Environmental Cadmium Exposure Induces an Increase in Systolic Blood Pressure by Its Effect on GFR

Urinary β2-Microglobulin Predicts the Risk of Hypertension in Populations Chronically Exposed to Environmental Cadmium

Chronic exposure to the pollutant cadmium (Cd) is inevitable for most people because it is present in nearly all food types. Concerningly, the risk of developing hypertension has been linked to dietary Cd exposure lower than 58 µg/day for a 70 kg person. The mechanisms involved are, however, unclear. Since the kidneys play an indispensable role in long-term blood pressure regulation, and they are also the main site of Cd accumulation and toxicity, a retrospective analysis was conducted to examine if kidney damage and malfunction, reflected by urinary β2-microglobulin excretion (Eβ2M), and the estimated glomerular filtration rate (eGFR), are related to Cd excretion (ECd) and blood pressure variation. Data were obtained from 689 Thai Nationals without diabetes or occupational exposure to Cd, of which 32.4% had hypertension and 7.3% had β2-microglobulinuria, defined as an increase in the β2M excretion rate ≥ 300 µg/g creatinine. Respective prevalence odds ratio (POR) and 95% confidence interval (CI) values for β2-microglobulinuria and hypertension were 10.7 (1.36-83.4), p = 0.024 and 2.79 (1.60-4.87) p < 0.001, comparing the top quartile of ECd with the bottom quartile. Only in subjects with eGFR below 90 mL/min/1.73 m2 did systolic blood pressure (SBP) and diastolic blood pressure (DBP) both increase linearly with Eβ2M (respective β = 0.182 and 0.192 for SBP and DBP) after adjustment for age, body mass index, gender, and smoking. The present study confirms the significant impact of Cd on the risk of having hypertension, following GFR loss induced by Cd. A simple mediation model analysis for cause-effect inference has provided, for the first time, evidence that may link rising SBP and DBP in Cd-exposed people to a novel role of β2M as a predictor of blood pressure variability.

Antioxidative Function of Zinc and Its Protection Against the Onset and Progression of Kidney Disease Due to Cadmium

Chronic kidney disease (CKD) is now the world's top seventh cause of death from a non-communicable disease, and its incidence is projected to increase further as its major risk factors, including obesity, diabetes, hypertension, and non-alcoholic fatty liver disease (NAFLD), continue to rise. Current evidence has linked the increased prevalence of CKD, diabetes, hypertension, and NAFLD to chronic exposure to the metal pollutant cadmium (Cd). Exposure to Cd is widespread because diet is the main exposure route for most people. Notably, however, the health risk of dietary Cd exposure is underappreciated, and the existing tolerable exposure guidelines for Cd do not afford health protection. New health-protective exposure guidelines are needed. From one's diet, Cd is absorbed by the intestinal epithelium from where it passes through the liver and accumulates within the kidney tubular epithelial cells. Here, it is bound to metallothionine (MT), and as it is gradually released, it induces tubular damage, tubulointerstitial inflammation and fibrosis, and nephron destruction. The present review provides an update on our knowledge of the exposure levels of Cd that are found to be associated with CKD, NAFLD, and mortality from cardiovascular disease. It discusses the co-existence of hypertension and CKD in people environmentally exposed to Cd. It highlights nuclear and mitochondrial targeting and zinc deficiency as the universal cytotoxic mechanisms of Cd. Special emphasis is placed on the novel antioxidative function of zinc involving de novo heme biosynthesis and the induced expression of heme oxygenase-1 (HO-1). Other exogenous biomolecules with promising anti-Cd toxicity are highlighted.

Albuminuria in People Chronically Exposed to Low-Dose Cadmium Is Linked to Rising Blood Pressure Levels

Exposure to low-dose environmental pollutant cadmium (Cd) increases the risks of both albuminuria and hypertension by mechanisms which are poorly understood. Here, multiple regression and mediation analyses were applied to data from 641 Thai subjects of whom 39.8%, 16.5%, 10.8%, and 4.8% had hypertension, albuminuria, diabetes, and chronic kidney disease (CKD), defined as the estimated glomerular filtration rate (eGFR) ≤ 60 mL/min/1.73 m2, respectively. To correct for interindividual differences in urine dilution and surviving nephrons, the excretion rates of Cd (ECd), albumin (Ealb), and β2-microglobulin (Eβ2M) were normalized to the creatinine clearance (Ccr) as ECd/Ccr, Ealb/Ccr, and Eβ2M/Ccr. The respective risks of having CKD and hypertension rose to 3.52 (95% CI: 1.75, 7.05) and 1.22 (95% CI: 1.12, 1.3) per doubling of the Cd body burden. The respective risk of having albuminuria increased 2.95-fold (p = 0.042) and 4.17-fold (p = 0.020) in subjects who had hypertension plus severe and extremely severe tubular dysfunction, defined according to the elevated β2M excretion rates. In multiple regression analysis, the Ealb/Ccr increased linearly with both the systolic blood pressure (SBP, β = 0.263) and diastolic blood pressure (DBP, β = 0.150), while showing an inverse association with eGFR (β = -0.180). The mediation model analyses inferred that a declining eGFR induced by Cd contributed to 80.6% of the SBP increment (p = 0.005), which then fully mediated an elevation of albumin excretion (p < 0.001). The present study provides, for the first time, evidence that causally links Cd-induced eGFR reductions to blood pressure elevations, which enhance albumin excretion.

Urinary N-acetylglucosaminidase in People Environmentally Exposed to Cadmium Is Minimally Related to Cadmium-Induced Nephron Destruction

Exposure to even low levels of the environmental pollutant cadmium (Cd) increases the risk of kidney damage and malfunction. The body burden of Cd at which these outcomes occur is not, however, reliably defined. Here, multiple-regression and mediation analyses were applied to data from 737 non-diabetic Thai nationals, of which 9.1% had an estimated glomerular filtration rate (eGFR) ≤ 60 mL/min/1.73 m2 (a low eGFR). The excretion of Cd (ECd), and renal-effect biomarkers, namely β2-microglobulin (Eβ2M), albumin (Ealb), and N-acetylglucosaminidase (ENAG), were normalized to creatinine clearance (Ccr) as ECd/Ccr Eβ2M/Ccr, Ealb/Ccr, and ENAG/Ccr. After adjustment for potential confounders, the risks of having a low eGFR and albuminuria rose twofold per doubling ECd/Ccr rates and they both varied directly with the severity of β2-microglobulinuria. Doubling ECd/Ccr rates also increased the risk of having a severe tubular injury, evident from ENAG/Ccr increments [POR = 4.80, p = 0.015]. ENAG/Ccr was strongly associated with ECd/Ccr in both men (β = 0.447) and women (β = 0.394), while showing a moderate inverse association with eGFR only in women (β = -0.178). A moderate association of ENAG/Ccr and ECd/Ccr was found in the low- (β = 0.287), and the high-Cd body burden groups (β = 0.145), but ENAG/Ccr was inversely associated with eGFR only in the high-Cd body burden group (β = -0.223). These discrepancies together with mediation analysis suggest that Cd-induced nephron destruction, which reduces GFR and the tubular release of NAG by Cd, involves different mechanisms and kinetics.