13 to -0.07; p < 0.00001) [
32].
Present study demonstrated that the 3-year-SGLT2is treatment significantly reduced TC and LDL-C, which completely disagreed with the results of meta-analyses. The inclusion of a relatively large number of short-term RCTs and patients of different races in such meta-analyses can explain a different result between ours and meta-analyses. Our previous study showed that SGLT2is did not show a significant effect on LDL-C and TG at 1, 2, 3, and 6 months after the start of SGLT2is [
33,
34], suggesting that a long-term treatment is required to evaluate the effects of SGLT2is on serum lipids. Our study showed that SGLT2is significantly increased HDL-C, which was observed in all four meta-analyses. We also showed that SGLT2is significantly reduced TG, which was observed in three of four meta-analyses.
Epidemiologic and clinical trials have shown that elevated TG and low levels of HDL-C are independent risk factors for coronary heart disease Therefore, the 2001 National Cholesterol Education Program Adult Treatment Panel III (ATP III) guidelines placed more emphasis on the importance of managing high TG and low HDL-C by identifying non-HDL-C (LDL-C + VLDL-C) [
35]. An effect of SGLT2is on non-HDL-C has not been sufficiently studied, and only present study and one meta-analysis reported an effect of SGLT2is on non-HDL-C. The meta-analysis showed that SGLT2is had a significant increase in non-HDL-C by 0.09 mmol/L (95 % CI, 0.06 to 0.12; p < 0.00001) [
32], which disagreed with our result that SGLT2is significantly reduced non-HDL-C. This difference may be due to the inclusion of a relatively large number of short-term RCTs and patients of different races in the meta-analysis. Most studies had shown that SGLT2is reduced TG [
30,
31,
32], which is included in non-HDL-C. SGLT2is may reduce TG, by reducing TG-rich lipoprotein-cholesterol which is included non-HDL-C. Therefore, our result may be considered more scientifically valid.
Present study showed that SGLT2is significantly reduced LDL-C, TG and non-HDL-C, and significantly increased HDL-C. The 3-year-SGLT2is treatment significantly reduced body weight and HbA1c, suggesting that the long-term SGLT2is treatment improved insulin resistance. Such beneficial changes in serum lipids may be induced by an improvement in insulin resistance by the 3-year-SGLT2is treatment. Changes in TG and non-HDL-C were significantly and positively correlated with changes in HbA1c, supporting our hypothesis.
The evidence on effects of SGLT2is on MASLD is very limited. We previously reported that SGLT2is significantly reduced the serum levels of AST and ALT at 3 and 6 months after the start of the SGLT2is in patients with type 2 diabetes [
33,
34]. We found that the FIB-4 index was significantly decreased at 12 months after the start of SGLT2i treatment in a high-risk (FIB-4 ≥ 2.67) group for advanced hepatic fibrosis [
36]. We also retrospectively studied 568 patients with MASLD and type 2 diabetes. At 96 weeks, the mean FIB-4 index had significantly decreased (from 1.79±1.10 to 1.56±0.75) in the SGLT2i group but not in the pioglitazone group [
37]. APRI significantly decreased in both groups. The body weight of the SGLT2i-treated group decreased by 3.2 kg; however, that of the pioglitazone group increased by 1.7 kg.
Present study is the first to report the long-term effects of SGLT2is on the markers for hepatic steatosis and hepatic fibrosis. Our study showed that the 3-year-SGLT2is treatment significantly reduced HSI as the marker for hepatic steatosis and APRI as the marker for hepatic fibrosis, and a significant reduction in FIB-4 index in high-risk group was also observed. Changes in both HSI and APRI were significantly and positively correlated with changes in body weight, BMI and HbA1c, suggesting that weight loss and an improvement of insulin resistance by the 3-year-SGLT2is treatment may improve hepatic steatosis and fibrosis. Change in HSI was significantly and negatively correlated with change in HDL-C and change in GGT was significantly and positively correlated with changes in TG and non-HDL-C, suggesting that an improvement in serum lipids by SGLT2is may also favorably influence on the progression of MASLD.
SGLT2is decrease plasma glucose without an increase in insulin secretion by reducing renal glucose reabsorption, resulting in an increase in the ratio of glucagon to insulin, which activates HSL in adipose tissue [127]. FFA release from adipose tissue increases due to an increase in the hydrolysis of TG, which reduces adipose tissue size, resulting in an improvement in insulin resistance due to reduced inflammatory cytokines and increased adiponectin [128]. FFA released from adipose tissue may be promptly used by skeletal muscles and liver because SGLT2is shift the energy metabolism towards FA utilization by the alteration of the glucose–FA cycle [130]. An increase in adiponectin levels has beneficial effects on glucose and lipid metabolism by activation of adenosine monophosphate-activated protein kinase (AMPK) [94]. SGLT2is has been reported to activate AMPK and inactivate acetyl-CoA carboxylase (ACC) which regulates FA synthesis in obese mice [129]. The activation of AMPK and resulting inactivation of ACC is also associated with the improvement of MASLD. The long-term SGLT2is treatment may reduce FA accumulation in liver, which reduces inflammation and oxidative stress and results in an improvement of MASLD.