Diabetic nephropathy is a worldwide-leading cause of end-stage kidney disease resulting in dialysis or renal transplantation in about 20–40% of patients with diabetes patients (1). Additionally, such patients with diabetic kidney disease (DKD) might also have exceptionally high rates of cardiovascular morbidity and mortality (2); furthermore, along with glycemic control, the renin-angiotensin-aldosterone system (RAAS) has also been shown as associated with the progression of DKD, promising advancements in chronic kidney disease (CKD) management in type 2 diabetes mellitus (T2D) including angiotensin-converting enzyme inhibitors (ACEi), angiotensin II receptor blockers (ARB), sodium-glucose cotransporter-2 inhibitors (SGLT2i), and nonsteroidal mineralocorticoid receptor antagonists (MRA).

Microalbuminuria is considered an important predictor of progression to proteinuria or nephropathy; however, its usefulness for early detection and monitoring of CKD progression is limited (3, 4). In certain cases, such as in patients with T2D, CKD can be observed even in the absence of albumin excretion, indicating that albuminuria alone is inadequate for monitoring diabetic nephropathy (3). This inconsistency has been documented in studies showing normal renal function in patients with microalbuminuria, while those with normoalbuminuria have renal disorders (5, 6). These findings highlight the limitations of using albuminuria or serum creatinine alone as early indicators of renal dysfunction; therefore, novel biomarker potential for early detection of patients in risk for CKD progression is warranted.

Adropin is a newly identified peptide hormone expressed in the liver and brain, and has been implicated in processes such as energy homeostasis and lipid metabolism (7). Demographic factors such as gender, age and body mass index (BMI) might affect the serum levels of adropin (8). Observation studies have reported decreased adropin levels in patients with obesity, metabolic syndrome, and type 2 diabetes, suggesting the correlation of adropin and insulin resistance (9); indeed, treatment with adropin lowered blood glucose levels and improved insulin resistance in type 2 diabetic animal models (10). Interestingly, conflicting serum adropin levels have been reported in some patients with T2D (11, 12) lead to the role of adropin plays in diabetes becoming much more complex as previously assumed.

In addition to its expression in the liver and brain, adropin has also been detected in the kidney, including glomeruli, peritubular interstitial cells, and peritubular capillary endothelial cells (13). Interestingly, adropin immunoreaction was enhanced in the kidney of streptozotocin (STZ)-induced diabetic rats compared with the controls (13); and subsequently, adropin was found to reduce the expression levels of inflammatory markers like tumor necrosis factor-alpha (TNF-α) and interleukin 6 (IL-6) in diabetic rats (10). On considering that inflammation plays important roles in the development of diabetic nephropathy, these findings suggest that adropin could exert a protective effect against the development of diabetic nephropathy through its anti-inflammatory actions; accordingly, given the evidence presented, this study aimed to examine serum adropin levels in subjects with diabetes at various stage of CKD and explore the potential application of adropin levels as an adjuvant indicator of CKD progression in individuals with T2D.

Although initially classified as a “promoting fat burning” peptide, adropin was later confirmed to enhance glucose oxidation and utility to reduce insulin resistance. Through knockout of the Enho gene, Kumar et al. demonstrated that adropin-deficiency was associated with increased risk of developing insulin resistance and diseases of metabolic syndrome (16); correspondingly, clinical observation studies corroborated this study and found that reduced circulating adropin was associated with insulin resistance, gestation and type 2 diabetes (9, 17, 18). Nevertheless, not all reports fit into this rule in the real world, where high-circulating adropin level was reported especially in patients with long-standing diabetes (11, 12, 19).

Herein, we also observed elevated adropin levels in subjects with advanced diabetes compared to those without diabetes in our study. Although lowered circulating adropin levels favor the development of diabetes from the standpoint of glucose homeostasis, it has been acknowledged that chronic oxidative stress and/or inflammation may stimulate adropin overexpression in advanced and/or poorly controlled subjects with diabetes, as has been found in rheumatic arthritis and systemic sclerosis (20–22). In correspondence, lines of evidence have documented increased serum (23) and tissue (pancreas, liver, kidney, heart, brain and cerebellar) adropin immunoreactivities (13) in diabetic rats induced by STZ. Therefore, we hypothesize a U-shaped risk association of adropin during the natural course of diabetes.

The published data suggesting oxidative stress may affect adropin expression through as yet unclarified meticulous networks. However, as mentioned above, we propose that the elevated adropin levels observed in our study may reflect a compensatory response to chronic oxidative stress and inflammation process in advanced CKD. This hypothesis is supported by evidence from chronic heart failure patients, where elevated adropin correlates with inflammation markers like IL-6 (24). Adropin may act to counterbalance the inflammatory status, which helps explain its paradoxical elevation in advanced disease stages.

Another possible interruption for serum adropin level in this study is medication, and it is worth noting that a significant proportion of our subjects with diabetes (79.3%) were taking anti-hypertensive agents, while a considerable number (62.1%) were on statin treatment, and 25.9% were using SGLT2 inhibitors. Previous research suggested that patients treated with anti-hypertensive medications like amlodipine and valsartan may have increased serum levels of adropin (25); additionally, an in vitro study demonstrated that atorvastatin might upregulate the expression of adropin (23), while another study demonstrated that dapagliflozin, an SGLT2 inhibitor, led to an increase in adropin level by up to 26.6% (26). Accordingly, multivariate linear regression analyses was performed to assess this correlation, with results still revealing borderline significance of higher adropin levels in subjects with diabetes after adjusting potential confounding factors including the use of the aforementioned drugs. In summary, the current study found significantly elevated serum adropin levels in subjects with diabetes, especially those with CKD, compared to those without diabetes for demographic differences.

In addition to the diabetogenic effect, lower serum adropin levels in subjects with diabetes were also found to be associated with higher possibility of developing renal and cardiovascular complications (27). Although the mechanisms as to how adropin preserves normal tissue function under hyperglycemic status remains obscure, it has been found that decreased adropin secretion in diabetic rats and adropin-knockout mice is associated with overexpressed inflammatory factors such as TNF-α, IL-6, as well as reduced phosphorylation level of eNOS (28, 29), while in contrast, increased generation and secretion of adropin promoted nitrogen oxide production and up-regulation of vascular endothelial growth factor receptors, potentially preventing endothelial cell damage in diabetic nephropathy (28, 29). Taken together, these findings suggest that adropin plays a crucial role in the pathophysiological development of diabetic nephropathy.

The association of adropin and the development and progression of diabetic nephropathy has been monitored by UACR and serum creatinine in individuals with T2D in clinical practice. In a literature review, most of the studies were focused on patients with early-stage CKD (average eGFR ranging from 68.80 to 109.13 mL/min/1.73 m²) (17, 18, 30). To further investigate these finding in more advanced CKD, our study focused on subjects with diabetes, with 69% classified as CKD stages 4-5 (average eGFR of 52.57 mL/min/1.73 m²), and as expected, our result is in contrast to the previous reports in early CKD, and we found that patients with advanced CKD had higher adropin levels than those with early CKD. Notably, our report is also the first study to demonstrate higher serum adropin levels; that is, those exceeding 6872.24 pg/ml, as being an independent risk factor for CKD progression.

Corresponding to our finding of elevated circulating adropin in advanced CKD, animal studies have already reported increased adropin immunoreaction documented in the kidney of STZ-induced diabetic rats with the intensities increasing with diabetic severity (13, 18). It is proposed that in response to chronic insults in poorly controlled T2D with advanced CKD, over-secretion of adropin plays a vital role in preventing further deterioration of diabetic nephropathy; therefore, higher levels of adropin in this study might represent an adaptive response with a potential protective effect against the progression of CKD. Nevertheless, as it was found in an in vivo study showing “adropin resistance” (31, 32), it seems that a threshold of adropin-associated renal protection exists, so when the environmental condition is exacerbated, renal function deterioration commences even when serum adropin is persistently high, just as it was observed in our progression DKD patients.

It has been well documented that the progression of CKD is associated with the nuclear factor erythroid 2-related factor 2 (Nrf2) which plays critical roles in maintaining normal redox homeostasis and renal function (33). A role of Nrf2 in DKD was demonstrated by reduced translocation of Nrf2 into the nucleus and diminished protection against inflammation, fibrosis and oxidative damage in renal tissues of diabetic rat studies (34). Nevertheless, clinical treatment of stage 4 diabetic CKD patients with a synthetic Nrf2 activator, bardoxolone methyl, in the BEACON study showed the recovery of eGFR within a narrow therapeutic range, accompanied by certain unfavorable side effects (35, 36). To integrate these findings into our study, we propose an Nrf2-mediated mechanism for “adropin resistance” in our study based on the following studies. Firstly, the naturally occurring endogenous Nrf2 activator, adropin bioactive peptides stimulated and activated Nrf2 in a dose-dependent manner in an adropin-knockout mice to prevent non-alcoholic steatohepatitis (37). Secondly, biphasic Nrf2 expression in CKD patients were reported in Denmark (38) and Italian (39) studies, where increased Nrf2 expression was detected in patients with mild to moderate renal impairment, whereas significant reduced Nrf2 was observed as CKD progressed in patients with severe renal impairment. To summarize, initially overexpressed adropin may protect against CKD progression through its dose- dependent induction and activation of Nrf2 until this protective mechanism is overwhelmed in more deteriorated conditions. Subsequently, Nrf2 exhaustion led to renal function decline and rapid progression, even with their serum adropin levels exceeding 6872.24 pg/ml.

Although we did not perform kidney biopsy or urinary analysis to check for the renal expression of adropin, Nrf2 and inflammatory markers, our results echo the findings of increased renal expression of adropin in STZ-induced diabetic rats (13, 40), which also highlights the complexity of the role of adropin in diabetic nephropathy and suggests that further research is needed to better understand its mechanisms and potential therapeutic implications.

Another important finding was the elevated osmolality in the subjects with diabetes and CKD, which was also well-correlated with the circulating adropin. In addition to the elevated blood glucose and urea nitrogen in the subjects with poorly controlled diabetes, there is a possibility that an elevated circulating adropin might also act on the brain to inhibit water intake (41, 42) cannot be excluded, and the higher osmolality might in part have contributed to the rapid progression of renal function in our DKD patients (43).

These findings suggest that serum adropin levels could potentially serve as a useful biomarker for predicting rapid deterioration of renal function, especially in individuals with poorly controlled T2D and pre-existing CKD. Several reviews and the hypothesis have well summarized the evidence to date on adropin in the development of diabetes and its complications, as displayed in Figure 4 .

This study has some limitations. Firstly, the sample size was relatively small, necessitating further studies for confirmation, while adropin concentrations were measured only under fasting conditions, which might not fully capture the fluctuations of adropin level(s) over time, particularly after different dietary intakes. Additionally, anthropologic parameters such as age, gender and body composition might also influence circulating adropin levels, while exploring the kinetics of adropin clearance from the circulation desirably measuring its concentration in various conditions could further help to clarify the possible mechanisms involved in the response of adropin to altered metabolic status. Additionally, we could not exclude the possibility that reduced nephron activity and impaired renal clearance may contribute to accumulated circulating adropin in our advanced CKD patients as a possible mechanism albeit whether adropin was excreted by urination was unclear. Finally, multicenter studies with larger sample sizes are necessary to validate our findings.

Despite the association between lack of circulating adropin and the development of diabetes and its renal and cardiovascular complications, this study observed significantly higher serum adropin levels in diabetes subjects with advanced nephropathy compared to those with mild diabetic nephropathy and subjects without diabetes in this study. These findings suggest that compensatory elevated adropin could be associated with proinflammation- and redox imbalance-induced renal dysfunction in patients with T2D. Adropin could potentially serve as a biomarker for early detection of renal Nrf2 insufficiency and diabetic nephropathy deterioration, especially in patients with advanced CKD. Further research is needed to better understand the exact role and mechanisms of adropin in the pathogenesis of diabetes and diabetic kidney disease. Such understanding can provide valuable insights for the development of targeted interventions and treatments in the future.