Targeting tumor suppressor p53 for organ fibrosis therapy

Fibrosis is a reparative and progressive process characterized by abnormal extracellular matrix deposition, contributing to organ dysfunction in chronic diseases. The tumor suppressor p53 (p53), known for its regulatory roles in cell proliferation, apoptosis, aging, and metabolism across diverse tissues, appears to play a pivotal role in aggravating biological processes such as epithelial-mesenchymal transition (EMT), cell apoptosis, and cell senescence. These processes are closely intertwined with the pathogenesis of fibrotic disease. In this review, we briefly introduce the background and specific mechanism of p53, investigate the pathogenesis of fibrosis, and further discuss p53's relationship and role in fibrosis affecting the kidney, liver, lung, and heart. In summary, targeting p53 represents a promising and innovative therapeutic approach for the prevention and treatment of organ fibrosis.

State of the Art of Pharmacological Activators of p53 in Ocular Malignancies

The pivotal role of p53 in the regulation of a vast array of cellular functions has been the subject of extensive research. The biological activity of p53 is not strictly limited to cell cycle arrest but also includes the regulation of homeostasis, DNA repair, apoptosis, and senescence. Thus, mutations in the p53 gene with loss of function represent one of the major mechanisms for cancer development. As expected, due to its key role, p53 is expressed throughout the human body including the eye. Specifically, altered p53 signaling pathways have been implicated in the development of conjunctival and corneal tumors, retinoblastoma, uveal melanoma, and intraocular melanoma. As non-selective cancer chemotherapies as well as ionizing radiation can be associated with either poor efficacy or dose-limiting toxicities in the eye, reconstitution of the p53 signaling pathway currently represents an attractive target for cancer therapy. The present review discusses the role of p53 in the pathogenesis of these ocular tumors and outlines the various pharmacological activators of p53 that are currently under investigation for the treatment of ocular malignancies.

Netrin-1 Overexpression Induces Polycystic Kidney Disease: A Novel Mechanism Contributing to Cystogenesis in Autosomal Dominant Polycystic Kidney Disease

Despite recent advances in understanding the pathogenesis of polycystic kidney disease (PKD), the underlying molecular mechanisms involved in cystogenesis are not fully understood. This study describes a novel pathway involved in cyst formation. Transgenic mice overexpressing netrin-1 in proximal tubular cells showed increased production and urinary excretion of netrin-1. Although no cysts were detectable immediately after birth, numerous small cysts were evident by the age of 4 weeks, and disease was accelerated along with age. Surprisingly, cyst formation in the kidney was restricted to male mice, with 80% penetrance. However, ovariectomy induced kidney cyst growth in netrin-1-overexpressing female mice. Cyst development in males was associated with albuminuria and polyuria and increased cAMP excretion in netrin-1 transgenic mice. Netrin-1 overexpression significantly increased extracellular signal-regulated kinase and focal adhesion kinase phosphorylation and vimentin expression. Interestingly, p53 expression was increased but in an inactive form. Furthermore, netrin-1 expression was increased in cystic epithelia and urine of various rodent models of PKD. siRNA-mediated suppression of netrin-1 significantly reduced cyst growth and improved kidney function in netrin-1 transgenic mice and in two genetic animal models of PKD. Together, these data demonstrate that netrin-1 up-regulation induced cyst formation in autosomal dominant PKD.

The master regulators Myc and p53 cellular signaling and functions in polycystic kidney disease

The transcription factors Myc and p53 associated with oncogenesis play determinant roles in a human genetic disorder, autosomal dominant polycystic kidney disease (ADPKD), that was coined early in ADPKD etiology a «neoplasia in disguise ». These factors are interdependent master cell regulators of major biological processes including proliferation, apoptosis, cell growth, metabolism, inflammation, fibrosis and differentiation that are all modulated in ADPKD. Myc and p53 proteins evolved to respond and carry out overlapping functions via opposing mechanisms of action. Studies in human ADPKD kidneys, caused by mutations in the PKD1 or PKD2 genes, reveal reduced p53 expression and high expression of Myc in the cystic tubular epithelium. Myc and p53 via direct interaction act respectively, as transcriptional activator and repressor of PKD1 gene expression, consistent with increased renal PKD1 levels in ADPKD. Mouse models generated by Pkd1 and Pkd2 gene dosage dysregulation reproduce renal cystogenesis with activation of Myc expression and numerous signaling pathways, strikingly similar to those determined in human ADPKD. In fact, upregulation of renal Myc expression is also detected in virtually all non-orthologous animal models of PKD. A definitive causal connection of Myc with cystogenesis was established by renal overexpression of Myc in transgenic mice that phenocopies human ADPKD. The network of activated signaling pathways in human and mouse cystogenesis individually or in combination can target Myc as a central node of PKD pathogenesis. One or many of the multiple functions of Myc upon activation can play a role in every phases of ADPKD development and lend credence to the notion of "Myc addiction" for cystogenesis. We propose that the residual p53 levels are conducive to an ADPKD biological program without cancerogenesis while a "p53 dependent annihilation" mechanism would be permissive to oncogenesis. Of major importance, Myc ablation in orthologous mouse models or direct inhibition in non-orthologous mouse model significantly delays cystogenesis consistent with pharmacologic or genetic inhibition of Myc upstream regulator or downstream targets in the mouse. Together, these studies on PKD proteins upon dysregulation not only converged on Myc as a focal point but also attribute to Myc upregulation a causal and « driver » role in pathogenesis. This review will present and discuss our current knowledge on Myc and p53, focused on PKD mouse models and ADPKD.

Cell Death in the Kidney

Apoptotic cell death is usually a response to the cell’s microenvironment. In the kidney, apoptosis contributes to parenchymal cell loss in the course of acute and chronic renal injury, but does not trigger an inflammatory response. What distinguishes necrosis from apoptosis is the rupture of the plasma membrane, so necrotic cell death is accompanied by the release of unprocessed intracellular content, including cellular organelles, which are highly immunogenic proteins. The relative contribution of apoptosis and necrosis to injury varies, depending on the severity of the insult. Regulated cell death may result from immunologically silent apoptosis or from immunogenic necrosis. Recent advances have enhanced the most revolutionary concept of regulated necrosis. Several modalities of regulated necrosis have been described, such as necroptosis, ferroptosis, pyroptosis, and mitochondrial permeability transition-dependent regulated necrosis. We review the different modalities of apoptosis, necrosis, and regulated necrosis in kidney injury, focusing particularly on evidence implicating cell death in ectopic renal calcification. We also review the evidence for the role of cell death in kidney injury, which may pave the way for new therapeutic opportunities.

Roles and regulation of histone methylation in animal development

Histone methylation can occur at various sites in histone proteins, primarily on lysine and arginine residues, and it can be governed by multiple positive and negative regulators, even at a single site, to either activate or repress transcription. It is now apparent that histone methylation is critical for almost all stages of development, and its proper regulation is essential for ensuring the coordinated expression of gene networks that govern pluripotency, body patterning and differentiation along appropriate lineages and organogenesis. Notably, developmental histone methylation is highly dynamic. Early embryonic systems display unique histone methylation patterns, prominently including the presence of bivalent (both gene-activating and gene-repressive) marks at lineage-specific genes that resolve to monovalent marks during differentiation, which ensures that appropriate genes are expressed in each tissue type. Studies of the effects of methylation on embryonic stem cell pluripotency and differentiation have helped to elucidate the developmental roles of histone methylation. It has been revealed that methylation and demethylation of both activating and repressive marks are essential for establishing embryonic and extra-embryonic lineages, for ensuring gene dosage compensation via genomic imprinting and for establishing body patterning via HOX gene regulation. Not surprisingly, aberrant methylation during embryogenesis can lead to defects in body patterning and in the development of specific organs. Human genetic disorders arising from mutations in histone methylation regulators have revealed their important roles in the developing skeletal and nervous systems, and they highlight the overlapping and unique roles of different patterns of methylation in ensuring proper development.

Metabolic pathways regulated by TAp73 in response to oxidative stress

Reactive oxygen species are involved in both physiological and pathological processes including neurodegeneration and cancer. Therefore, cells have developed scavenging mechanisms to maintain redox homeostasis under control. Tumor suppressor genes play a critical role in the regulation of antioxidant genes. Here, we investigated whether the tumor suppressor gene TAp73 is involved in the regulation of metabolic adaptations triggered in response to oxidative stress. H₂O₂ treatment resulted in numerous biochemical changes in both control and TAp73 knockout (TAp73−/−) mouse embryonic fibroblasts, however the extent of these changes was more pronounced in TAp73−/− cells when compared to control cells. In particular, loss of TAp73 led to alterations in glucose, nucleotide and amino acid metabolism. In addition, H₂O₂ treatment resulted in increased pentose phosphate pathway (PPP) activity in null mouse embryonic fibroblasts. Overall, our results suggest that in the absence of TAp73, H₂O₂ treatment results in an enhanced oxidative environment, and at the same time in an increased pro-anabolic phenotype. In conclusion, the metabolic profile observed reinforces the role of TAp73 as tumor suppressor and indicates that TAp73 exerts this function, at least partially, by regulation of cellular metabolism.

Regulation of kidney development by the Mdm2/Mdm4-p53 axis

While p53 activity is required for tumour suppression, unconstrained p53 activity on the other hand is detrimental to the organism, resulting in inappropriate cellular death or proliferation defects. Unimpeded p53 activity is lethal in the developing embryo, underlining the need for maintaining a tight control on p53 activity during this period. The critical role of the negative regulators of p53, Mdm2 and Mdm4, in vertebrate development came to light by fatal disruption of embryogenesis that was observed with Mdm2 and Mdm4 gene deletions in mice. Embryonic lethality was rescued only by superimposing p53 removal. Here we summarize the contribution of the Mdm2/Mdm4–p53 axis that occurs at multiple steps of kidney development. Conditional, cell type-specific deletions reveal distinct functions of these proteins in renal morphogenesis. The severe impact on the renal phenotype from targeted gene deletions underscores the critical role played by the Mdm2/Mdm4–p53 nexus on nephrogenesis, and emphasizes the need to monitor patients with aberrations in this pathway for kidney function defects and associated cardiovascular dysfunction.

Evaluation of promoter hypomethylation and expression of p73 as a diagnostic and prognostic biomarker in Wilms' tumour

AIMS

A member of the p53 family, the p73 gene is essential for the maintenance of genomic stability, DNA repair and apoptosis regulation. This study was designed to evaluate the utility of expression and DNA methylation patterns of the p73 gene in the early diagnosis and prognosis of Wilms' tumour (WT).

METHODS

Methylation-specific PCR, semi-quantitative (sq-PCR), real-time quantitative PCR (qRT-PCR), receiver operating characteristic (ROC), and survival and hazard function curve analyses were utilised to measure the expression and DNA methylation patterns of p73 in WT tissue samples with a view to assessing diagnostic and prognostic value.

RESULTS

The relative expression of p73 mRNA was higher, while the promoter methylation level was lower in the WT than the control group (p<0.05) and closely associated with poor survival prognosis in children with WT (p<0.05). Increased expression and decreased methylation of p73 were correlated with increasing tumour size, clinical stage and unfavourable histological differentiation (p<0.05). ROC curve analysis showed areas under the curve of 0.544 for methylation and 0.939 for expression in WT venous blood, indicating the higher diagnostic yield of preoperative p73 expression.

CONCLUSIONS

Preoperative venous blood p73 level serves as an underlying biomarker for the early diagnosis of WT. p73 overexpression and concomitantly decreased promoter methylation are significantly associated with poor survival in children with WT.

Histone deacetylase 1 and 2 regulate Wnt and p53 pathways in the ureteric bud epithelium

Histone deacetylases (HDACs) regulate a broad range of biological processes through removal of acetyl groups from histones as well as non-histone proteins. Our previous studies showed that Hdac1 and Hdac2 are bound to promoters of key renal developmental regulators and that HDAC activity is required for embryonic kidney gene expression. However, the existence of many HDAC isoforms in embryonic kidneys raises questions concerning the possible specificity or redundancy of their functions. We report here that targeted deletion of both the Hdac1 and Hdac2 genes from the ureteric bud (UB) cell lineage of mice causes bilateral renal hypodysplasia. One copy of either Hdac1 or Hdac2 is sufficient to sustain normal renal development. In addition to defective cell proliferation and survival, genome-wide transcriptional profiling revealed that the canonical Wnt signaling pathway is specifically impaired in UB(Hdac1,2-/-) kidneys. Our results also demonstrate that loss of Hdac1 and Hdac2 in the UB epithelium leads to marked hyperacetylation of the tumor suppressor protein p53 on lysine 370, 379 and 383; these post-translational modifications are known to boost p53 stability and transcriptional activity. Genetic deletion of p53 partially rescues the development of UB(Hdac1,2-/-) kidneys. Together, these data indicate that Hdac1 and Hdac2 are crucial for kidney development. They perform redundant, yet essential, cell lineage-autonomous functions via p53-dependent and -independent pathways.

The MDM2-p53 pathway: multiple roles in kidney development

The molecular basis of nephron progenitor cell renewal and differentiation into nascent epithelial nephrons is an area of intense investigation. Defects in these early stages of nephrogenesis lead to renal hypoplasia, and eventually hypertension and chronic kidney disease. Terminal nephron differentiation, the process by which renal epithelial precursor cells exit the cell cycle and acquire physiological functions is equally important. Failure of terminal epithelial cell differentiation results in renal dysplasia and cystogenesis. Thus, a better understanding of the transcriptional frameworks that regulate early and late renal cell differentiation is of great clinical significance. In this review, we will discuss evidence implicating the MDM2-p53 pathway in cell fate determination during development. The emerging central theme from loss- and gain-of-function studies is that tight regulation of p53 levels and transcriptional activity is absolutely required for nephrogenesis. We will also discuss how post-translational modifications of p53 (e.g., acetylation and phosphorylation) alter the spatiotemporal and functional properties of p53 and thus cell fate during kidney development. Mutations and polymorphisms in the MDM2-p53 pathway are present in more than 50 % of cancers in humans. This raises the question of whether sequence variants in the MDM2-p53 pathway increase the susceptibility to renal dysgenesis, hypertension or chronic kidney disease. With the advent of whole exome sequencing and other high throughput technologies, this hypothesis is testable in cohorts of children with renal dysgenesis.

The regulation of apoptosis in kidney development: implications for nephron number and pattern?

Apoptosis is essential to remodel developing structures and eliminate superfluous cells in a controlled manner during normal development, and continues to be an important component of tissue remodeling and regeneration during an organism's lifespan, or as a response to injury. This mini review will discuss recent studies that have provided insights into the roles of apoptosis in the determination of nephron number and pattern, during normal and abnormal kidney development. The regulation of congenital nephron endowment has implications for risk of chronic kidney disease in later life, whereas abnormalities in nephron pattern are associated with congenital anomalies of the kidney and urinary tract (the leading cause of renal disease in children). Tight regulation of apoptosis is required in normal renal morphogenesis, although many questions remain regarding the regulation of apoptosis by genetic, epigenetic, and environmental factors, in addition to the functional requirement of different components of the apoptotic pathway.

The ciliary protein cystin forms a regulatory complex with necdin to modulate Myc expression

Cystin is a novel cilia-associated protein that is disrupted in the cpk mouse, a well-characterized mouse model of autosomal recessive polycystic kidney disease (ARPKD). Interestingly, overexpression of the Myc gene is evident in animal models of ARPKD and is thought to contribute to the renal cystic phenotype. Using a yeast two-hybrid approach, the growth suppressor protein necdin, known to modulate Myc expression, was found as an interacting partner of cystin. Deletion mapping demonstrated that the C-terminus of cystin and both termini of necdin are required for their mutual interaction. Speculating that these two proteins may function to regulate gene expression, we developed a luciferase reporter assay and observed that necdin strongly activated the Myc P1 promoter, and cystin did so more modestly. Interestingly, the necdin effect was significantly abrogated when cystin was co-transfected. Chromatin immunoprecipitation and electrophoretic mobility shift assays revealed a physical interaction with both necdin and cystin and the Myc P1 promoter, as well as between these proteins. The data suggest that these proteins likely function in a regulatory complex. Thus, we speculate that Myc overexpression in the cpk kidney results from the dysregulation of the cystin-necdin regulatory complex and c-Myc, in turn, contributes to cystogenesis in the cpk mouse.

Genome-wide analysis of the p53 gene regulatory network in the developing mouse kidney

Despite mounting evidence that p53 senses and responds to physiological cues in vivo, existing knowledge regarding p53 function and target genes is largely derived from studies in cancer or stressed cells. Herein we utilize p53 transcriptome and ChIP-Seq (chromatin immunoprecipitation-high throughput sequencing) analyses to identify p53 regulated pathways in the embryonic kidney, an organ that develops via mesenchymal-epithelial interactions. This integrated approach allowed identification of novel genes that are possible direct p53 targets during kidney development. We find the p53-regulated transcriptome in the embryonic kidney is largely composed of genes regulating developmental, morphogenesis, and metabolic pathways. Surprisingly, genes in cell cycle and apoptosis pathways account for <5% of differentially expressed transcripts. Of 7,893 p53-occupied genomic regions (peaks), the vast majority contain consensus p53 binding sites. Interestingly, 78% of p53 peaks in the developing kidney lie within proximal promoters of annotated genes compared with 7% in a representative cancer cell line; 25% of the differentially expressed p53-bound genes are present in nephron progenitors and nascent nephrons, including key transcriptional regulators, components of Fgf, Wnt, Bmp, and Notch pathways, and ciliogenesis genes. The results indicate widespread p53 binding to the genome in vivo and context-dependent differences in the p53 regulon between cancer, stress, and development. To our knowledge, this is the first comprehensive analysis of the p53 transcriptome and cistrome in a developing mammalian organ, substantiating the role of p53 as a bona fide developmental regulator. We conclude p53 targets transcriptional networks regulating nephrogenesis and cellular metabolism during kidney development.

p53 Attenuates the oncogenic Ras-induced epithelial-mesenchymal transition in human mammary epithelial cells

Inactivation of the tumor suppressor p53 and activation of the oncogene Ras are the two most pivotal events in tumor development. However, potential intersection between p53 and Ras activity during an EMT process, which plays a crucial role during malignant tumor progression, remains elusive. Here, we report that increased expression of wild type p53 suppressed H-Ras(V12)-induced EMT phenotypes and restrained stem cell properties, through downregulation of MEK-ERK signaling pathways. In vivo experiments showed that p53 was able to inhibit H-Ras(V12)-induced tumor growth of human mammary epithelial cells. This study elucidates a novel correlation between the tumor suppressor gene p53 and the oncogene Ras in regulating EMT program, and expands the knowledge about the function of p53 in EMT process.

Mechanisms of p53 activation and physiological relevance in the developing kidney

The tumor suppressor protein p53 is a short-lived transcription factor due to Mdm2-mediated proteosomal degradation. In response to genotoxic stress, p53 is stabilized via posttranslational modifications which prevent Mdm2 binding. p53 activation results in cell cycle arrest and apoptosis. We previously reported that tight regulation of p53 activity is an absolute requirement for normal nephron differentiation (Hilliard S, Aboudehen K, Yao X, El-Dahr SS Dev Biol 353: 354-366, 2011). However, the mechanisms of p53 activation in the developing kidney are unknown. We show here that metanephric p53 is phosphorylated and acetylated on key serine and lysine residues, respectively, in a temporal profile which correlates with the maturational changes in total p53 levels and DNA-binding activity. Site-directed mutagenesis revealed a differential role for these posttranslational modifications in mediating p53 stability and transcriptional regulation of renal function genes (RFGs). Section immunofluorescence also revealed that p53 modifications confer the protein with specific spatiotemporal expression patterns. For example, phos-p53(S392) is enriched in maturing proximal tubular epithelial cells, whereas acetyl-p53(K373/K382/K386) are expressed in nephron progenitors. Functionally, p53 occupancy of RFG promoters is enhanced at the onset of tubular differentiation, and p53 loss or gain of function indicates that p53 is necessary but not sufficient for RFG expression. We conclude that posttranslational modifications are important determinants of p53 stability and physiological functions in the developing kidney. We speculate that the stress/hypoxia of the embryonic microenvironment may provide the stimulus for p53 activation in the developing kidney.

Ontogeny of bradykinin B1 receptors in the mouse kidney

Kinins are vasoactive peptides that stimulate two G-protein coupled bradykinin receptors (B1R and B2R). B2R-knockout mice are salt sensitive and develop renal dysgenesis and hypertension if salt stressed during embryogenesis. B1R-knockout mice, on the other hand, are protected from inflammation and fibrosis. This study examined the spatiotemporal expression of B1R during renal organogenesis. The segmental nephron identity of B1R immunoreactivity was determined by costaining with markers of the collecting duct (Dolichos biflorus), proximal tubule (Dolichos tetraglonus), and nephron progenitors (Pax2). At E14.5, the B1R was confined to few cells in the metanephric mesenchyme. Abundance of B1R increased progressively during development. On E17.5, B1R was enriched in differentiating proximal tubular cells and by postnatal day 1, B1R was clearly expressed on the luminal aspect of the proximal tubule. Quantitative real-time PCR revealed that the levels of B1R mRNA more than double during renal maturation. We conclude that 1) B1R expression correlates closely with nephron maturation; 2) lack of B1R in nephron progenitors suggests that B1R is unlikely to play a role in early nephrogenesis; and 3) enrichment of B1R in maturing proximal tubule suggests a potential role for this receptor in terminal differentiation of the proximal nephron.

Caspases and p53 modulate FOXO3A/Id1 signaling during mouse neural stem cell differentiation

Neural stem cells (NSCs) differentiate into neurons and glia, and a large percentage undergoes apoptosis. The engagement and activity of apoptotic pathways may favor either cell death or differentiation. In addition, Akt represses differentiation by up-regulating the inhibitor of differentiation 1 (Id1), through phosphorylation of its repressor FOXO3A. The aim of this study was to investigate the potential cross-talk between apoptosis and proliferation during mouse NSC differentiation. We determined the time of neurogenesis and gliogenesis using neuronal beta-III tubulin and astroglial GFAP to confirm that both processes occurred at approximately 3 and 8 days, respectively. p-Akt, p-FOXO3A, and Id1 were significantly reduced throughout differentiation. Caspase-3 processing, p53 phosphorylation, and p53 transcriptional activation increased at 3 days of differentiation, with no evidence of apoptosis. Importantly, in cells exposed to the pancaspase inhibitor z-VAD.fmk, p-FOXO3A and Id1 were no longer down-regulated, p53 phosphorylation and transcriptional activation were reduced, while neurogenesis and gliogenesis were significantly delayed. The effect of siRNA-mediated silencing of p53 on FOXO3A/Id1 was similar to that of z-VAD.fmk only at 3 days of differentiation. Interestingly, caspase inhibition further increased the effect of p53 knockdown during neurogenesis. In conclusion, apoptosis-associated factors such as caspases and p53 temporally modulate FOXO3A/Id1 signaling and differentiation of mouse NSCs.

Crosstalk of Notch with p53 and p63 in cancer growth control

Understanding the complexity of cancer depends on an elucidation of the underlying regulatory networks, at the cellular and intercellular levels and in their temporal dimension. This Opinion article focuses on the multilevel crosstalk between the Notch pathway and the p53 and p63 pathways. These two coordinated signalling modules are at the interface of external damaging signals and control of stem cell potential and differentiation. Positive or negative reciprocal regulation of the two pathways can vary with cell type and cancer stage. Therefore, selective or combined targeting of the two pathways could improve the efficacy and reduce the toxicity of cancer therapies.

Transcriptional control of terminal nephron differentiation

Terminal differentiation of epithelial cells into more specialized cell types is a critical step in organogenesis. Throughout the process of terminal differentiation, epithelial progenitors acquire or upregulate expression of renal function genes and cease to proliferate, while expression of embryonic genes is repressed. This exquisite coordination of gene expression is accomplished by signaling networks and transcription factors which couple the external environment with the new functional demands of the cell. While there has been much progress in understanding the early steps involved in renal epithelial cell differentiation, a major gap remains in our knowledge of the factors that control the steps of terminal differentiation. A number of signaling molecules and transcription factors have been recently implicated in determining segmental nephron identity and functional differentiation. While some of these factors (the p53 gene family, hepatocyte nuclear factor-1beta) promote the terminal epithelial differentiation fate, others (Notch, Brn-1, IRX, KLF4, and Foxi1) tend to regulate differentiation of specific nephron segments and individual cell types. This review summarizes current knowledge related to these transcription factors and discusses how diverse cellular signals are integrated to generate a transcriptional output during the process of terminal differentiation. Since these transcriptional processes are accompanied by profound changes in nuclear chromatin structure involving the genes responsible for creating and maintaining the differentiated cell phenotype, future studies should focus on identifying the nature of these epigenetic events and factors, how they are regulated temporally and spatially, and the chromatin environment they eventually reside in.

Mouse models of polycystic kidney disease

Polycystic kidney disease (PKD) is a diverse group of human monogenic lethal conditions inherited as autosomal dominant (AD) or recessive (AR) traits. Recent development of genetically engineered mouse models of ADPKD, ARPKD, and nephronophthisis/medullary cystic disease (NPHP) are providing additional insights into the molecular mechanisms governing of these disease processes as well as the developmental differentiation of the normal kidney. Genotypic and phenotypic mouse models are discussed and provide evidence for the fundamental involvement of cell-matrix, cell-cell, and primary cilia-lumen interactions, as well as epithelial proliferation, apoptosis, and polarization. Structure/function relationships between the PKD1, PKD2, PKHD1, and NPHP genes and proteins support the notion of a regulatory multiprotein cystic complex with a mechanosensory function that integrates signals from the extracellular environment. The plethora of intracellular signaling cascades that can impact renal cystic development suggest an exquisitely sensitive requirement for integrated downstream transduction and provide potential targets for therapeutic intervention. Appropriate genocopy models that faithfully recapitulate the phenotypic characteristics of the disease will be invaluable tools to analyze the effects of modifier genes and small molecule inhibitor therapies.

Activation and involvement of p53 in cisplatin-induced nephrotoxicity

Cisplatin, a widely used chemotherapy drug, induces acute kidney injury, which limits its use and efficacy in cancer treatment. However, the molecular mechanism of cisplatin-induced nephrotoxicity is currently unclear. Using pharmacological and gene knockout models, we now demonstrate a pathological role for p53 in cisplatin nephrotoxicity. In C57BL/6 mice, cisplatin treatment induced p53 phosphorylation and protein accumulation, which was accompanied by the development of acute kidney injury. p53 was induced in both proximal and distal tubular cells and partially colocalized with apoptosis. Pifithrin-alpha, a pharmacological inhibitor of p53, suppressed p53 activation and ameliorated kidney injury during cisplatin treatment. Moreover, cisplatin-induced nephrotoxicity was abrogated in p53-deficient mice. Compared with wild-type animals, p53-deficient mice showed a better renal function, less tissue damage, and fewer apoptotic cells. In addition, cisplatin induced less apoptosis in proximal tubular cells isolated from p53-deficient mice than the cells from wild-type animals. Together these results suggest the involvement of p53 in cisplatin-induced renal cell apoptosis and nephrotoxicity.

NGF-mediated transcriptional targets of p53 in PC12 neuronal differentiation

BACKGROUND

p53 is recognized as a critical regulator of the cell cycle and apoptosis. Mounting evidence also suggests a role for p53 in differentiation of cells including neuronal precursors. We studied the transcriptional role of p53 during nerve growth factor-induced differentiation of the PC12 line into neuron-like cells. We hypothesized that p53 contributed to PC12 differentiation through the regulation of gene targets distinct from its known transcriptional targets for apoptosis or DNA repair.

RESULTS

Using a genome-wide chromatin immunoprecipitation cloning technique, we identified and validated 14 novel p53-regulated genes following NGF treatment. The data show p53 protein was transcriptionally activated and contributed to NGF-mediated neurite outgrowth during differentiation of PC12 cells. Furthermore, we describe stimulus-specific regulation of a subset of these target genes by p53. The most salient differentiation-relevant target genes included wnt7b involved in dendritic extension and the tfcp2l4/grhl3 grainyhead homolog implicated in ectodermal development. Additional targets included brk, sdk2, sesn3, txnl2, dusp5, pon3, lect1, pkcbpb15 and other genes.

CONCLUSION

Within the PC12 neuronal context, putative p53-occupied genomic loci spanned the entire Rattus norvegicus genome upon NGF treatment. We conclude that receptor-mediated p53 transcriptional activity is involved in PC12 differentiation and may suggest a contributory role for p53 in neuronal development.

DeltaNp73 modulates nerve growth factor-mediated neuronal differentiation through repression of TrkA

p73, a member of the p53 family, expresses two classes of proteins: the full-length TAp73 and the N-terminally truncated DeltaNp73. While TAp73 possesses many p53-like features, DeltaNp73 is dominant negative towards TAp73 and p53 and appears to have distinct functions in tumorigenesis and neuronal development. Given its biological importance, we investigated the role of DeltaNp73 in nerve growth factor (NGF)-mediated neuronal differentiation in PC12 cells. We show that overexpression of DeltaNp73alpha or DeltaNp73beta inhibits NGF-mediated neuronal differentiation in both p53-dependent and -independent manners. In line with this, we showed that the level of endogenous DeltaNp73 is progressively diminished in differentiating PC12 cells upon NGF treatment and knockdown of DeltaNp73 promotes NGF-mediated neuronal differentiation. Interestingly, we found that the ability of DeltaNp73 to suppress NGF-mediated neuronal differentiation is correlated with its ability to regulate the expression of TrkA, the high-affinity NGF receptor. Specifically, we found that DeltaNp73 directly binds to the TrkA promoter and transcriptionally represses TrkA expression, which in turn attenuates the NGF-mediated mitogen-activated protein kinase pathway. Conversely, the steady-state level of TrkA is increased upon knockdown of DeltaNp73. Furthermore, we found that histone deacetylase 1 (HDAC1) and HDAC2 are recruited by DeltaNp73 to the TrkA promoter and act as corepressors to suppress TrkA expression, which can be relieved by trichostatin A, an HDAC inhibitor. Taken together, we conclude that DeltaNp73 negatively regulates NGF-mediated neuronal differentiation by transrepressing TrkA.

Susceptibility to metanephric apoptosis in bradykinin B2 receptor null mice via the p53-Bax pathway

In response to gestational high salt intake, BdkrB2−/− embryos acquire an aberrant renal phenotype mimicking renal dysplasia in humans. Genetic analysis identified p53 as a mediator of the renal dysplasia in salt-stressed BdkrB2−/− mice, acting partly via repression of terminal epithelial differentiation genes. The present study tested the hypothesis that inactivation of BdkrB2 predisposes the salt-stressed embryo to p53-mediated metanephric apoptosis. Newborn BdkrB2−/− pups exhibited hyperphosphorylation of metanephric p53 on serine 20 (mouse serine 23), a modification known to increase p53 stability and apoptotic activity. As a result, there was widespread, ectopic expression of p53 in the BdkrB2−/− kidney. However, no differences were found in the apoptosis index or gene expression in BdkrB2−/− and +/+ kidneys, indicating that p53 stabilization as a result of BdkrB2 inactivation is not sufficient to induce metanephric apoptosis. On gestational salt stress, fulminant metanephric apoptosis and enhanced Bax gene expression occurred in BdkrB2−/− but not their +/− or +/+ littermates. Germline deletion of p53 from BdkrB2−/− mice prevented Bax activation and normalized the apoptosis index. Rescue of metanephric apoptosis in BdkrB2−/− mice was similarly achieved by Bax gene deletion. Aberrant apoptosis in salt-stressed BdkrB2−/− mice was triggered on embryonic day E15.5 and involved both ureteric bud (UB) and metanephric mesenchyme-derived nephron elements. Cultured E12.5 salt-stressed BdkrB2−/− metanephroi manifested stunted UB branching compared with +/− and +/+ littermates; the abnormal UB branching was corrected by p53 deletion. Our results suggest a model whereby a seemingly silent genetic mutation of BdkrB2 predisposes mice to renal dysplasia by creating a “preapoptotic” state through p53 activation.

Spatiotemporal Switch from ΔNp73 to TAp73 Isoforms during Nephrogenesis

p73 is a member of the p53 gene family, which also includes p53 and p63. These proteins share sequence similarity and target genes but also have divergent roles in cancer and development. Unlike p53, transcription of the p73 gene yields multiple full-length (transactivation (TA) domain) and amino terminus-truncated (DeltaN) isoforms. DeltaNp73 acts in a dominant negative fashion to inhibit the actions of TAp73 and p53 on their target genes, promoting cell survival and proliferation and suppressing apoptosis. The balance between TAp73 and its negative regulator, DeltaNp73, may therefore represent an important determinant of developmental cell fate. There is little if anything known regarding the developmental regulation of the p73 gene. In this study, we showed that TAp73 and DeltaNp73 exhibit reciprocal spatiotemporal expression and functions during nephrogenesis. TAp73 was predominantly expressed in the differentiation domain of the renal cortex in an overlapping manner with the vasopressin-sensitive water channel aquaporin-2 (AQP-2). Chromatin immunoprecipitation assays demonstrated that the endogenous AQP-2 promoter was occupied by TAp73 in a developmentally regulated manner. Furthermore TAp73 stimulated AQP-2 promoter-driven reporter expression. TAp73 also activated the bradykinin B2 receptor (B2R) promoter, a developmentally regulated gene involved in regulation of sodium excretion. The transcriptional effects of TAp73 on AQP-2 and B2R were independent of p53. In marked contrast to TAp73, DeltaNp73 isoforms were induced early in development and were preferentially expressed in proliferating nephron precursors. Moreover DeltaNp73 was a potent repressor of B2R gene transcription. We conclude that the p73 gene is developmentally regulated during kidney organogenesis. The spatiotemporal switch from DeltaNp73 to TAp73 may play an important role in the terminal differentiation program of the developing nephron.

Combinatorial control of the bradykinin B2 receptor promoter by p53, CREB, KLF-4, and CBP: implications for terminal nephron differentiation

Despite a wealth of knowledge regarding the early steps of epithelial differentiation, little is known about the mechanisms responsible for terminal nephron differentiation. The bradykinin B2 receptor (B2R) regulates renal function and integrity, and its expression is induced during terminal nephron differentiation. This study investigates the transcriptional regulation of the B2R during kidney development. The rat B2R 5′-flanking region has a highly conserved cis-acting enhancer in the proximal promoter consisting of contiguous binding sites for the transcription factors cAMP response element binding protein (CREB), p53, and Krüppel-like factor (KLF-4). The B2R enhancer drives reporter gene expression in inner medullary collecting duct-3 cells but is considerably weaker in other cell types. Site-directed mutagenesis and expression of dominant negative mutants demonstrated the requirement of CREB DNA binding and Ser-133 phosphorylation for optimal enhancer function. Moreover, helical phasing experiments showed that disruption of the spatial organization of the enhancer inhibits B2R promoter activity. Several lines of evidence indicate that cooperative interactions among the three transcription factors occur in vivo during terminal nephron differentiation: 1) CREB, p53, and KLF-4 are coexpressed in B2R-positive differentiating cells; 2) the maturational expression of B2R correlates with CREB/p53/KLF-4 DNA-binding activity; 3) assembly of CREB, p53, and KLF-4 on chromatin at the endogenous B2R promoter is developmentally regulated and is accompanied by CBP recruitment and histone hyperacetylation; and 4) CREB and p53 occupancy of the B2R enhancer is cooperative. These results demonstrate that combinatorial interactions among the transcription factors, CREB, p53, and KLF-4, and the coactivator CBP, may be critical for the regulation of B2R gene expression during terminal nephron differentiation.

The occurrence of c-myc, p53 and Bcl-2 family proteins in the early phase of development of duodenal epithelium

In last few years, numerous groups of proteins participating in the regulation of cell proliferation, differentiation and death during ontogenesis have been described. In this study we compared the occurrence of Bcl-2, p53 and myc protein families with the level of proliferative activity and apoptosis during development of duodenal epithelium. Paraffin embedded tissues of eight human embryos and foetuses aged from the 6th-18th week of IUD were used. For the detection of apoptotic cells the TUNEL method was performed, the proliferative marker PCNA and all the proteins studied were detected by means of indirect three-step immunohistochemical method. In the 6th and 8th week of intrauterine development we observed isolated TUNEL positive epithelial cells only and this was accompanied by the disperse presence of PCNA as well as by all the studied proteins: Bcl-2, Bax, Bcl-XL, c-myc, N-myc, p53, p63 and p73. In the early foetal period of duodenal development we registered changes in PCNA and TUNEL positivity in accordance with the constitution of the stem cell pool on base of villi, where more numerous Bcl-2 positive cells were also found. The separation of primitive crypts and villi was not accompanied by any differences in distribution of Bax, Bcl-XL, c-myc, N-myc, p63 and p73 proteins between those compartments: all the studied proteins showed dispersed character. P53 rapidly decreased in this period. In the 18th week of intrauterine development the balance between proliferation in crypts and apoptosis of villi epithelium was well established and no p53 positive cells were found. In the presence of Bcl-2, Bax, Bcl-XL, p63 and p73 we did not find any dramatic changes. The myc proteins were restricted within the epithelium of the Lieberkuhn crypts only.

p53 is a regulator of macrophage differentiation

While it is well accepted that p53 plays a role in apoptosis, less is known as to its involvement in cell differentiation. Here we show that wild-type p53 facilitates IL-6-dependent macrophage differentiation. Treatment of M1/2 cells expressing the temperature-sensitive p53 143 (Val to Ala) mutant, at the wild-type conformation, facilitated the appearance of mature macrophages that exhibited phagocytic activity. Enhancement of differentiation by the p53 143 (Val to Ala) in the wild-type conformation was coupled with the inhibition of apoptosis induction by this protein. In agreement with previous studies, we found that p53 levels were reduced during p53-dependent macrophage differentiation. This occurred when p53 levels before IL-6 stimuli were high. Interestingly, the p53 143 (Val to Ala) protein, at the mutant conformation, enhanced macrophage differentiation, as did the wild-type conformation, whereas the p53 273 (Arg to His) core mutant exerted an inhibitory effect on this pathway. The transcription-deficient p53 molecules, p53 (22-23) and p53 22,23,143, could not induce p53-dependent differentiation. Moreover, the p53 (22-23) protein inhibited the p53-independent differentiation pathway. Interestingly, the p53 (22-23) protein not only blocked IL-6-mediated differentiation, but also induced significant apoptotic cell death, upon IL-6 stimulation. Taken together, our data show that wild-type p53 enhances macrophage differentiation, while various p53 mutant types exert different effects on this differentiation pathway.

A novel pathological role of p53 in kidney development revealed by gene-environment interactions

Gene-environment interactions are implicated in congenital human disorders. Accordingly, there is a pressing need to develop animal models of human disease, which are the product of defined gene-environment interactions. Previously, our laboratory demonstrated that gestational salt stress of bradykinin B(2) receptor (B(2)R)-null mice induces renal dysgenesis and early death of the offspring. In contrast, salt-stressed B(2)R +/+ or +/- littermates have normal development. The present study investigates the mechanisms underlying the susceptibility of B(2)R-null mice to renal dysgenesis. Proteomic and conventional Western blot screens identified E-cadherin among the differentially repressed proteins in B(2)R-/- kidneys, whereas the checkpoint kinase Chk1 and its substrate P-Ser(20) p53 were induced. We tested the hypothesis that p53 mediates repression of E-cadherin gene expression and is causally linked to the renal dysgenesis. Genetic crosses between B(2)R -/- and p53+/- mice revealed that germline reduction of p53 gene dosage rescues B(2)R-/- mice from renal dysgenesis and restores kidney E-cadherin gene expression. Furthermore, gamma-irradiation induces repression of E-cadherin gene expression in p53+/+ but not -/- cells. In transient transfection assays, p53 repressed human E-cadherin promoter-driven reporter activity, whereas a mutant p53, which cannot bind DNA, did not. Functional promoter analysis indicated the presence of a p53-responsive element in exon 1, which partially mediates p53-induced repression. Chromatin immunoprecipitation assays revealed that p53 inhibits histone acetylation of the E-cadherin promoter. Treatment with a histone deacetylase inhibitor reversed both p53-mediated promoter repression and deacetylation. In conclusion, this study demonstrates that gene-environment interactions cooperate to induce congenital defects through p53 activation.

Two functionally divergent p53-responsive elements in the rat bradykinin B2 receptor promoter

Although p53 is known to have dual functions as a transcriptional activator and repressor, there has not been an example where both p53-activating and -repressing elements reside within one target promoter. Previous work from this laboratory defined two different p53 response elements, termed P1 and P2, located at nucleotide positions -70 and -707, respectively, in the rat bradykinin B2 receptor promoter. In this study, through manipulation of the DNA sequence and context, we demonstrate opposing roles for P1 and P2 as transcriptional activator and repressor, respectively. Deletion of P1 abrogates p53-mediated activation. P1 maintains its role as an activator upon relocation to the P2 site and activates transcription from a heterologous promoter construct. Thus, P1 is a bona fide positive p53-response element. In contrast, deletion of P2 enhances P1-induced activation. P2 represses transcription when substituted for P1 or when relocated midway between P1 and P2. P2-mediated repression is sequence-dependent, because it is reversed to activation when P2 is substituted by the P1 or p53 consensus sequences. Moreover, site-directed mutagenesis that converts P2 to a higher affinity p53-binding site results in transcriptional activation rather than repression. Surprisingly, P2 strongly activates a heterologous promoter. Thus, P2-mediated transcriptional repression is both sequence- and context-dependent. Investigations into the mechanisms of P2-mediated repression indicate that it is trichostatin-insensitive and unaffected by CBP or mutation of the minimal repression C-terminal domain of p53. However, gel shift assays suggest that p53 competes with other transcriptional activators for binding to overlapping binding sequences within the P2 element. In conclusion, this study provides a rare example of a transcription factor having two divergent functional effects that are sequence- and context-dependent. The interplay of P1 and P2 may be important in the regulation of bradykinin B2 receptor gene expression in response to inflammatory stress and during development.

The p53 tumor suppressor gene is regulated in vivo by nuclear factor 1-C2 in the mouse mammary gland during pregnancy

The p53 tumor suppressor protein plays an important role in preventing cancer development by arresting or killing potential tumor cells. Downregulated p53 levels, or mutations within the p53 gene, leading to the loss of p53 activity, are found in many breast carcinomas. Here we demonstrate that the p53 gene is transcriptionally upregulated in the normal mouse mammary gland at midpregnancy. We show that the specific isoform nuclear factor 1-C2 (NF1-C2) plays an important role in this activation. Functional mutation of the NF1-binding site in the mouse p53 promoter resulted in a reduction of the gene expression to less than 30% in mammary epithelial cells. By the use of two powerful techniques, chromatin immunoprecipitation and oligonucleotide decoy, we verify the importance of NF1-C2 in p53 gene activation in vivo. These findings demonstrate a broader role for NF1-C2 in the mammary gland at midpregnancy, beyond its earlier reported activation of milk protein genes. We also demonstrate that NF1-A1 proteins are produced in the mouse mammary gland. However, due to their lower affinity to the NF1-binding site, these proteins are not involved in the transcriptional upregulation of p53 at midpregnancy. This paper constitutes the first report demonstrating the importance of NF1 proteins in the p53 gene activation in the mouse mammary gland. It is also the first time that p53 gene activation is coupled to a specific, endogenously expressed NF1 isoform.

Spatial repression of PCNA by p53 during kidney development

Transcriptional repression is a key mechanism for the spatial specification of gene expression and cell fate determination. During kidney development, proliferating cell nuclear antigen (PCNA) is expressed in the nephrogenic zone and is downregulated rapidly as renal epithelial cells enter terminal differentiation and acquire functional characteristics. Our laboratory reported that the transcription factor p53 stimulates the terminal differentiation of renal epithelial cells by means of transcriptional activation of renal function genes (Saifudeen Z, Dipp S, and El-Dahr SS. J Clin Invest 109: 1021-1030, 2002). Because p53-induced growth arrest correlates with downregulation of PCNA gene expression, we examined the impact of p53 inactivation on PCNA expression in mice and evaluated the effect of p53 on PCNA transcription. Immunohistochemistry revealed that the transition from nephrogenesis to terminal epithelial cell differentiation correlates with accumulation of the transcription factor p53. Importantly, the spatially restricted pattern of PCNA expression is disrupted in kidneys of p53-deficient pups, in which there was a redistribution of PCNA expression into the differentiation zone (without a change in total kidney PCNA content) and distortion of the tubular architecture. Electrophoretic mobility shift assays revealed that the binding of kidney nuclear extracts to the p53 response elements in human and rat PCNA promoters is developmentally regulated. Transient transfection assays performed in p53-deficient HeLa cells revealed that exogenous p53 strongly represses transcription from human PCNA promoter-reporter constructs. Interestingly, deletion of the p53-binding site confers enhanced responsiveness to p53-mediated repression, suggesting that transcriptional repression of PCNA by p53 is achieved by a mechanism other than direct DNA binding. On the basis of these results, we propose the hypothesis that p53-mediated transcriptional repression plays a role in the spatial restriction of PCNA gene expression during normal renal development.