Steroidogenic acute regulatory-related lipid transfer domain protein 5 localization and regulation in renal tubules

Abstract

STARD5 is a cytosolic sterol transport protein that is predominantly expressed in liver and kidney. This study provides the first report on STARD5 protein expression and distribution in mouse kidney. Immunohistochemical analysis of C57BL/6J mouse kidney sections revealed that STARD5 is expressed in tubular cells within the renal cortex and medullar regions with no detectable staining within the glomeruli. Within the epithelial cells of proximal renal tubules, STARD5 is present in the cytoplasm with high staining intensity along the apical brush-border membrane. Transmission electron microscopy of a renal proximal tubule revealed STARD5 is abundant at the basal domain of the microvilli and localizes mainly in the rough endoplasmic reticulum (ER) with undetectable staining in the Golgi apparatus and mitochondria. Confocal microscopy of STARD5 distribution in HK-2 human proximal tubule cells showed a diffuse punctuate pattern that is distinct from the early endosome marker EEA1 but similar to the ER membrane marker GRP78. Treatment of HK-2 cells with inducers of ER stress increased STARD5 mRNA expression and resulted in redistribution of STARD5 protein to the perinuclear and cell periphery regions. Since recent reports show elevated ER stress response gene expression and increased lipid levels in kidneys from diabetic rodent models, we tested STARD5 and cholesterol levels in kidneys from the OVE26 type I diabetic mouse model. Stard5 mRNA and protein levels are increased 2.8- and 1.5-fold, respectively, in OVE26 diabetic kidneys relative to FVB control kidneys. Renal free cholesterol levels are 44% elevated in the OVE26 mice. Together, our data support STARD5 functioning in kidney, specifically within proximal tubule cells, and suggest a role in ER-associated cholesterol transport.

The mammalian START domain protein family in lipid transport in health and disease

“…Filipin staining to detect distribution of free cholesterol in the macrophages revealed high cholesterol concentration within the Golgi, suggesting localization of STARD5 with membranes enriched in free cholesterol. In mouse kidney sections, STARD5 protein was detected by immunohistochemistry in the proximal tubules, but not in the glomeruli (Chen et al 2009). The staining pattern indicated diffuse cytoplasmic distribution with concentrated expression at the apical membrane.…”

“…Although no data have directly demonstrated START proteins in this directional cholesterol trafficking pathway, STARD5 is presented as shuttling cholesterol from the PM to the ER as an example for a START protein in this process based on the apical membrane association renal proximal tubule cells (Chen et al 2009). Cholesterol transport to mitochondria by STARD1 and STARD4.…”

“…STARD5 binds both cholesterol and 25HC and is expressed predominantly in liver and kidney , Chen et al 2009). In liver, STARD5 is localized to the Kupffer cells and is not expressed in hepatocytes (Rodriguez-Agudo et al 2006).…”

“…Stard5/STARD5 mRNA expression is increased by agents that promote ER stress, such as in thapsigargin-treated NIH-3T3 and HK-2 cells or cholesterol-loaded mouse macrophages (Soccio et al 2005, Chen et al 2009). In HK-2 human proximal tubule cells, chemically induced ER stress promotes STARD5 redistribution from a diffuse to a more prominent perinuclear and cell membrane localization (Chen et al 2009).…”

“…In HK-2 human proximal tubule cells, chemically induced ER stress promotes STARD5 redistribution from a diffuse to a more prominent perinuclear and cell membrane localization (Chen et al 2009). The role for STARD5 during ER stress is not known but chronic ER stress and inflammation are underlying metabolic disorders in many disease states, including NAFLD, type II diabetes, and cancer (Tsai & Weissman 2010, Malhi & Kaufman 2011.…”