C-reactive protein promotes acute renal inflammation and fibrosis in unilateral ureteral obstructive nephropathy in mice

C-reactive protein promotes acute kidney injury via Smad3-dependent inhibition of CDK2/cyclin E

Acute kidney injury (AKI) is exacerbated in C-reactive protein transgenic mice but alleviated in Smad3 knockout mice. Here we used C-reactive protein transgenic/Smad3 wild-type and C-reactive protein transgenic/Smad3 knockout mice to investigate the signaling mechanisms by which C-reactive protein promotes AKI. Serum creatinine was elevated, and the extent of tubular epithelial cell necrosis following ischemia/reperfusion-induced AKI was greater in C-reactive protein transgenics but was blunted when Smad3 was deleted. Exacerbation of AKI in C-reactive protein transgenics was associated with increased TGF-β/Smad3 signaling and expression of the cyclin kinase inhibitor p27, but decreased phosphorylated CDK2 and expression of cyclin E. Concomitantly, tubular epithelial cell proliferation was arrested at the G1 phase in C-reactive protein transgenics with fewer cells entering the S-phase cell cycle as evidenced by fewer bromodeoxyuridine-positive cells. In contrast, the protection from AKI in C-reactive protein transgenic/Smad3 knockout mice was associated with decreased expression of p27 and promotion of CDK2/cyclin E-dependent G1/S transition of tubular epithelial cells. In vitro studies using tubular epithelial cells showed that C-reactive protein activates Smad3 via both TGF-β-dependent and ERK/MAPK cross talk mechanisms, Smad3 bound directly to p27, and blockade of Smad3 or the Fc receptor CD32 prevented C-reactive protein-induced p27-dependent G1 cell cycle arrest. In vivo, treatment of C-reactive protein transgenics with a Smad3 inhibitor largely improved AKI outcomes. Thus, C-reactive protein may promote AKI by impairing tubular epithelial cell regeneration via the CD32-Smad3-p27-driven inhibition of the CDK2/cyclin E complex. Targeting Smad3 may offer a new treatment approach for AKI.

C-Reactive Protein Promotes Diabetic Kidney Disease in db/db Mice via the CD32b-Smad3-mTOR signaling Pathway

C-reactive protein (CRP) is associated with progressive diabetic nephropathy in patients with type-2 diabetes (T2DN). However, role of CRP in T2DN remains unclear. We report here that CRP is pathogenic in T2DN in db/db mice that express human CRP (CRPtg-db/db). Compared to the littermate db/db mice, CRPtg-db/db developed more severe T2DN, showing higher levels of fasting blood glucose and microalbuminuria and more progressive renal inflammation and fibrosis. Enhanced T2DN in CRPtg-db/db mice were associated with over-activation of CRP-CD32b, NF-κB, TGF-β/Smad3, and mTOR signaling. Further studies in vitro defined that CRP activated Smad3 directly at 15 mins via the CD32b- ERK/p38 MAP kinase crosstalk pathway and indirectly at 24 hours through a TGF-β1-dependent mechanism. Importantly, CRP also activated mTOR signaling at 30 mins via a Smad3-dependent mechanism as Smad3 bound mTOR physically and CRP-induced mTOR signaling was abolished by a neutralizing CD32b antibody and a specific Smad3 inhibitor. Finally, we also found that CRP induced renal fibrosis through a CD32b-Smad3-mTOR pathway because blocking mTOR signaling with rapamycin inhibited CRP-induced CTGF and collagen I expression. Thus, CRP is pathogenic in T2DN. CRP may promote CD32b- NF-κB signaling to mediate renal inflammation; whereas, CRP may enhance renal fibrosis in T2DN via CD32b-Smad3-mTOR signaling.

Global miRNA expression and correlation with mRNA levels in primary human bone cells

MicroRNAs (miRNAs) are important post-transcriptional regulators that have recently introduced an additional level of intricacy to our understanding of gene regulation. The aim of this study was to investigate miRNA-mRNA interactions that may be relevant for bone metabolism by assessing correlations and interindividual variability in miRNA levels as well as global correlations between miRNA and mRNA levels in a large cohort of primary human osteoblasts (HOBs) obtained during orthopedic surgery in otherwise healthy individuals. We identified differential expression (DE) of 24 miRNAs, and found 9 miRNAs exhibiting DE between males and females. We identified hsa-miR-29b, hsa-miR-30c2, and hsa-miR-125b and their target genes as important modulators of bone metabolism. Further, we used an integrated analysis of global miRNA-mRNA correlations, mRNA-expression profiling, DE, bioinformatics analysis, and functional studies to identify novel target genes for miRNAs with the potential to regulate osteoblast differentiation and extracellular matrix production. Functional studies by overexpression and knockdown of miRNAs showed that, the differentially expressed miRNAs hsa-miR-29b, hsa-miR-30c2, and hsa-miR-125b target genes highly relevant to bone metabolism, e.g., collagen, type I, α1 (COL1A1), osteonectin (SPARC), Runt-related transcription factor 2 (RUNX2), osteocalcin (BGLAP), and frizzled-related protein (FRZB). These miRNAs orchestrate the activities of key regulators of osteoblast differentiation and extracellular matrix proteins by their convergent action on target genes and pathways to control the skeletal gene expression.

The long pentraxin PTX3 promotes fibrocyte differentiation

Monocyte-derived, fibroblast-like cells called fibrocytes are associated with fibrotic lesions. The plasma protein serum amyloid P component (SAP; also known as pentraxin-2, PTX2) inhibits fibrocyte differentiation in vitro, and injections of SAP inhibit fibrosis in vivo. SAP is a member of the pentraxin family of proteins that includes C-reactive protein (CRP; PTX1) and pentraxin-3 (PTX3). All three pentraxins are associated with fibrosis, but only SAP and CRP have been studied for their effects on fibrocyte differentiation. We find that compared to SAP and CRP, PTX3 promotes human and murine fibrocyte differentiation. The effect of PTX3 is dependent on FcγRI. In competition studies, the fibrocyte-inhibitory activity of SAP is dominant over PTX3. Binding competition studies indicate that SAP and PTX3 bind human FcγRI at different sites. In murine models of lung fibrosis, PTX3 is present in fibrotic areas, and the PTX3 distribution is associated with collagen deposition. In lung tissue from pulmonary fibrosis patients, PTX3 has a widespread distribution, both in unaffected tissue and in fibrotic lesions, whereas SAP is restricted to areas adjacent to vessels, and absent from fibrotic areas. These data suggest that the relative levels of SAP and PTX3 present at sites of fibrosis may have a significant effect on the ability of monocytes to differentiate into fibrocytes.

Rosuvastatin ameliorates inflammation, renal fat accumulation, and kidney injury in transgenic spontaneously hypertensive rats expressing human C-reactive protein

Recently, we derived "humanized" spontaneously hypertensive rats (SHR-CRP) in which transgenic expression of human CRP induces inflammation, oxidative stress, several features of metabolic syndrome and target organ injury. In addition, we found that rosuvastatin treatment of SHR-CRP transgenic rats can protect against pro-inflammatory effects of human CRP and also reduce cardiac inflammation and oxidative damage. In the current study, we tested the effects of rosuvastatin (5 mg/kg) on kidney injury in SHR-CRP males versus untreated SHR-CRP and SHR controls. All rats were fed a high sucrose diet. In SHR-CRP transgenic rats, treatment with rosuvastatin for 10 weeks, compared to untreated transgenic rats and SHR controls, was associated with significantly reduced systemic inflammation which was accompanied with activation of antioxidative enzymes in the kidney, lower renal fat accumulation, and with amelioration of histopathological changes in the kidney. These findings provide evidence that, in the presence of high CRP levels, rosuvastatin exhibits significant anti-inflammatory, anti-oxidative, and renoprotective effects.

ASK1/p38 signaling in renal tubular epithelial cells promotes renal fibrosis in the mouse obstructed kidney

Stress-activated kinases p38 MAPK and JNK promote renal fibrosis; however, how the pathways by which these kinases are activated in kidney disease remain poorly defined. Apoptosis signal-regulating kinase 1 (ASK1/MAPKKK5) is a member of the MAPKKK family that can induce activation of p38 and JNK. The present study examined whether ASK1 induces p38/JNK activation and renal fibrosis in unilateral ureteric obstruction (UUO) using wild-type (WT) and Ask1-deficient ( Ask1−/−) mice. Basal p38 and JNK activation in WT kidneys was increased three- to fivefold in day 7 UUO mice in association with renal fibrosis. In contrast, there was no increase in p38 activation in Ask1−/− UUO mice, whereas JNK activation was only partially increased. The progressive increase in kidney collagen (hydroxyproline) content seen on days 7 and 12 of UUO in WT mice was significantly reduced in Ask1−/− UUO mice in association with reduced α-smooth muscle actin-positive myofibroblast accumulation. However, cultured WT and Ask1−/− renal fibroblasts showed equivalent proliferation and matrix production, indicating that ASK1 acts indirectly on fibroblasts. Tubular epithelial cells are the main site of p38 activation in the obstructed kidney. Angiotensin II and H2O2, but not IL-1 or lipopolysaccharide, induced p38 activation and upregulation of transforming growth factor-β1, platelet-derived growth factor-B, and monocyte chemoattractant protein-1 production was suppressed in Ask1−/− tubular epithelial cells. In addition, macrophage accumulation was significantly inhibited in Ask1−/− UUO mice. In conclusion, ASK1 is an important upstream activator of p38 and JNK signaling in the obstructed kidney, and ASK1 is a potential therapeutic target in renal fibrosis.

C-reactive protein promotes acute kidney injury by impairing G1/S-dependent tubular epithelium cell regeneration

There are three new and significant findings from the present study: (i) CRP is not only a biomarker for, but also a pathogenic mediator in, AKI; (ii) CRP promotes AKI by inhibiting G1/S-dependent TEC regeneration; and (iii) blockade of CRP signalling attenuates AKI.

Pentraxins: structure, function, and role in inflammation

The pentraxins are an ancient family of proteins with a unique architecture found as far back in evolution as the Horseshoe crab. In humans the two members of this family are C-reactive protein and serum amyloid P. Pentraxins are defined by their sequence homology, their pentameric structure and their calcium-dependent binding to their ligands. Pentraxins function as soluble pattern recognition molecules and one of the earliest and most important roles for these proteins is host defense primarily against pathogenic bacteria. They function as opsonins for pathogens through activation of the complement pathway and through binding to Fc gamma receptors. Pentraxins also recognize membrane phospholipids and nuclear components exposed on or released by damaged cells. CRP has a specific interaction with small nuclear ribonucleoproteins whereas SAP is a major recognition molecule for DNA, two nuclear autoantigens. Studies in autoimmune and inflammatory disease models suggest that pentraxins interact with macrophage Fc receptors to regulate the inflammatory response. Because CRP is a strong acute phase reactant it is widely used as a marker of inflammation and infection.

The microRNA miR-433 promotes renal fibrosis by amplifying the TGF-β/Smad3-Azin1 pathway

The TGF-β/Smad3 pathway plays a major role in tissue fibrosis, but the precise mechanisms are not fully understood. Here we identified microRNA miR-433 as an important component of TGF-β/Smad3-driven renal fibrosis. The miR-433 was upregulated following unilateral ureteral obstruction, a model of aggressive renal fibrosis. In vitro, overexpression of miR-433 enhanced TGF-β1-induced fibrosis, whereas knockdown of miR-433 suppressed this response. Furthermore, Smad3, but not Smad2, bound to the miR-433 promoter to induce its expression. Delivery of an miR-433 knockdown plasmid to the kidney by ultrasound microbubble-mediated gene transfer suppressed the induction and progression of fibrosis in the obstruction model. The antizyme inhibitor Azin1, an important regulator of polyamine synthesis, was identified as a target of miR-433. Overexpression of miR-433 suppressed Azin1 expression, while, in turn, Azin1 overexpression suppressed TGF-β signaling and the fibrotic response. Thus, miR-433 is an important component of TGF-β/Smad3-induced renal fibrosis through the induction of a positive feedback loop to amplify TGF-β/Smad3 signaling, and may be a potential therapeutic target in tissue fibrosis.

C-reactive protein exacerbates renal ischemia-reperfusion injury

Renal ischemia-reperfusion injury (IRI) is a common cause of acute kidney injury (AKI), occurring with hypotension and cardiovascular surgery and inevitably during kidney transplantation. Mortality from AKI is high due to incomplete knowledge of the pathogenesis of IRI and the lack of an effective therapy. Inflammation accompanies IRI and increases the blood level of C-reactive protein (CRP), a biomarker of worsened outcomes in AKI. To test if CRP is causal in AKI we subjected wild-type mice (WT) and human CRP transgenic mice (CRPtg) to bilateral renal IRI (both pedicles clamped for 30 min at 37°C then reperfused for 24 h). Serum human CRP level was increased approximately sixfold after IRI in CRPtg (10.62 ± 1.31 μg/ml at baseline vs. 72.01 ± 9.41 μg/ml at 24 h) but was not elevated by sham surgery wherein kidneys were manipulated but not clamped. Compared with WT, serum creatinine, urine albumin, and histological evidence of kidney damage were increased after IRI in CRPtg mice. RT-PCR analysis of mRNA isolated from whole kidneys of CRPtg and WT subjected to IRI revealed that in CRPtg kidneys 1) upregulation of markers of macrophage classical activation (M1 markers) was blunted, 2) downregulation of markers of macrophage alternative activation (M2 markers) was more robust, and 3) expression of the activating receptor FcγRI was increased. Our finding that CRP exacerbates IRI-induced AKI, perhaps by shifting the balance of macrophage activation and FcγR expression towards a detrimental portfolio, might make CRP a promising therapeutic target for the treatment of AKI.

Pentraxins in nephrology: C-reactive protein, serum amyloid P and pentraxin-3

Every clinician uses C-reactive protein (CRP) levels as a biomarker for systemic inflammation in acute disorders. Nephrologists also consider CRP levels as a predictor for overall mortality in patients with chronic kidney disease or end-stage renal disease. But what is the biological function of CRP? CRP is a member of the family of pentraxins, which are small pentameric innate immunity effector proteins. Pentraxins are absent or weakly expressed during homeostasis. However, the pro-inflammatory cytokines interleukin (IL)-1, IL-6 and tumour necrosis factor induce CRP and serum amyloid P (SAP) in hepatocytes, whereas the long pentraxins, such as pentraxin (PTX)-3, are produced in peripheral tissues and monocytic phagocytes. Pentraxins opsonize pathogens or other particles such as dead cells, for their phagocytic clearance or induce pathogen killing in extracellular compartments. In this review, we discuss the immunoregulatory properties of the different members of the pentraxin family. We discuss the evolving evidence demonstrating their roles in acute and chronic forms of kidney disease and the significance of SAP and PTX3 as additional biomarkers of innate immune activation and systemic inflammation.

Vasoactive intestinal peptide enhances TNF-α-induced IL-6 and IL-8 synthesis in human proximal renal tubular epithelial cells by NF-κB-dependent mechanism

Vasoactive intestinal polypeptide (VIP) is a 28-amino acid neuropeptide with vasodilator, bronchodilator, and anti-inflammatory effects. But little is known about its pro-inflammatory effects. We investigated the effect of VIP on the secretion of interleukin-6 (IL-6) and interleukin-8 (IL-8), two pro-inflammatory cytokines, in TNF-α-activated proximal renal tubular epithelial cell line (HK-2 cells). Cultured HK-2 cells were treated with TNF-α in the presence or absence of VIP with a dose range from 1 to 100 nM, followed by analysis of pro-inflammatory cytokines (IL-6 and IL-8) induction and their signal events including activation of the NF-κB pathway. We report here that tumor necrosis factor-α (TNF-α) increased IL-6 and IL-8 production, and that these effects were potentiated by VIP at 10 nM in HK-2 cells. However, VIP at 1 and 100 nM did not display this function. Consistent with these observations, we were able to show that VIP at 10 nM upregulated TNF-α-induced phosphorylation of IκB-α, leading to IκB-α degradation and the subsequent nuclear translocation of NF-κB. Furthermore, VIP-enhanced activation of NF-κB transcription activity was demonstrated using a NF-κB reporter construct upon transient transfection into HK-2 cells. These results strongly suggest that VIP synergistically enhances TNF-α-stimulated IL-6 and IL-8 synthesis via activating the NF-κB pathway in HK-2 cells.