The PPARδ ligand GW501516 reduces growth but not apoptosis in mouse inner medullary collecting duct cells

Peroxisome proliferator-activated receptor delta protects against obesity-related glomerulopathy through the P38 MAPK pathway

OBJECTIVE

Obesity is a prominent component of metabolic syndrome and a major risk factor for renal disease. The aim of this study was to explore the effect of cross-talk between peroxisome proliferator-activated receptor (PPAR)δ and p38 mitogen-activated protein kinase (p38 MAPK) on obesity-related glomerulopathy.

DESIGN AND METHODS

Male Wistar rats were randomly assigned to standard laboratory chow or a high-fat diet for 32 weeks. Glomerular mesangial cells HBZY-1 and mature differentiation 3T3-L1 cells were cocultured and were transfected with PPARδ-expressing vectors or treated with agonist or inhibitor of PPARδ or p38 MAPK.

RESULTS

Rats on a high-fat diet showed typical characteristics of metabolic syndrome including obesity, dyslipidemia, insulin resistance, and hypertension. Rats on a high-fat diet also had significant glomerular hypertrophy and extracellular matrix accumulation, which were accompanied by increased p38 MAPK phosphorylation and decreased PPARδ expression in the kidney tissue. The roles of p38 MAPK and PPARδ in a coculture system of mesangial cells and mature differentiation 3T3-L1 cells were further explored. PPARδ suppression promoted laminin and type IV collagen secretion through p38 MAPK phosphorylation in mesangial cells, whereas PPARδ overexpression or PPARδ agonist attenuated phosphorylation of p38 MAPK and laminin and type IV collagen secretion.

CONCLUSIONS

The characteristics of obesity-related glomerulopathy, which might be partly caused by PPARδ suppression-induced p38 MAPK activation and laminin and type IV collagen secretion was demonstrated.

Peroxisome proliferator-activated receptor δ downregulates the expression of the receptor for advanced glycation end products and pro-inflammatory cytokines in the kidney of streptozotocin-induced diabetic mice

Activation of peroxisome proliferator-activated receptor δ (PPARδ) plays board beneficial effects in treating metabolic syndrome. The aim of this study is to examine whether PPARδ alters the expression of the receptor for advanced glycation end products (RAGE) and downstream pro-inflammatory cytokines in diabetic nephropathy. Streptozotocin-induced diabetic mice (STZ mice) were injected with a PPARδ agonist, L-165041 (5 μM/kg, intraperitoneal) once daily for 10 days and high glucose-treated cultured HEK cells were also used. After L-165041 treatment, serum TNFα, IL-6 and IL-1 levels were significantly decreased in STZ mice. RAGE mRNA and protein expression were both decreased by L-165041 in kidney tissues of STZ mice. The high glucose incubation increased NF-κB, RAGE and IL-6 expressions in HEK293 cells. These effects were inhibited by L-165041 and specific RAGE siRNA transfection. This study demonstrated that PPARδ may play a beneficial role in preventing diabetic nephropathy. Its downstream signaling may include RAGE and NF-κB pathway. Target on PPARδ will provide new meaningful therapies to patients with diabetic nephropathy.

Thiazolidinediones alter growth and epithelial cell integrity, independent of PPAR-γ and MAPK activation, in mouse M1 cortical collecting duct cells

Peroxisome proliferator-activated receptor (PPAR)-γ is highly expressed in the collecting duct (CD), yet little is known about the effects of PPAR-γ ligands, thiazolidinediones (TZDs), on CD cell structure and function. M1 mouse cortical CD cells were treated with 5 μM troglitazone (TRO) and rosiglitazone (ROSI). First, growth was measured by [(3)H]thymidine and [(3)H]leucine incorporation, as well as analysis of cyclin D1 and the CDK inhibitor p27 by Western blot. [(3)H]thymidine incorporation was reduced by 56 and 24% by TRO and ROSI at 6 h, and [(3)H]leucine by 21 and 10%. A similar growth inhibition was also observed after 24 h for thymidine, but leucine was reduced by 48 and 24%, respectively. Likewise, cyclin D1 was diminished 60% by TRO, and p27 was elevated 1.6- and 1.7-fold in response to TRO and ROSI. Next, epithelial cell integrity was assessed by measuring different markers by Western blot analysis. While fibronectin and α-smooth muscle actin levels were unchanged, by 24 h E-cadherin was decreased by 50%, and β-catenin levels were reduced 2- and 1.5-fold in response to TRO and ROSI, respectively. GW9662, a PPAR-γ antagonist, did not reverse any of the TZD responses in M1 cells. Of interest, phosho-p38 levels were also elevated 2-fold in response to TRO and 2.3-fold to ROSI, but MAPK inhibition by PD98059 or SB203580 caused an additive inhibition of cell growth and did not alter E-cadherin or β-catenin in response to TZDs. Finally, apoptotic death was assessed by Western blot, but cleaved caspase-3 levels were unchanged from 15 min to 24 h in response to TZDs, and TRO did not affect cell viability or reactive oxygen species generation. Our data suggest that TZDs cause a disruption of M1 cell integrity that is preceded by an inhibition of cell growth. This response is independent of p38 or PPAR-γ activation.

Sorting out the functional role(s) of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) in cell proliferation and cancer

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) has many beneficial physiological functions ranging from enhancing fatty acid catabolism, improving insulin sensitivity, inhibiting inflammation and increasing oxidative myofibers allowing for improved athletic performance. Thus, given the potential for targeting PPARbeta/delta for the prevention and/or treatment of diseases including diabetes, dyslipidemias, metabolic syndrome and cancer, it is critical to clarify the functional role of PPARbeta/delta in cell proliferation and associated disorders such as cancer. However, there is considerable controversy whether PPARbeta/delta stimulates or inhibits cell proliferation. This review summarizes the literature describing the influence of PPARbeta/delta on cell proliferation, with an emphasis toward dissecting the data that give rise to opposing hypotheses. Suggestions are offered to standardize measurements associated with these studies so that interlaboratory comparisons can be accurately assessed.