Diabetes mellitus with poor glycemic control increases risk of death due to endothelial dysfunction, increased inflammatory activity and albuminuria [15]. This study treated T2D patients with poor glycemic control and investigated changes in serum endotrophin levels after target HbA1c levels were reached for the first time.
In our study, significant falls were observed in endotrophin, HbA1c, FBG, CRP and UACR in T2D patients with poor control of glycemic regulation after 3 months of lifestyle changes and medical treatment. The reduction in endotrophin levels was found to be associated with HbA1c and UACR.
In addition to the lipid storage capacity of adipose tissue, it is a very active endocrine and metabolic organ. Many studies have shown adipokines (leptin, adiponectin, resistin, etc.) play an important role in insulin resistance and T2D development.
Endotrophin is a Col 6 subunit secreted by adipose tissue. There are studies showing a positive correlation between Col 6 levels and insulin resistance [16, 17]. However, there is still insufficient data about the effects of Col 6 and endotrophin on glucose metabolism in patients with T2D. A study by Rodriguez et al. found Col 6 and endotrophin causes a dose-linked increase in glucose uptake by muscle cells. This effect was shown to be mediated by integrin receptors [18].
Pasarica et al. in a study of adipose tissue biopsy in obese groups with diabetes and without diabetes found no difference in COL6A3 (endotrophin) levels; and, they stated that COL6A3 did not contribute to diabetes development. They concluded that these results were associated with insulin resistance in both obese groups, but that beta cell injury in obese cases with T2D was additionally not associated with COL6A3 [19]. However, this study did not state HbA1c levels. Additionally, the blood glucose levels of patient group in the study had regulated and were close to levels in the control group.
The oxygen free radicals caused by glucotoxicity have been shown to increase TGF-β synthesis. Collagen VI (endotrophin) has an active role in various biological processes such as inflammation, angiogenesis, fibrosis and epithelial-mesenchymal transition (EMT) by increasing TGF-β synthesis just like glycotoxicity [20, 21]. However, there is insufficient data on the direct effect of glucose on endotrophin metabolis. In an invitro study conducted by Pölivi muona et al with an electron microscope at the cellular level, it was concluded that “high glucose concentrations increase collogen VI concentration in many cell types”. At high glucose concentrations, it was determined that fibrillar structures were formed in the ECM around the adipose tissue and this impaired microenvironment induced collagen VI increase. Macrophage and fibroblast suppression can be achieved by regulation of blood glucose. This creates a more stable microenvironment. Therefore, it is expected that the concentration of endotrophin will decrease in good glycemic regulation [17].
In our study, we included patients with poorly controlled T2D and HbA1c levels of 74.9 mmol/mol (9%) and above. In this way, we looked at the relationship between serum endotrophin levels and diabetes regulation. Our study data shown that endotrophin levels were initially high in T2D patients with poor glycemic control, but this significantly reduced after treatment. We also found a positive correlation between endotrophin and HbA1c levels. We think low insulin reserves in the pancreas and resulting poor blood glucose control disrupts adipose tissue functions creating a negative effect on endotrophin levels, together with obesity. We assumed that despite the catabolic effects of poor glycemic control in patients included in our study, weight loss could not be achieved despite the appropriate diet due to anabolic effects caused by glycemic control and insulin use. In conclusion, there was no significant change in BMI in our overweight and obese patients over a 3-month period. The lack of no weight loss allowed us to evaluate the parameters that changed after blood glucose regulation independently of weight. We thought that blood glucose control may be responsible for the decrease in endotrophin levels, independent of BMI.
Some studies have reported moderate-degrees of increase in the inflammation marker of CRP predicts the development of insulin resistance [22–24]. Additionally, Tanigaki et al. showed moderate elevations in CRP caused insulin resistance in mice [25]. When we examine the inflammation marker of CRP in our study, as expected, the CRP levels reduced along with glycemic regulation. The reduction in serum endotrophin levels with blood glucose regulation and similar reduction in CRP levels leads to the consideration that there was a positive effect on inflammation.
Endotrophin increases the synthesis of TGF-β and regulates insulin gene transcription via the SMAD3 pathway of the TGF-β signal. TGF-β superfamily expression levels were shown to control blood glucose levels [20, 21]. As is known, TGF-β is associated with diabetic cardiomyopathy [26] and diabetic nephropathy [27] development. For this reason, studies about the TGF-β signal pathway as a treatment modality to reduce islet cell inflammation and preserve beta cell differentiation have gained importance [21]. Endotrophin increasing TGF- β synthesis are worth investigating for their effect on diabetes and complications in this sense.
Endotrophin is an adipokine with a basic role in fibrosis and inflammation. At the same time, it is one of the most commonly found proteins in glomerular ECM and increases in renal injury [28, 29]. Renal fibrosis is known to be the basic pathologic route in chronic renal injury. Fenton et al. [30] in a 500-cohort study found a significant inverse correlation between EGFR and serum endotrophin levels. Here they stated the increase in Col-6 and endotrophin may be explained by increased fibrotic tissue production with renal injury. The same study found high endotrophin levels caused an increase in the progression of end stage renal failure and found it was associated with increased mortality [30]. In accordance with this data, our patient group with poor glycemic control were identified to have a significant reduction in UACR with blood glucose regulation. Additionally, we identified a positive correlation between the reduction in UACR and endotrophin levels. Regression analysis found the δ-UACR value significantly explained the δ-endotrophin level. This means that if there is greater variation in UACR, the variation in endotrophin levels will be similarly high, and if the variation in UACR is low, the variation in endotrophin levels in patients will be minimal. Additionally, this variation is independent of HbA1c variation and hence independent of blood glucose. These results lead to the consideration that there is a tight and independent correlation between endotrophin and the microalbuminuria, one of the microvascular complications of diabetes and we think that studies targeting endotrophin will contribute to preventing the progression of diabetic nephropathy in the future.
Limitations of our study include the relatively small sample and short follow-up duration. This study dealt with the correlation of endotrophin levels in T2D and is the first study to research the longitudinal correlation between endotrophin levels, glycemic control and microalbuminuria.