We observed important associations between lipid and inflammatory biomarkers and renal function in a cross-sectional study of 732 men with type 2 diabetes. Limited data are available regarding these biomarkers and renal function in type 2 diabetes; this paper reports that several lipid and inflammatory markers were independently associated with reduced renal function in adult onset diabetes. Recent investigations have reported that higher baseline levels of sTNFR-2, IL-6, CRP [18
], E-selectin, and ICAM [19
] are independent predictors of incident type 2 diabetes in prospective nested case-control analyses of the Nurses’ Health Study. The accumulating evidence that a chronic inflammation is a harbinger of adult-onset diabetes raises provocative questions of which, if any, of these same mechanisms lead to renal dysfunction in diabetes and whether any of these mechanisms also underlie non-diabetic nephropathy.
Overall, triglycerides, fibrinogen, sTNFR-2, and VCAM levels were consistently and strongly inversely associated with GFR in our study of men with type 2 diabetes, whereas HDL was positively correlated with renal function and CRP. These findings suggest that elevated risk for macrovascular and microvascular disease in type 2 diabetes may be progressive endothelial cell dysfunction and atherosclerosis mediated by elevated levels of these biomarkers.
In secondary analyses of data from 1785 participants of the MDRD study, investigators reported lower cholesterol levels at GFR<25 ml/min/1.73 m2
], but direct comparison to our findings of no association between total cholesterol and GFR are difficult because the MDRD study excluded all but diet-controlled diabetics and encompassed a much lower range of GFR than in our current study. Most previous studies have found no relation between total cholesterol and kidney dysfunction or dialysis-dependency [1
]. All of these studies, however, had less than 100 subjects and excluded diabetics. In contrast, several of these same studies reported higher triglyceride levels in subjects with kidney dysfunction or who were dialysis-dependent compared to controls [1
]. Furthermore, a cross-sectional analysis of 5808 elderly participants in the Cardiovascular Heart Study (CHS) also revealed higher triglycerides levels in those with renal insufficiency (defined as sCr ≥1.5 mg/dl in men and ≥1.3 mg/dl in women) compared to those without renal insufficiency; participants with diabetes comprised only 12–14% of this cohort [4
We also noted that higher HDL levels were more common in those without moderate renal insufficiency, defined as GFR < 60 ml/min/1.73 m2
. This observation is consistent with previous investigations demonstrating that those with kidney dysfunction have 11% to 32% lower HDL levels [1
]. Our findings are also consistent with a recent prospective analysis of 4,517 members of Physicians’ Health Study demonstrating that those with an HDL < 40 mg/dl at baseline had a two-fold higher risk of renal insufficiency (defined a serum creatinine ≥ 1.5 mg/dl) at 14 years of follow-up [5
]. Since all of these previously published studies excluded diabetics or had <15% diabetics, our data confirm the inverse relation between HDL levels and renal dysfunction in type 2 diabetes.
Previously published studies on the association between LDL and presence of kidney disease have been mixed, with a couple of investigations reporting a positive association [21
], and others reporting no association [1
]. The finding that LDL was inversely associated with GFR < 60 ml/min/1.73 m2
was opposite to our expectations especially because in our cohort, the highest quartile of LDL was associated with a −2.8 ml/min/1.73m2
difference in estimated GFR when compared to the lowest quartile. On closer scrutiny, these OR estimates were based on only a few patients who fell within the highest quartile of LDL and had estimated GFR < 60 ml/min/1.73 m2
. LDL was not statistically associated with estimated GFR in any of our other analyses, and therefore, we felt this represented a chance finding.
We did not see significant or consistent associations between renal function and lipoprotein (a) (Lp (a)) or apoprotein B (Apo B). Whereas previous publications have reported higher Lp(a) in subjects with kidney disease, the studies either excluded individuals with diabetes or included diabetics who had much lower levels of renal function (CrCl 2 to 19 ml/min) [24
] than those in our current study. We found no previously published literature on the relation between Apo B and kidney function. In contrast, we found fibrinogen levels were consistently inversely correlated with GFR. Fibrinogen has been noted to be positively associated with albuminuria in individuals with diabetes [25
] and to be elevated in subjects with renal insufficiency [1
We found no association between GFR and C-reactive protein (CRP) levels. Although some published investigations have found higher CRP levels in those with chronic renal failure or on dialysis compared to healthy controls, it should be noted that diabetic subjects were excluded [1
] or comprised a minority (11 to 13%) of the study subjects [27
]. Our results are consistent with those recently published from the MDRD study (comprised of only 5% non-insulin dependent diabetic subjects), which reported that age-stratified CRP levels in subjects with mean GFR 32.7 ml/min/1.73 m2
(range 12 to 55 ml/min/1.73 m2
) approximated those in the general population as measured in the NHANES III study [29
]. Of note, two studies looking at CRP specifically in people with diabetes have reported an association with albuminuria and higher CRP levels, but did not address the relation between CRP and renal function [25
Tumor necrosis factor alpha (TNF-α) is an inflammatory cytokine with a short plasma half-life; therefore, the soluble form of the TNF receptor type 2 (sTNFR-2) is frequently used as a more stable surrogate marker. Since TNF-α is reported to be elevated in obese subjects [30
], our multivariable models included BMI, and we observed a strong and consistent inverse association between sTNFR-2 and renal function. One small study of 64 pre-dialysis patients (mean sCr 6.0 mg/dl) also reported a two-fold elevation in TNF-α levels when compared with compared with 40 healthy controls [2
]. The inverse association between GFR and sTNFR-2 may result from decreased clearance of this inflammatory cytokine in renal dysfunction as previously reported in a bilateral nephrectomized mouse model [31
We found no consistent relation between ICAM and GFR. Higher ICAM levels in dialysis-dependent subjects have been reported [1
], as well as in small studies of subjects with advanced renal dysfunction (mean sCr 6.0 mg/dl, range 1.3–6.7 mg/dl) [2
]. The cross-sectional CHS investigation reported 6% higher ICAM levels in those with renal dysfunction compared with those without [4
]. These CHS results may not be generalizable to subjects with better preserved kidney function since the mean CrCl in the renal dysfunction group was 39 ml/min, which is lower than that in our current cohort where only two subjects had a GFR < 30 ml/min/1.73 m2
. No previously published reports on ICAM and higher GFR levels were found.
We observed a strong and consistent inverse association between VCAM levels and GFR. Like ICAM, elevated VCAM levels have been observed in obese women [30
] and in dialysis-dependent subjects [1
]. Although one prospective observational study of 363 type 2 diabetics reported 1.5 to 2.0 increased relative risk for mortality in those with higher VCAM levels, it did not examine its relation to GFR [26
]. Interestingly, although both VCAM and ICAM expression are upregulated in inflammatory states, ICAM is constitutively expressed at low levels on normal endothelial cells [33
], and VCAM appears to be a more important factor in the initiation of atherosclerotic lesions [34
]. Our study found an inverse relation between VCAM and renal function, which may suggest an intensifying atherosclerotic process in those with decreased GFR, in a large cohort of people with type 2 diabetes.
By entering combinations of two significant biomarkers simultaneously into regression models, we observed that VCAM was not an independent predictor of GFR < 60 ml/min/m2
after controlling for sTNFR-2, suggesting that these molecules may be involved in the same inflammatory pathway. TNF-α infusion stimulates VCAM expression in the endothelial cells from a rabbit aorta [35
], but we found no clinical human studies examining this relation.
Limitations to our study include the cross-sectional design, which limits conclusions about mechanism or temporal relation. A prospective study would provide information about how these biomarkers may change with declining kidney function over time; however, the lack of previous data on how these biomarkers relate to GFR in diabetes makes these findings informative. The rare use of ACE inhibitors and statin medications in 1994 in this cohort may limit the generalizability of our results under current medical practices since these therapies are much more prevalent today in diabetics. Importantly, however, our study allows for a purer physiological examination of the association between GFR and biomarkers, since ACE-inhibitors can elevate serum creatinine and statins usually lower blood lipids. Unfortunately, no information is available on measured albuminuria, which is considered a marker for generalized endothelial vascular injury in the kidney as well as nephrotic range proteinuria, which can increase total cholesterol and LDL. Lastly, renal function was estimated from creatinine –based prediction equations since radio-isotope clearance GFR measurements would have been difficult and impractical in this cohort of >700 men. Estimated CrCl using the Cockcroft-Gault formula has been shown to be as accurate as creatinine clearance measured by 24 hour urine collections in diabetics [36
]. Whereas we chose to present the kidney function as GFR estimated by the simplified MDRD equation, the results were very similar when CrCl by Cockcroft-Gault was invoked. There is a growing literature on underestimation of true measured GFR by the MDRD equations in people with serum creatinine values in the normal range [37
]. We used categories of estimated GFR rather than GFR as a continuous variable, and therefore the relative association between these biomarkers and estimated GFR < 60 ml/min/1.73 m2
should not be influenced by a systematic bias in the estimation equation. Moreover, the associations see were consistent whether GFR was a continuous () or a categorical ( and ) variable. The relatively high CV of plasma creatinine in this study would presumably result in random misclassification and bias the results towards the null.
In conclusion, triglycerides, fibrinogen, sTNFR-2, and VCAM levels are inversely associated with GFR in a large cohort of men with type 2 diabetes, while HDL is positively correlated with renal function. The potential impact of drug therapies on altering these biomarkers and thereby slowing the progression of renal dysfunction is of great clinical interest and importance. Further investigation is also needed to determine whether the elevated levels of triglycerides and inflammatory biomarkers are due to increased production, decreased degradation, decreased excretion, or some combination in the setting of declining renal function. Nevertheless, an exciting new and more focused picture of elevated lipids and chronic endothelial inflammation is emerging as the potential mechanism behind the manifold risk of renal dysfunction and CVD in individuals with type 2 diabetes.