Alterations of proteins in MDCK cells during acute potassium deficiency

K+ deficiency caused defects in renal tubular cell proliferation, oxidative stress response, tissue repair and tight junction integrity, but enhanced energy production, proteasome function and cellular K+ uptake

Hypokalemia is a common electrolyte disorder in hospitalized patients and those with chronic diseases and is associated with renal tubular injury. Our recent expression proteomics study revealed changes in levels of several proteins in renal tubular cells during K⁺ deficiency. However, functional significance and mechanisms underlying such changes remained unclear. The present study, thus, aimed to investigate functional changes of renal tubular cells induced by K⁺ deficiency. MDCK cells were maintained in normal-K⁺ (ANK; [K⁺] = 5.0 mM), Low-K⁺ (ALK; [K⁺] = 2.5 mM), or K⁺-depleted (AKD; [K⁺] = 0 mM) medium. Cell count and cell death assay showed that ALK and AKD groups had marked decrease in cell proliferation without significant change in cell death. Other functional investigations revealed that AKD cells had significantly increased levels of carbonylated proteins (by OxyBlot assay), impaired tissue repair (by scratch assay), defective tight junction (by Western blotting, immunofluorescence staining and measuring transepithelial electrical resistance), increased intracellular ATP level (by ATP measurement), decreased levels of ubiquitinated proteins (by Western blotting), and increased level of Na⁺/K⁺-ATPase (by Western blotting), which was consistent with the increased cellular K⁺ uptake after K⁺ repletion. Our findings have shown that AKD caused defects in cell proliferation, oxidative stress response, tissue repair and tight junction integrity, but on the other hand, enhanced energy production, proteasome function and cellular K⁺ uptake. These findings may shed light onto cellular response to K⁺ deficiency and better understanding of both pathogenic and compensatory mechanisms in hypokalemic nephropathy.

Role of HSP60 (HSPD1) in diabetes-induced renal tubular dysfunction: regulation of intracellular protein aggregation, ATP production, and oxidative stress

Because underlying mechanisms of diabetic nephropathy/tubulopathy remained poorly understood, we aimed to define a key protein involving in hyperglycemia-induced renal tubular dysfunction. All altered renal proteins identified from previous large-scale proteome studies were subjected to global protein network analysis, which revealed heat shock protein 60 (HSP60, also known as HSPD1) as the central node of protein-protein interactions. Functional validation was performed using small interfering RNA (siRNA) to knock down HSP60 (siHSP60). At 48 h after exposure to high glucose (HG) (25 mM), Madin-Darby canine kidney (MDCK) renal tubular cells transfected with controlled siRNA (siControl) had significantly increased level of HSP60 compared to normal glucose (NG) (5.5 mM), whereas siHSP60-transfected cells showed a dramatically decreased HSP60 level. siHSP60 modestly increased intracellular protein aggregates in both NG and HG conditions. Luciferin-luciferase assay showed that HG modestly increased intracellular ATP, and siHSP60 further enhanced such an increase. OxyBlot assay showed significantly increased level of oxidized proteins in HG-treated siControl-transfected cells, whereas siHSP60 caused marked increase of oxidized proteins under the NG condition. However, the siHSP60-induced accumulation of oxidized proteins was abolished by HG. In summary, our data demonstrated that HSP60 plays roles in regulation of intracellular protein aggregation, ATP production, and oxidative stress in renal tubular cells. Its involvement in HG-induced tubular cell dysfunction was most likely via regulation of intracellular ATP production.-Aluksanasuwan, S., Sueksakit, K., Fong-ngern, K., Thongboonkerd, V. Role of HSP60 (HSPD1) in diabetes-induced renal tubular dysfunction: regulation of intracellular protein aggregation, ATP production, and oxidative stress.