Proximal tubule PPARα attenuates renal fibrosis and inflammation caused by unilateral ureteral obstruction.

We examined the effects of increased expression of proximal tubule peroxisome proliferator-activated receptor (PPAR)α in a mouse model of renal fibrosis. After 5 days of unilateral ureteral obstruction (UUO), PPARα expression was significantly reduced in kidney tissue of wild-type mice but this downregulation was attenuated in proximal tubules of PPARα transgenic (Tg) mice. When compared with wild-type mice subjected to UUO, PPARα Tg mice had reduced mRNA and protein expression of proximal tubule transforming growth factor (TGF)-ß1, with reduced production of extracellular matrix proteins including collagen 1, fibronectin, α-smooth muscle actin, and reduced tubulointerstitial fibrosis. UUO-mediated increased expression of microRNA 21 in kidney tissue was also reduced in PPARα Tg mice. Overexpression of PPARα in cultured proximal tubular cells by adenoviral transduction reduced aristolochic acid-mediated increased production of TGF-ß, demonstrating PPARα signaling reduces epithelial TGF-ß production. Flow cytometry studies of dissociated whole kidneys demonstrated reduced macrophage infiltration to kidney tissue in PPARα Tg mice after UUO. Increased expression of proinflammatory cytokines including IL-1ß, IL-6, and TNF-α in wild-type mice was also significantly reduced in kidney tissue of PPARα Tg mice. In contrast, the expression of anti-inflammatory cytokines IL-10 and arginase-1 was significantly increased in kidney tissue of PPARα Tg mice when compared with wild-type mice subjected to UUO. Our studies demonstrate several mechanisms by which preserved expression of proximal tubule PPARα reduces tubulointerstitial fibrosis and inflammation associated with obstructive uropathy.

Reduced kidney lipoprotein lipase and renal tubule triglyceride accumulation in cisplatin-mediated acute kidney injury.

Peroxisome proliferator-activated receptor-α (PPARα) activation attenuates cisplatin (CP)-mediated acute kidney injury by increasing fatty acid oxidation, but mechanisms leading to reduced renal triglyceride (TG) accumulation could also contribute. Here, we investigated the effects of PPARα and CP on expression and enzyme activity of kidney lipoprotein lipase (LPL) as well as on expression of angiopoietin protein-like 4 (Angptl4), glycosylphosphatidylinositol-anchored-HDL-binding protein (GPIHBP1), and lipase maturation factor 1 (Lmf1), which are recognized as important proteins that modulate LPL activity. CP caused a 40% reduction in epididymal white adipose tissue (WAT) mass, with a reduction of LPL expression and activity. CP also reduced kidney LPL expression and activity. Angptl4 mRNA levels were increased by ninefold in liver and kidney tissue and by twofold in adipose tissue of CP-treated mice. Western blots of two-dimensional gel electrophoresis identified increased expression of a neutral pI Angptl4 protein in kidney tissue of CP-treated mice. Immunolocalization studies showed reduced staining of LPL and increased staining of Angptl4 primarily in proximal tubules of CP-treated mice. CP also increased TG accumulation in kidney tissue, which was ameliorated by PPARα ligand. In summary, a PPARα ligand ameliorates CP-mediated nephrotoxicity by increasing LPL activity via increased expression of GPHBP1 and Lmf1 and by reducing expression of Angptl4 protein in the proximal tubule.

The beta-agonist isoproterenol attenuates EGF-stimulated wound closure in human airway epithelial cells.

Asthma is a disease characterized by reversible airway obstruction. An additional hallmark of chronic asthma is altered wound healing that leads to airway remodeling. Although beta-agonists are effective in treating the bronchospasm associated with asthma, their effects on airway wound healing, which are related to airway remodeling, are unknown. It has been demonstrated that beta-agonists can alter the signaling of epidermal growth factor (EGF) receptors, which are important in timely wound healing. Therefore, we hypothesized that the beta-agonist isoproterenol would affect wound healing. Using an in vitro scrape wound assay, we demonstrated that isoproterenol attenuates EGF-stimulated wound healing in 16HBE airway epithelial cell cultures. Through experiments with forskolin and cells overexpressing beta2-adrenergic receptor-yellow fluorescent protein, we show that attenuation is due to the accumulation of cAMP and the involvement of at least one additional pathway. Furthermore, attenuation is not due to a direct effect on the EGF receptor or to an alteration of the ERK/MAPK signaling cascade. Based on these results, we propose that isoproterenol may exert its effects through other MAPK signaling pathways (JNK and/or p38) or through parallel mechanisms. These results also demonstrate a problem of potential therapeutic relevance in which a commonly prescribed medication may alter wound healing and contribute to the remodeling of asthmatic airways.

EGF receptor downregulation depends on a trafficking motif in the distal tyrosine kinase domain.

On binding to its receptor, epidermal growth factor (EGF) initiates a cascade of events leading to cell proliferation or differentiation. In addition, the EGF receptor itself is downregulated to attenuate mitogenic signaling. Downregulation occurs through trafficking of receptors to lysosomes, culminating in proteolytic destruction of both the receptor and ligand; however, endocytic sorting mechanisms that underlie lysosomal targeting remain obscure. The goal of this study was to explore one aspect of the molecular basis for ligand-induced lysosomal targeting and degradation of EGF receptors. In this study, we identify a tyrosine-leucine motif ((954)YLVI) that is essential for transit of ligand-receptor complexes to lysosomes. When this motif is mutated, HEK 293 cells expressing the mutant receptors demonstrate impaired lysosomal targeting and downregulation compared with wild-type receptors. (954)YLVI is highly conserved among EGF receptors from various mammalian and invertebrate species and is critical for receptor downregulation. We propose that (954)YLVI works in concert with at least two additional regions within the EGF receptor cytoplasmic domain that are essential for efficiently targeting ligand-receptor complexes to the lysosome.