In Vitro Evaluation of the Effects of Cadmium on Endocytic Uptakes of Proteins into Cultured Proximal Tubule Epithelial Cells

Abstract

Cadmium (Cd) is an environmental pollutant known to cause dysfunctions of the tubular reabsorption of biomolecules in the kidney. Elevated levels of urinary excretion of low-molecular-weight proteins such as β2-microglobulin (β2-MG) have been used as an indicator of Cd-induced renal tubular dysfunctions. However, very few studies have examined the direct effects of Cd on the reabsorption efficiency of proteins using cultured renal cells. Here, we developed an in vitro assay system for quantifying the endocytic uptakes of fluorescent-labeled proteins by flow cytometry in S1 and S2 cells derived from mouse kidney proximal tubules. Endocytic uptakes of fluorescent-labeled albumin, transferrin, β2-MG, and metallothionein into S1 cells were confirmed by fluorescence imaging and flow cytometry. The exposure of S1 and S2 cells to Cd at 1 and 3 µM for 3 days resulted in significant decreases in the uptakes of β2-MG and metallothionein but not in those of albumin or transferrin. These results suggest that Cd affects the tubular reabsorption of low-molecular-weight proteins even at nonlethal concentrations. The in vitro assay system developed in this study to evaluate the endocytic uptakes of proteins may serve as a useful tool for detecting toxicants that cause renal tubular dysfunctions.

The pathogenesis of albuminuria in cadmium nephropathy

“…Additional observations are compatible with the concept that Cd affects some aspect of megalin homeostasis. In S1 cells, exposure to sublethal Cd doses impeded uptake of β 2 M much more than that of albumin ( Fujishiro et al, 2020 ). This observation is expected if a small fraction of filtered albumin and most or all filtered β 2 M enter tubular cells by a shared mechanism.…”

Single-Cell Imaging for Studies of Renal Uranium Transport and Intracellular Behavior

“…The gene expression of metal transporters and antioxidant enzymes differ in these segments [23]. The mechanism of segment-specific cadmium absorption and excretion has been clarified [24,25]. Microbeam scanning elemental analysis, which is also known as μ-PIXE (particle-induced X-ray emission with microprobe) analysis, is one of the suitable analytical techniques for detecting the precise distribution of elements in a small specimen with good detection of light elements, such as phosphorus, potassium, and calcium.…”

Editorial to Special Issue Toxic Metals, Chronic Diseases and Related Cancers

“…In this Special Issue, entitled “Toxic Metals, Chronic Diseases and Related Cancers”, there are 19 published manuscripts, including reports of environmental exposure monitoring [ 1 , 2 ] and food safety surveillance [ 3 , 4 ]; reviews focusing on health risks of chronic exposure to cadmium and lead [ 5 ], experimental studies on the toxicity of cadmium, lead or mercury in gonads [ 6 ], and evidence that cadmium exposure may be one of the environmental factors contributing to hyperglycemia, insulin resistance and diabetes [ 7 ]; a mathematical model of the oral glucose tolerance test [ 8 ]; epidemiological studies examining health impacts of cadmium, lead, and mercury [ 9 , 10 , 11 , 12 , 13 , 14 , 15 ]; and reports concerning the immunotoxicity of high exposure to lead [ 16 ], the effects of cadmium on protein reabsorption by kidney tubular epithelial cells [ 17 ], the cytotoxicity of hexavalent chromium [ 18 ], and the carcinogenicity of aluminum in vivo [ 19 ].…”

“…Fujishiro et al report that reabsorption of β 2 MG and metallothionein (MT) by immortalized cells derived from human proximal tubule dropped after incubation with cadmium for 3 days [ 17 ]. A similar result was seen when S1 and S2 cell culture models of mouse proximal tubule were tested.…”