The mechanisms underlying the pathogenesis of DPN have been attributed to impaired glucose metabolism and dyslipidemia [15]. The dysfunctions of metabolic pathways characterized by hyperglycemia and dyslipidemia cause an imbalance of the mitochondrial redox state and inflammatory processes, thereby leading to neuronal and glial cell injuries, which are accepted as crucial mechanisms of the pathogenesis of DPN [3]. Increasing evidence has demonstrated the roles of LCN2 in modulating the activities of glial cells, recruiting immune cells and amplifying neuroinflammation, consequently resulting in neuronal demyelination and apoptosis [4]. Besides, as an iron-binding protein, LCN2-mediated oxidative stress promotes neuronal injury[10, 16]. The LCN2 levels are known to be elevated in circulation in diabetes [5-9, 17]. Consistently, increased LCN2 expressions have also been described in the brain regions in both ob/ob mice and mice fed high-fat diets, the two classical models with metabolic disorders characterized by obesity, hyperglycemia, dyslipidemia, systemic inflammation, and neuroinflammation [10, 18, 19]. LCN2-related reactive oxygen species genes, which contribute to neurodegeneration, has been shown to be differentially expressed in the hippocampus of wild-type and ob/ob mice [10]. Noteworthily, the involvement of LCN2 has recently been implied in the neurological disorders from the studies in diabetic rodent models [10, 11]. Bhusal et al. found that the expression of LCN2 in the hippocampus was increased in STZ-induced diabetic mice models [11, 20]. Deletion of Lcn2 gene ameliorated diabetes-induced reactive gliosis and expression of pro-inflammatory cytokines in the hippocampus, subsequently decreasing neuronal loss in the hippocampus. Moreover, diabetes-associated cognitive deficits were improved in LCN2 KO mice compared to wild-type mice in diabetic conditions.
Preclinical studies strongly suggest the presumable role of LCN2 in the pathogenesis of DPN in humans. This study herein firstly reported the association of LCN2 with DPN in humans. In this study, serum LCN2 levels were shown to be elevated in individuals with DPN (Fig 1), and the ratio of individuals with DPN to all diabetic individuals increased with the increase in the serum LCN2 levels (Fig 2). Furthermore, multivariable regression analysis showed that serum LCN2 level was independently correlated with the occurrence of DPN in individuals with T2D. Additionally, we found that with the increase in serum LCN2 levels, the TCNS scores for DPN were increased (Fig 3). Application of TCNS in clinical studies has confirmed its role in documenting and monitoring DPN [14, 21, 22]. A higher score indicates more severe disability. Given that LCN2 has been recently shown to be stable in the circulation, this study suggests LCN2 as a biomarker in the evaluation of DPN severity [23].
Coincident with our observations, a most recent study on DPN in mice model has revealed that LCN2 expressions in both DRG and sciatic nerve increase significantly in DPN mice [12]. Under the conditions of diabetes, LCN2 from satellite glial cells mediates macrophage infiltration into DRG, stimulates the, release of inflammatory cytokines such as tumor necrosis factor-α, and enhances neuronal inflammatory response. PDK2, the key regulator of mitochondrial function, is typically up-regulated in diabetic conditions and promotes glycolytic metabolism, along with increased DRG lactic acid production, consequently leading to neurotoxicity. LCN2 contributes to the pathogenesis of DPN via PDK2-lactic axis in DRG of diabetic mice. These findings above may provide mechanic interpretations for our clinical observations.
A variety of factors including renal function, proteinuria, blood glucose, lipid and blood pressure have been extensively described to influence serum LCN2 levels in individuals with T2D [5, 17, 24, 25]. To minimize the potential differences in the comparison of serum LCN2 levels between DPN and NDPN groups, we carefully characterized and matched participants according to sex, age and BMI. Notably, no statistically significant differences were indicated in other parameters including HbA1c, FBG, FCP, TC, TG, UACR, eGFR and blood pressure. The only statistically significant difference in the two groups was longer diabetes duration for individuals in the DPN group. This could be explained by the fact that diabetes duration is the risk factor for DPN in T2D [26].
In this study, both BMI and TG were shown to be independent factors associated with serum LCN2 levels, consistently with previous clinical studies [24, 25, 27]. Dysregulation of LCN2 has been tied to obesity, metabolic syndrome and cardiovascular diseases, mainly through its ability to bind to lipids like fatty acids [28]. For example, LCN2 could bind to the fatty acid retinoic acid to mediate thermogenesis and lipid metabolism in adipose tissue [29]. Additionally, down-regulation of LCN2 was shown to attenuate the metabolism of arachidonic acid, impairing energy homeostasis in mice study [30]. This study indicated a negative association of LCN2 with DBP, whereas such association was shown to be positive in previous studies [25, 31]. This discrepancy may be attributed to the heterogeneity of the studied population across the distinct studies.
A recent study has indicated that a LCN2 monoclonal antibody significantly reduces cerebral infarction and neurological deficits after stroke, suggesting targeting LCN2 as a promising intervention for the therapy of neurological diseases. Another study showed that treatment with an anti-LCN2 antibody prevented LCN2-related neuroinflammation and neuronal death in vitro [20]. Therefore, the association of LCN2 with DPN described in this study suggests a presumable strategy for the treatment of DPN.
Limitations should be noted in this study. This was a cross-sectional study, which could not provide the causal relationship between increased serum LCN2 levels and the development and progression of DPN. Moreover, the sample size of this study was limited. Prospective studies with larger sample size are required to unravel the role of LCN2 in the pathogenesis of DPN.