We had explored causal pathway of fetuin-A and kidney function through BMI, UA, DM, and HT using multiple mediation analysis. Our findings indicated that fetuin-A directly associated with decreasing eGFR. In addition, effects of fetuin-A on eGFR was found to pass through the following mediators: BMI, UA (fetuin-A→BMI→UA→eGFR ) and HT (fetuin-A→BMI→HT→eGFR ), i.e., every unit of fetuin-A increased would increase BMI and UA risk resulting in a decrease in eGFR of 0.00132 ml/min/1.73 m2. Likewise, increasing fetuin-A would increase BMI, HT risk, and decrease eGFR by 0.00139 ml/min/1.73 m2. Furthermore, effects of fetuin-A could be mediated through DM and HT resulting in the lowering of eGFR.
Our finding with regards to the association of fetuin-A level with eGFR is similar to a previous study by Ix et al(29), which showed negative association between fetuin-A and predominantly non-diabetic subjects with stage 3 or 4 CKD, i.e., high fetuin-A level and low eGFR. Contrastingly, previous observational studies(14, 15, 30) found positive association between fetuin-A and kidney function, i.e., decreasing serum fetuin-A would decrease eGFR. It is worth noting that the functions and regulatory mechanisms of fetuin-A are complex, and may seem to differ according to the pathophysiologic characteristics of the population being studied(14). Several studies have demonstrated that inflammatory processes are increased in CKD, even in the early stages of CKD, and that the inflammatory processes triggered by inflammatory markers such as CRP and adiponectin are linked to endothelial dysfunction(14, 31, 32)
Fetuin-A, an anti-inflammatory protein acts as a negative acute phase reactant in the extra-cellular space to attenuate inflammatory responses, as such in patients with less advanced stages of CKD, and likely in the early phase of inflammation, fetuin-A levels may be normal or raised, however its expression is negatively regulated by pro-inflammatory cytokines such as CRP, which downregulates its synthesis during inflammation.(33, 34) Therefore, in a sustained inflammatory response, circulating fetuin-A levels are progressively depleted and its protective role in halting further decline of kidney function is undermined. Moreover, though not consistently demonstrated(16), variations in fetuin-A levels may also be determined by genetic polymorphisms independent of inflammation(15, 35).
Overweight/ obesity are known risk factors of cardiovascular disease (CVD)(36) and declining kidney function(4, 37). In addition, overweight/obesity is also highly associated with other CVD risks such as UA(38), DM(39) and HT (40), which may impact on kidney function as demonstrated by our findings. However, our mediation analysis showed positive causal effect of fetuin-A on eGFR that was mediated through BMI, i.e., BMI was a protective factor on kidney function. This might imply that the kidney function of subjects with high BMI could still be in good condition, if there are no associated risk factors such as hyperuricemia, DM, and HT.
Diabetes and hypertension are comorbid conditions frequently associated with kidney functions(41, 42). The role of DM in the pathogenesis of kidney disease has been established by epidemiological studies(9). Older subjects with longer duration of DM have a higher risk of developing CKD(43) and about 40% of patients with DM develop impaired kidney function, albuminuria, or both(44). Common kidney diseases which are associated with DM include CKD, ischemic nephropathy related to diabetic vascular disease and hypertensive nephrosclerosis(45, 46)
Fetuin-A is secreted predominantly by the hepatocytes and is encoded by the alpha Heremans-Schmidt glycoprotein (AHSG) gene, which is located on chromosome (3q27)(47). Its physiologic role includes the regulation of bone metabolism and the inhibition of vascular calcification. It has been implicated in vascular inflammatory processes as well as in the etiology of complex diseases such as CVDs(48) and DM(19, 20). Though some studies have been conducted to assess the relation of fetuin-A to CKD morbidity and progression (14, 49), and mortality(30, 33, 50) however its role in the etiology of kidney disease remains unclear.
Some observational studies have shown that increasing fetuin-A levels is associated with both improvements in the CKD status(31, 33) and endothelial dysfunction (ED)(31). ED is considered to be one of the major causal pathomechanisms of CKD(14, 51). Additionally, ED has equally been implicated in the pathophysiology of different forms of complex phenotypes like hypertension / coronary artery disease(52), DM(53) and CKD(54), which might be associated with the ED vascular inflammatory processes.
Out study have some strength. We assessed not only the direct causal effect of fetuin-A on kidney function but also the effects that were mediated through known risk factors of kidney function including BMI, UA, DM, and HT. We applied a multiple-mediation analysis model to determine possible causal pathways and effects of fetuin-A on eGFR. We used the EGAT prospective cohort to demonstrate the causal pathways that fetuin-A could have on kidney function through multiple mediator pathways, adjusting for covariables, which were obtained during the baseline and follow-up visits. A few limitations should however be addressed. Although we used longitudinal data from the EGAT cohort, fetuin-A was measured only once at baseline because of budget limitation. We considered intermediate mediators and also surrogate outcome of eGFR instead off end-clinical outcomes because the cohort had been followed up for only 5 years.
In conclusion, fetuin-A might have direct effect on declining kidney function, in which increasing fetuin-A might reduce kidney function. In addition, its effects might be mediated through multiple mediators including BMI, UA, DM, and HT, resulting in the lowering of eGFR. High level fetuin-A might increase BMI, however among this EGAT cohort that were studied, raised BMI on its own had no effect on declining kidney. The effect of raised BMI in declining kidney function was observed with the inclusion of other risk factors such as DM, HT, or high UA. These findings would however need to be further assessed in a cohort with a longer follow-up period.