PPARgamma induces cell cycle withdrawal: inhibition of E2F/DP DNA-binding activity via down-regulation of PP2A

Peroxisome-proliferator-activated receptor gamma suppresses Wnt/beta-catenin signalling during adipogenesis

The Wnt/beta-catenin signalling pathway appears to operate to maintain the undifferentiated state of preadipocytes by inhibiting adipogenic gene expression. To define the mechanisms regulating suppression of Wnt/beta-catenin signalling, we analysed the beta-catenin expression in response to activation of transcription factors that regulate adipogenesis. The results show an extensive down-regulation of nuclear beta-catenin that occurs during the first few days of differentiation of 3T3-L1 preadipocytes and coincides with the induction of the adipogenic transcription factors, C/EBPbeta (CCAAT-enhancer-binding protein) and PPARgamma (peroxisome-proliferator-activated receptor). To assess the role of each of these factors in this process, we conditionally overexpressed C/EBPbeta in Swiss mouse fibroblasts using the TET-off system. Abundant expression of C/EBPbeta alone had minimal effect on beta-catenin expression, whereas expression of C/EBPbeta, in the presence of dexamethasone, induced PPARgamma expression and caused a measurable decrease in beta-catenin. In addition, exposure of cells expressing both C/EBPbeta and PPARgamma to a potent PPARgamma ligand resulted in an even greater decrease in beta-catenin by mechanisms that involve the proteasome. Our studies also suggest a reciprocal relationship between PPARgamma activity and beta-catenin expression, since ectopic production of Wnt-1 in preadipocytes blocked the induction of PPARgamma gene expression. Moreover, by suppressing beta-catenin expression, ectopic expression of PPARgamma in Wnt-1-expressing preadipocytes rescued the block in adipogenesis after their exposure to the PPARgamma ligand, troglitazone.

Tumor Necrosis Factor–Mediated E2F1 Suppression in Endothelial Cells

After balloon angioplasty, locally expressed tumor necrosis factor (TNF)-α disrupts endothelial cell (EC) proliferation and reendothelialization of the injured vessel. We have previously reported that TNF inhibits the EC cycle and downregulates the transcription factor E2F1. Ectopic expression of E2F1 at the site of injury improves reendothelialization of the injured vessel. In this study, we report that c-Jun N-terminal kinase (JNK) 1 and p38 mitogen-activated protein kinases (MAPKs) are differentially required for E2F1 expression and activity in ECs. Overexpression of constitutively active JNK1 mimicked TNF-mediated inhibitory events, whereas dominant-negative JNK1 prevented these effects. E2F cis elements in the promoter of E2F1 gene mediate suppressive actions of TNF, because removal of these sites rendered E2F1 promoter activity insensitive to TNF. JNK1 physically interacted with E2F1 and inactivated it via direct phosphorylation. Additionally, TNF inhibited Rb phosphorylation and dissociation from E2F1. Overexpression of constitutively active p38 MAPK facilitated Rb-E2F1 dissociation, whereas that of dominant-negative p38 MAPK did not. Taken together, these data suggest a differential requirement of JNK1 and p38 MAPK in TNF regulation of E2F1. Targeted inactivation of JNK1 at arterial injury sites may represent a potential therapeutic intervention for ameliorating TNF-mediated EC dysfunction.

The effect of PPAR?? ligands on the proliferation and apoptosis of human melanoma cells

The peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of the nuclear receptor family of ligand-activated transcription factors. PPARgamma ligands have been shown to inhibit the growth of cells from different cancer lineages. This study was designed to evaluate the effects of PPARgamma receptor activation in human melanoma cell lines. The effects of its expression and activation on cell proliferation and apoptosis in human melanoma cell lines (early stage cancer [WM35] and metastatic tumour [A375]) were investigated. Reverse transcription-polymerase chain reaction and Western blot analysis showed that both human melanoma cell lines expressed PPARgamma mRNA and protein. Treatment of cells transfected with the luciferase gene ligated to PPAR response element constructed promoter showed that ciglitazone and prostaglandin J2 (PGJ2), selective ligands for PPARgamma, increased the luciferase activity, proving the induction of the PPARgamma reporter gene. Ciglitazone and PGJ2 inhibited melanoma cell proliferation in a dose-dependent manner. Analysis of the cellular morphology and apoptosis assayed by fluorescence microscopy after incubation of A375 cells with 10 micro M ciglitazone for 24 h indicated that this ligand not only inhibited cell proliferation but also induced apoptosis.

PPARdelta activation induces COX-2 gene expression and cell proliferation in human hepatocellular carcinoma cells

Cyclooxygenase-2 (COX-2) has been suggested to be associated with carcinogenesis. Recently, many studies have shown increased expression of COX-2 in a variety of human malignancies, including hepatocellular carcinoma (HCC). Therefore, it becomes important to know more about what determines COX-2 expression. In this work, we have studied the effect of PPARdelta activation on COX-2 expression using a selective agonist (GW501516) in human hepatocellular carcinoma (HepG2) cells. Activation of PPARdelta resulted in increased COX-2 mRNA and protein expression. The mechanism behind the induction seems to be increased activity of the proximal promoter of the COX-2 gene, spanning nucleotides -327 to +59. The increased COX-2 protein expression and promoter activity induced by the GW501516 was also confirmed in the monocytic cell line THP-1. Induced levels of COX-2 have previously been associated with resistance to apoptosis and increased cell proliferation in many cell types. In HepG2 cells, we observed a dose-dependent increase in cell number by GW501516 treatment for 72h. The levels of PCNA, used as an indicator of cell division were induced, and the cell survival promoting complex p65 (NF-kappaB) was phosphorylated under GW501516 treatment. We conclude that PPARdelta activation in HepG2 cells results in induced COX-2 expression and increased cellular proliferation. These results may suggest that PPARdelta plays an important role in the development of HCC by modulating expression of COX-2.

Human invasive trophoblasts transformed with simian virus 40 provide a new tool to study the role of PPARgamma in cell invasion process

Invasive cytotrophoblasts play a key role in the development of human placenta and is therefore essential for subsequent development of the embryo. Human implantation is characterized by a major trophoblastic invasion that offers a unique model of a controlled and oriented tumor-like process. The ligand-activated nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) modulates cell growth and differentiation and might be therefore considered as a tumor suppressor. We have recently reported that PPARgamma, in synergy with its dimerization partner retinoid X receptor (RXR)alpha, controls the invasion of human primary cytotrophoblasts. Because these cells are unable to replicate in culture, we have, in the present study, transformed these primary cells with the simian virus 40 large T antigen for studying the role of PPARgamma in cell invasion process. Our results show that the cell line human invasive proliferative extravillous cytotrophoblast (HIPEC) 65 expressed markers of human invasive primary cytotrophoblast as determined by immunocytochemistry, immunobloting and real-time RT-PCR, and were highly invasive in vitro. We have next studied the role of PPARgamma/RXRalpha heterodimers in cell proliferation and invasion. Our results show that PPARgamma and RXRalpha are co-expressed by HIPEC 65 and that, as commonly observed, activation of PPARgamma/RXRalpha heterodimers with the specific PPARgamma agonist rosiglitazone induced lipid droplet accumulation as revealed by oil red O staining. Treatment with rosiglitazone or with the natural PPARgamma agonist 15-deoxy-delta-(12,14) PGJ2 did not modify cell growth, but interestingly, activation of PPARgamma by this synthetic (rosiglitazone) or natural (15d-PGJ2) ligand markedly inhibited cell invasion in a concentration-dependent manner. Finally, we showed that other potential natural PPARgamma ligand such as oxidized-but not native-low-density lipoprotein inhibited cell invasion. This proliferative and invasive human cytotrophoblast cell line from extravillous origin provides a new tool for studying specifically the role of PPARgamma in the control of cell invasion.

PPARgamma controls cell proliferation and apoptosis in an RB-dependent manner

The nuclear receptor PPARgamma is implicated in the control of cell proliferation and apoptosis. However, the molecular mechanisms by which it controls these processes remain largely elusive. We show here that PPARgamma activation in the presence of the retinoblastoma protein (RB) results in the arrest of cells at the G1 phase of the cell cycle, whereas in the absence of RB, cells accumulate in G2/M, endoreduplicate, and undergo apoptosis. Through the use of HDAC inhibitors and coimmunoprecipitations, we furthermore demonstrate that the effects of RB on PPARgamma-mediated control of the cell cycle and apoptosis depend on the recruitment of histone deacetylase 3 (HDAC3) to PPARgamma. In combination, these data hence demonstrate that the effects of PPARgamma on cell proliferation and apoptosis are dependent on the presence of an RB-HDAC3 complex.

PPARgamma ligands inhibit cholangiocarcinoma cell growth through p53-dependent GADD45 and p21 pathway

Ligands of peroxisome proliferator-activated receptor-gamma (PPARgamma) induce differentiation and growth inhibition in several human cancers. However, the role of PPARgamma ligands in the growth control of human cholangiocarcinoma cells remains unknown. This study was designed to investigate the biological functions and molecular mechanisms of PPARgamma ligands in the growth regulation of human cholangiocarcinoma cells. Western blot analysis showed that PPARgamma is expressed in all of the three human cholangiocarcinoma cell lines used in this study (SG231, CC-LP-1, and HuCCT1). Transient transfection assays using a peroxisome proliferator response element (PPRE) reporter construct showed that the PPARgamma expressed in human cholangiocarcinoma cells is functional as a transcription activator. Exposure of SG231, CC-LP-1, and HuCCT1 cells to PPARgamma ligands 15-deoxy-delta12, 14-prostaglandin J(2) (15d-PGJ(2)) and troglitazone for 24 to 96 hours resulted in a dose-dependent inhibition of cell growth. Flow cytometry analysis showed that 15d-PGJ(2) and troglitazone-induced cell cycle arrest at the G2/M checkpoint. Consistent with these findings, both 15d-PGJ(2) and troglitazone significantly inhibited the G2/M cyclin-dependent kinase (CDK) Cdc2 activity. Furthermore, cells treated with 15d-PGJ(2) and troglitazone showed elevated expression of p53 and two p53-controlled downstream genes, GADD45 and p21(WAF1/Cip1). Dominant negative inhibition of p53 in SG231 cells significantly blocked the 15d-PGJ(2) and troglitazone-induced growth inhibition, G2/M arrest, and GADD45/p21 induction. 15d-PGJ(2) and troglitazone failed to directly inhibit Cdc2 activity in a cell-free system in spite of direct association between GADD45 and PPARgamma proteins. In conclusion, these results show a novel p53-dependent mechanism in the PPARgamma ligand-mediated inhibition of cholangiocarcinoma growth and suggest a potential therapeutic role of PPARgamma ligands in the treatment of human cholangiocarcinoma.

Enhancement of carcinogen-induced malignant cell transformation by prostaglandin F(2 alpha)

The enhancement of carcinogen-induced malignant transformation of C3H/M2 mouse fibroblasts by the tumor promoters 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 12-O-tetradecanoylphorbol-13-acetate (TPA) is associated with the induction of cyclooxygenase expression and the stimulation of prostaglandin (PG) formation. Therefore, the potential of PGs, i.e., PGF(2alpha) and PGE(2), for tumor promotion was studied in the two-step C3H/M2 cell transformation assay, a model of the multi-step process of carcinogenesis. The transformation of fibroblasts was clearly enhanced by the addition of PGF(2alpha) in the promotion phase after pretreatment with a subthreshold dose of a carcinogen (3-methylcholanthrene or N-methyl-N'-nitro-N-nitrosoguanidine). No enhancement of cell transformation was observed in cells without carcinogen-pretreatment, i.e., PGF(2alpha) had no tumor initiating potential. The promotional effect was dose-dependent with a maximum at 16 nM PGF(2alpha). PGE(2) had no significant effect in this assay. Furthermore, PGF(2alpha) (but not PGE(2)) clearly reduced the inhibition of TPA-induced promotion by NS-398, an isozyme-specific inhibitor of cyclooxygenase-2. The inhibition of TPA- or TCDD-induced promotion by the non-specific cyclooxygenase inhibitor indomethacin was not affected by co-treatment with PGF(2alpha) and PGE(2). Our data suggest that PGF(2alpha) acts as an endogenous promoter of cell transformation implying that it may also be critically involved in tumor promoter-induced signalling transfer cascades ultimately triggering the process of carcinogenesis.

Peroxisome proliferator-activated receptor gamma-mediated differentiation: a mutation in colon cancer cells reveals divergent and cell type-specific mechanisms

Activation of the nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits cell growth and induces differentiation in both adipocyte and epithelial cell lineages, although it is unclear whether this occurs through common or cell-type specific mechanisms. We have identified four human colon cancer cell lines that do no undergo growth inhibition or induce markers of differentiation after exposure to PPARgamma agonists. Sequence analysis of the PPARgamma gene revealed that all four cell lines contain a previously unidentified point mutation in the ninth alpha-helix of the ligand binding domain at codon 422 (K422Q). The mutant receptor did not exhibit any defects in DNA binding or retinoid X receptor heterodimerization and was transcriptionally active in an artificial reporter assay. However, only retroviral transduction of the wild-type (WT), but not mutant, receptor could restore PPARgamma ligand-induced growth inhibition and differentiation in resistant colon cancer cell lines. In contrast, there was no difference in the ability of fibroblast cells expressing WT or K422Q mutant receptor to undergo growth inhibition, express adipocyte differentiation markers, or uptake lipid after treatment with a PPARgamma agonist. Finally, analysis of direct PPARgamma target genes in colon cancer cells expressing the WT or K422Q mutant allele suggests that the mutation may disrupt the ability of PPARgamma to repress the basal expression of a subset of genes in the absence of exogenous ligand. Collectively, these data argue that codon 422 may be a part of a co-factor(s) interaction domain necessary for PPARgamma to induce terminal differentiation in epithelial, but not adipocyte, cell lineages and argues that the receptor induces growth inhibition and differentiation via cell lineage-specific mechanisms.

PPARγ pathway activation results in apoptosis and COX-2 inhibition in HepG2 cells

AIM: To investigate whether troglitazone (TGZ), the peroxisome proliferator-activated receptor (PPAR) gamma ligand, can induce apoptosis and inhibit cell proliferation in human liver cancer cell line HepG2 and to explore the molecular mechanisms.

METHODS: [3-(4, 5)-dimethylthiazol-2-yl]-2, 5-diphenyl tetrazolium bromide (MTT), [³H] Thymidine incorporation, Hochest33258 staining, DNA ladder, enzyme-linked immunosorbent assay (ELISA), RT-PCR, Northern and Western blotting analyses were employed to investigate the effect of TGZ on HepG2 cells and related molecular mechanisms.

RESULTS: TGZ was found to inhibit the growth of HepG2 cells and to induce apoptosis. During the process, the expression of COX-2 mRNA and protein and Bcl-2 protein was down-regulated, while that of Bax and Bak proteins was up-regulated, and the activity of caspase-3 was elevated. Furthermore, the level of PGE₂ was decreased transiently after 12 h of treatment with 30 μM troglitazone.

CONCLUSION: TGZ inhibits cell proliferation and induces apoptosis in HepG2 cells, which may be associated with the activation of caspase-3-like proteases, down-regulation of the expression of COX-2 mRNA and protein, Bcl-2 protein, the elevation of PGE₂ levels, and up-regulation of the expressions of Bax and Bak proteins.

Peroxisome proliferator-activated receptor gamma inhibits expression of minichromosome maintenance proteins in vascular smooth muscle cells

Using a cDNA array consisting only of cell cycle genes, we found that a novel nonthiazolidinedione partial peroxisome proliferator-activated receptor gamma (PPARgamma) agonist (nTZDpa) inhibited expression of minichromosome maintenance (MCM) proteins 6 and 7 in vascular smooth muscle cells. MCM proteins are required for the initiation and elongation stages of DNA replication and are regulated by the transcription factor E2F. Mitogen-induced MCM6 and MCM7 mRNA expression was potently inhibited by nTZDpa and to a lesser degree by the full PPARgamma agonist, rosiglitazone. Inhibition of MCM6 and MCM7 expression by nTZDpa and rosiglitazone paralleled their effect to inhibit phosphorylation of the retinoblastoma protein and cell proliferation. Transient transfection experiments revealed that the nTZDpa inhibited mitogen-induced MCM6 and MCM7 promoter activity, implicating a transcriptional mechanism. Adenoviral-mediated E2F overexpression reversed the suppressive effect of nTZDpa on MCM6 and MCM7 expression. Furthermore, activity of a luciferase reporter plasmid driven by multiple E2F elements was inhibited by nTZDpa, indicating that their down-regulation by nTZDpa involves an E2F-dependent mechanism. Overexpression of dominant-negative PPARgamma or addition of a PPARgamma antagonist, GW 9662, blocked nTZDpa inhibition of MCM7 transcription. Adenovirus-mediated overexpression of constitutively active PPARgamma inhibited MCM7 expression in a similar manner as the nTZDpa. These findings provide strong evidence that activation of PPARgamma attenuates MCM7 transcription and support the important role of this nuclear receptor in regulating vascular smooth muscle cell proliferation.

AhR- and ERK-dependent pathways function synergistically to mediate 2,3,7,8-tetrachlorodibenzo-p-dioxin suppression of peroxisome proliferator-activated receptor-gamma1 expression and subsequent adipocyte differentiation

Activation of the aryl-hydrocarbon receptor (AhR) by pretreatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) blocks hormone (IDM/BRL)-induced adipocyte differentiation of C3H10T1/2 cells in proportion to the suppression of the elevation of the key mediator, peroxisome proliferator-activated receptor (PPARgamma1). Inhibition of MEK-induced ERK phosphorylation had no effect on adipogenesis but prevented this TCDD suppression. Initiation of MEK inhibition up to 6 h after IDM/BRL stimulation in combination with serum addition completely reversed the TCDD-mediated suppression but declined to ineffectiveness when delayed to 24 h after stimulation. This period occurs well after the decline of serum-induced ERK activation, at a time when ERK phosphorylation is low, and prior to the onset of IDM/BRL-stimulated PPARgamma1 expression. This temporal separation of ERK activation from the affected PPARgamma1 expression suggests that ERK does not act directly on either PPARgamma1 transcription or receptor function. Thus, ERK activation and TCDD/AhR stimulation work synergistically to inhibit adipocyte differentiation. Nonrenewal of serum at the time of IDM/BRL addition removed most of the ERK activation and also the TCDD-mediated suppressions of PPARgamma1 expression and adipocyte differentiation. Transfection of a vector expressing constitutively active MEK1 generated a constant, high level of phosphorylated ERK comparable to the peak serum-induced level and fully restored TCDD suppression without a TCDD-mediated effect on ERK phosphorylation. We conclude that low levels of activated MEK and ERK cooperate with AhR-induced factor(s) to generate a suppressor that prevents PPARgamma1 transcription and then differentiation.

Mechanisms involved in growth inhibition induced by clofibrate in hepatoma cells

Low concentrations of some peroxisome proliferators have been found to decrease apoptosis in rat liver cells, whereas higher but pharmacological concentrations have been found to inhibit cell proliferation or to induce apoptosis in human and rat hepatoma cells. The highly deviated JM2 rat hepatoma cell line was used to examine the mechanisms underlying the inhibitory effect on cell proliferation. Clofibrate chiefly inhibited cell proliferation in these cells. Parallel to the decrease in cell proliferation there was an increase of peroxisome proliferator activated receptor (PPAR) gamma and of protein phosphatase 2A, whose importance was confirmed, respectively, by using antisense oliginucleotides (AS-ODN) or okadaic acid. The increase of protein phosphatase 2A induced by PPARgamma caused a decrease of MAPK, an intracellular signaling transduction pathway, as shown by evaluation of Erk1,2 and c-myc. In light of these results, clofibrate, like conventional synthetic ligands of PPARgamma, may be regarded as a possible prototype anti-tumour drug.

The role of E2F4 in adipogenesis is independent of its cell cycle regulatory activity

The E2F and pocket protein families are known to play an important role in the regulation of both cellular proliferation and terminal differentiation. In this study, we have used compound E2F and pocket protein mutant mouse embryonic fibroblasts to dissect the role of these proteins in adipogenesis. This analysis shows that loss of E2F4 allows cells to undergo spontaneous differentiation. The ability of E2F4 to prevent adipogenesis seems to be quite distinct from the known properties of E2F. First, it can be separated from any change in either E2F-responsive gene expression or cell cycle regulation. Second, it is a specific property of E2F4, and not other E2Fs, and it occurs independently of E2F4's ability to interact with pocket proteins. In addition, E2F4 loss does not override the differentiation defect resulting from pRB loss even though it completely suppresses the proliferation defect of Rb(-/-) mouse embryonic fibroblasts. This finding definitively separates the known, positive role of pRB in adipogenesis from its cell cycle function and shows that this pocket protein is required to act downstream of E2F4 in the differentiation process.

GATA-6 is involved in PPARgamma-mediated activation of differentiated phenotype in human vascular smooth muscle cells

OBJECTIVE

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a member of the nuclear receptor superfamily involved in the growth and differentiation of many cell types. Although the activation of PPARgamma in human vascular smooth muscle cells (VSMCs) inhibits the growth of these cells, the precise mechanism of this effect is unknown. PPARgamma-mediated growth inhibition of VSMCs is associated with the induction of the differentiated phenotype. A zinc finger transcription factor, GATA-6, has been implicated in the maintenance of the differentiated phenotype in VSMCs.

METHODS AND RESULTS

The administration of 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2), a naturally occurring PPARgamma ligand, and troglitazone, a thiazolidinedione derivative, induced the expression of smooth muscle myosin heavy chain and smooth muscle alpha-actin, highly specific markers for differentiated VSMCs. Stimulation of proliferative VSMCs with PPARgamma ligands also increased the activity of the transfected wild-type smooth muscle myosin heavy chain promoter but not that of the mutant promoter, in which a GATA-6 binding site was mutated. Compatible with the role of GATA-6, both 15d-PGJ2 and troglitazone upregulated the DNA binding activity of GATA-6 in proliferative VSMCs.

CONCLUSIONS

The activation of PPARgamma-dependent pathways induces the differentiated phenotype in proliferative VSMCs, and this induction is mediated, in part, through a GATA-6-dependent transcriptional mechanism.

CCAAT/enhancer-binding protein beta is required for mitotic clonal expansion during adipogenesis

Hormonal induction of growth-arrested 3T3-L1 preadipocytes triggers a signaling cascade that culminates in adipogenesis. CCAATenhancer-binding protein (CEBP)beta is expressed immediately but gains DNA-binding activity only after a long lag as the cells synchronously begin mitotic clonal expansion (MCE). After MCE, a process required for adipogenesis, CEBPbeta activates expression of CEBPalpha and peroxisome proliferator-activated receptor gamma, which then transcriptionally activate genes that produce the adipocyte phenotype. When mouse embryo fibroblasts (MEFs) are subjected to the same differentiation protocol, a subset of the MEFs undergoes a similar program of events. Similar to 3T3-L1 preadipocytes, the MEFs reenter the cell cycle (as indicated by the synchronous expression of cyclin A) and undergo MCE as evidenced by the incorporation of BrdUrd into DNA and the formation of mitotic foci of cells that undergo adipogenesis. CEBPbeta is expressed immediately after induction but exhibits delayed acquisition of DNA-binding activity followed by expression of adipocyte markers and the accumulation of cytoplasmic triglyceride. MEFs from CEBPbeta(-/-) mice, however, neither undergo MCE nor differentiate into adipocytes. Forced expression of CEBPbeta (LAP) but not dominant-negative CEBPbeta (LIP) in CEBPbeta(-/-) MEFs restores MCE, expression of adipocyte markers, and the capacity to form mitotic foci of cells that undergo adipogenesis. These findings demonstrate that expression of CEBPbeta is a prerequisite for MCE in the adipocyte-differentiation program.

Mitotic clonal expansion: a synchronous process required for adipogenesis

When induced to differentiate, growth-arrested 3T3-L1 preadipocytes synchronously reenter the cell cycle and undergo mitotic clonal expansion (MCE) followed by expression of genes that produce the adipocyte phenotype. The preadipocytes traverse the G(1)S checkpoint synchronously as evidenced by the expressionactivation of cdk2-cyclin-EA, turnover of p27kip1, hyperphosphorylation of Rb, translocation of cyclin D(1) from nuclei to cytoplasm and GSK-3beta from cytoplasm to nuclei, and incorporation of [(3)H]thymidine into DNA. As the cells cross the G(1)S checkpoint, CEBPbeta acquires DNA-binding activity, initiating a cascade of transcriptional activation that culminates in the expression of adipocyte proteins. The mitogen-activated protein kinaseextracellular signal-regulated kinase kinase (MEK) inhibitor PD98059 delays, but does not block, MCE and differentiation, the extent of the delay causing a comparable delay in the expression of cell-cycle markers, MCE, and adipogenesis. The more potent and specific MEK inhibitor UO126 and the cyclin-dependent kinase inhibitor roscovitine, which inhibit the cell cycle at different points, block MCE, expression of cell cycle and adipocyte markers, as well as adipogenesis. These results show that MCE is a prerequisite for differentiation of 3T3-L1 preadipocytes into adipocytes.

Stimulation of proximal tubular cell apoptosis by albumin-bound fatty acids mediated by peroxisome proliferator activated receptor-gamma

In nephrotic syndrome, large quantities of albumin enter the kidney tubule. This albumin carries with it a heavy load of fatty acids to which the proximal tubule cells are exposed at high concentration. It is postulated that exposure to fatty acids in this way is injurious to proximal tubule cells. This study has examined the ability of fatty acids to interact with peroxisome proliferator-activated receptors (PPAR) in primary cultures of human proximal tubule cells. Luciferase reporter assays in transiently transfected human proximal tubule cells were used to show that albumin bound fatty acids and other agonists activate PPARgamma in a dose-dependent manner. One of the consequences of this activation is apoptosis of the cells as determined by changes in cell morphology, evidence of PARP cleavage, and appearance of DNA laddering. Overexpression of PPARgamma in these cells also results in enhanced apoptosis. Both fatty acid-induced PPAR activation and apoptosis in these cells can be blocked by PPAR response element decoy oligonucleotides. Activation of PPARgamma by the specific agonist PGJ(2) is associated with inhibition of cell proliferation, whereas activation by albumin bound fatty acids is accompanied by increased proliferation. However, the net balance of apoptosis/proliferation favors deletion of cells. These results implicate albumin-bound fatty acids as important mediators of tubular injury in nephrosis and provide fresh impetus for pursuit of lipid-lowering strategies in proteinuric renal disease.

Proliferative action of mast-cell tryptase is mediated by PAR2, COX2, prostaglandins, and PPARgamma : Possible relevance to human fibrotic disorders

Mast-cell products can stimulate fibroblast proliferation, implying that these cells are key players in fibrosis. One mast-cell product, the serine protease tryptase, is known to activate protease-activated receptor 2 (PAR2) and cause proliferation of fibroblasts. We found that recombinant tryptase, human mast-cell (HMC-1) supernatant, which contains tryptase, and the PAR2-activating peptide SLIGKV exert fibroproliferative actions in human fibroblasts. Here we report insights into this action, which after activation of PAR2 leads to increased expression of cyclooxygenase 2 (COX2), a key enzyme in the biosynthesis of prostaglandins, and consequently to enhanced prostaglandin synthesis. Subsequent cell proliferation is mediated by the prostaglandin 15-deoxy-Delta(12,14)-prostaglandin J(2), which acts via the nuclear peroxisome proliferator-activated receptor gamma (PPARgamma). Fibroblast proliferation induced by tryptase and PAR2 agonist peptide can be blocked by antagonists of COX2 and PPARgamma, implying that the proliferative effect of tryptase is PAR2-initiated but depends on COX2, 15-deoxy-Delta(12,14)-prostaglandin J(2), and PPARgamma. This previously uncharacterized pathway could be of relevance for human fibrotic diseases. For instance, increased numbers of activated mast cells are correlated with fibrosis in testes of infertile men. In these cases all components of the signaling pathway of tryptase were detected as well as expression of COX2. Therefore, our study describes as-yet-unknown interactions between mast cells and fibroblasts, which could be relevant for human fibrotic diseases.

Agonists of peroxisome proliferator-activated receptor gamma inhibit cell growth in malignant melanoma

Peroxisome proliferator-activated receptor gamma is a member of the nuclear receptor superfamily involved in adipocyte differentiation and glucose homeostasis. There is evidence that peroxisome proliferator-activated receptor gamma may also act as a tumor suppressor. Here, we demonstrate expression of peroxisome proliferator-activated receptor gamma in benign melanocytic naevi, different variants of primary cutaneous melanomas, and melanoma metastases. Peroxisome proliferator-activated receptor gamma protein and peroxisome proliferator-activated receptor gamma1 mRNA were also detected in human melanoma cell lines. The peroxisome proliferator-activated receptor gamma specific agonists 15-deoxy-Delta12,14-prostaglandin J2, troglitazone, and rosiglitazone dose-dependently inhibited cell proliferation in four melanoma cell lines, whereas a specific agonist of peroxisome proliferator-activated receptor alpha had no such effect. At a concentration of 50 microM rosiglitazone, the most potent peroxisome proliferator-activated receptor gamma agonist tested suppressed cell growth by approximately 90%. Apoptosis could be induced in melanoma cell lines by incubation with tumor-necrosis-factor-related apoptosis-inducing ligand. In contrast, the growth inhibitory effect of peroxisome proliferator-activated receptor gamma activation was independent of apoptosis and seemed to occur primarily through induction of cell cycle arrest. Our data indicate that melanoma cell growth may be modulated through peroxisome proliferator-activated receptor gamma.

Adipogenesis and fat-cell function in obesity and diabetes

Normal metabolic balance is maintained by a complex homeostatic system involving multiple tissues and organs. Acquired or inherited defects in any part of this system can lead to metabolic disorders, such as diabetes and obesity. Adipose tissue, once thought to function primarily as a passive depot for the storage of excess lipid, is now understood to play a much more active role in metabolic regulation, secreting a variety of metabolic hormones and actively functioning to prevent deleterious lipid accumulation in other tissues. Here, we review new advances in our understanding of adipogenesis and fat-cell function, primarily from the perspective of the transcription factor peroxisome proliferator-activated receptor gamma.

Differential expression of NAT1 translational repressor during development of bovine intramuscular adipocytes

This study was undertaken to test for differential gene expression in intramuscular adipocytes during fat deposition of feedlot steers. Angus x Hereford steers (n = 50) were fed a high-energy concentrate ration ad libitum for 20 (n = 5), 86 (n = 15), 121 (n = 15), and 146 days (n = 15) to obtain various degrees of intramuscular adipocyte development. Carcass traits were significantly different (P < 0.05) between the groups. Intramuscular adipose tissue was excised from the longissimus dorsi and snap frozen in liquid nitrogen. Pooled samples of total RNA representing each group were analyzed by differential-display polymerase chain reaction using 200 primer combinations comprising 20 arbitrary (5') and 10 anchor (3') oligonucleotides. Bands (n = 70) representing putative differences among treatment groups were excised, sequenced, and subjected to BLAST homology search. From these, 40 contained significant homology to known genes. One was of particular interest, the translational repressor NAT1 (novel APOBEC-1 target-1). NAT1 mRNA was quantified in individual animals to confirm differential expression among treatment groups. Results indicate that NAT1 message is more abundant (P < 0.05) in intramuscular adipocytes of younger/leaner animals.

Controversy: PPARgamma as a target for treatment of colorectal cancer

Colorectal cancer (CRC) represents a significant cause of morbidity and mortality worldwide. Recently, ligands for the nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPARgamma) have exhibited promise in the treatment of CRC. For example, activation of PPARgamma reduces the proliferation of cultured CRC cells grown in vitro or in vivo using the nude mouse xenograft model of tumor growth. Furthermore, agonists of the receptor also reduce the development of preneoplastic lesions in a model of carcinogen-induced CRC in rats. However, ligands for the receptor paradoxically enhance intestinal adenoma formation in another murine model of intestinal polyposis, the APC(Min) mice. These disparate results may be due to the inherent limitations of the APC(Min) mouse as a model for humans with CRC. Finally, genetic studies identifying loss of function mutations of PPARgamma in human CRC specimens strongly suggest a tumor suppressive role for the receptor during the development of CRC.

A vitamin D receptor-Ser/Thr phosphatase-p70 S6 kinase complex and modulation of its enzymatic activities by the ligand

We provide evidence of a cross-talk between nuclear receptor and Ser/Thr protein phosphatases and show that vitamin D receptor (VDR) interacts with the catalytic subunit of protein phosphatases, PP1c and PP2Ac, and induces their enzymatic activity in a ligand-dependent manner. PP1c specifically interacts with VDR but not retinoic acid receptor alpha and retinoid X receptor alpha in yeast. Although VDR-PP1c and VDR-PP2Ac interaction is ligand-independent in vivo, 1alpha,25-dihydroxy-vitamin D(3) induces VDR-associated phosphatase activity. Further, VDR modulation of PP1c/PP2Ac activity results in a rapid and specific dephosphorylation and inactivation of their substrate, p70 S6 kinase (p70(S6k)). Finally, we demonstrate that the endogenous VDR, PP1c or PP2Ac, and p70(S6k) are present in a ternary complex in vivo, and the interaction of p70(S6k) with the VDR-PP complex is modulated by the phosphorylation state of the kinase. Since p70(S6k) is essential for G(1)-S transition, our results provide a molecular basis of 1alpha,25-dihydroxyvitamin D(3)-induced G(1) block in colon cancer cells.

Down-regulation of aortic carboxypeptidase-like protein during the early phase of 3T3-L1 adipogenesis

Aortic carboxypeptidase-like protein (ACLP) is a 175-kDa protein that is expressed in vascular smooth muscle cells and contains a signal peptide sequence, a lysine- and proline-rich repeating motif, a discoidin-like domain with 35% identity to discoidin I, and a carboxypeptidase-like domain that is 39% identical with carboxypeptidase E. It is secreted into the extracellular matrix and may play a role in abdominal wall development and dermal wound healing. ACLP is also expressed in adipose tissue, but at lower levels. In this study we demonstrate that ACLP protein and mRNA are severely down-regulated in the early phase of 3T3-L1 preadipocyte differentiation induced by insulin, dexamethasone, and isobutylmethylxanthine. Neither dexamethasone, isobutylmethylxanthine, nor insulin treatment alone reduced the level of ACLP protein, suggesting that ACLP down-regulation is a differentiation-associated event. ACLP down-regulation coincided with the onset of the postconfluent mitotic clonal expansion phase of adipogenesis. In contrast, subconfluent 3T3-L1 cell proliferation did not alter ACLP expression, suggesting a specific linkage between ACLP and differentiation-induced clonal expansion. Stable overexpression of ACLP had no effect on preadipocyte differentiation assessed by triacylglycerol accumulation and peroxisome proliferator-activated receptor-gamma levels. The role of ACLP and its marked reduction during adipogenesis merit further study.

E2Fs regulate adipocyte differentiation

When preadipocytes reenter the cell cycle, PPAR gamma expression is induced, coincident with an increase in DNA synthesis, suggesting the involvement of the E2F family of cell cycle regulators. We show here that E2F1 induces PPAR gamma transcription during clonal expansion, whereas E2F4 represses PPARg amma expression during terminal adipocyte differentiation. Using a combination of in vivo experiments with knockout and chimeric animals and in vitro experiments, we demonstrate that the absence of E2F1 impairs, whereas depletion of E2F4 stimulates, adipogenesis. E2Fs hence represent the link between proliferative signaling pathways, triggering clonal expansion, and terminal adipocyte differentiation through regulation of PPAR gamma expression. This underscores the complex role of the E2F protein family in the control of both cell proliferation and differentiation.

PPARgamma ligands suppress proliferation of human urothelial basal cells in vitro

Expression of peroxisome proliferator-activated receptor (PPAR) gamma in the human urinary tract through embryonic development suggests its possible roles in the development, proliferation, and differentiation of uroepithelium. Little is known, however, about physiological roles of PPARgamma in the urinary tract. We investigated effects of PPARgamma ligands on the proliferation of normal human urothelial cells and stromal cells cultivated from surgical specimens. Active proliferation in vitro as well as high molecular weight cytokeratin expression indicated that cultured urothelial cells possess basal cell phenotype. PPARgamma protein, expressed predominantly in the epithelial layer of the normal human urinary tract in vivo, was abundantly expressed in urothelial cells but barely detectable in stromal cells in vitro. Natural ligand for PPARgamma, 15-deoxy-Delta(12,14) prostaglandin J(2) (15d-PGJ(2)), as well as synthetic ones, troglitazone and pioglitazone, suppressed proliferation of the urothelial cells dose-dependently. These effects were PPARgamma specific because clofibrate or PGF(2alpha) did not affect proliferation of urothelial cells. Neither 9-cis retinoic acid or all-trans retinoic acid (ATRA) at 1 microM showed any synergism on the antiproliferative effects of PPARgamma ligands. Urothelial cells treated with PPARgamma ligands showed drastic morphologic changes and cell cycle arrest at G0/G1 phase accompanied with increased mRNA level of a cyclin-dependent kinase inhibitor p21(WAF1/CIP1). Since 15d-PGJ(2) is present in vivo during the resolution phase of inflammation, these results indicated that PPARgamma might be involved in the terminal phase of urothelial re-epithelialization processes.

Expression of peroxisome proliferator activated receptor-gamma in non-small cell lung carcinoma: correlation with histological type and grade

Peroxisome Proliferator Activated Receptor-gamma (PPAR-gamma) is a ligand-activated transcription factor belonging to the steroid receptor superfamily. It is a key regulator of adipogenic differentiation and glucose homeostasis, the ligands of which have also been demonstrated to induce differentiation in human breast, lung and colon cancer cell lines. In the present study, PPAR-gamma expression in cases of non-small cell lung carcinoma (NSCLC) was examined immunohistochemically and was correlated with tumor histological type and grade. Primary tumor samples from 147 patients with NSCLC were immunostained using a monoclonal antibody against PPAR-gamma. Positive PPAR-gamma immunostaining was prominent in 61 out of 147 cases (42%) and negative in the rest. PPAR-gamma positivity was prominent in 37 out of 79 cases (47%) of squamous cell lung carcinoma and in 24 out of 68 ones (35%) of lung adenocarcinoma. PPAR-gamma positivity was most frequently observed in squamous cell tumors (P=0.021) and in tumors of high histological grade of both histological types (P=0.041). Well-differentiated adenocarcinoma cases presented increased frequency for PPAR-gamma positivity compared with moderately and poorly differentiated ones (P=0.001). The intensity and pattern of PPAR-gamma staining in tumor cells were not correlated with histopathological parameters in PPAR-gamma positive cases of NSCLC examined. Our findings support evidence for participation of this protein in the biological mechanisms underlying the carcinogenic evolution in the lung, suggesting also the importance of specific PPAR-gamma ligands as future therapeutic approach in lung cancer.

The mechanisms of action of PPARs

The peroxisome proliferator-activated receptors (PPARs) are a group of three nuclear receptor isoforms, PPAR gamma, PPAR alpha, and PPAR delta, encoded by different genes. PPARs are ligand-regulated transcription factors that control gene expression by binding to specific response elements (PPREs) within promoters. PPARs bind as heterodimers with a retinoid X receptor and, upon binding agonist, interact with cofactors such that the rate of transcription initiation is increased. The PPARs play a critical physiological role as lipid sensors and regulators of lipid metabolism. Fatty acids and eicosanoids have been identified as natural ligands for the PPARs. More potent synthetic PPAR ligands, including the fibrates and thiazolidinediones, have proven effective in the treatment of dyslipidemia and diabetes. Use of such ligands has allowed researchers to unveil many potential roles for the PPARs in pathological states including atherosclerosis, inflammation, cancer, infertility, and demyelination. Here, we present the current state of knowledge regarding the molecular mechanisms of PPAR action and the involvement of the PPARs in the etiology and treatment of several chronic diseases.

Novel genes regulated by the insulin sensitizer rosiglitazone during adipocyte differentiation

Thiazolidinediones (TZDs) are a new class of compounds that improve insulin sensitivity in type 2 diabetic patients as well as in rodent models of this disease. These compounds act as ligands for a member of the nuclear hormone receptor superfamily, peroxisome proliferator-activated receptor-gamma (PPAR-gamma), which is highly expressed in adipose tissue and, moreover, has been shown to play an important role in adipocyte differentiation. The strong correlation between the antidiabetic activity of TZDs and their ability to activate PPAR-gamma suggests that PPAR-gamma, through downstream-regulated genes, mediates the effects of TZDs. In this report, we present the isolation and characterization of 81 genes, encoding proteins of known function, differentially expressed during TZD-stimulated differentiation of 3T3-L1 cells. By the use of different reverse- Northern blot techniques, the differential expression of 50 of these genes could be verified, and 21 genes were specifically regulated by a potent TZD during the course of adipocyte differentiation, whereas no effect of a PPAR-gamma antagonist could be observed in mature adipocytes. The differential expression of a large fraction of the isolated genes was also shown to occur in white adipose tissue of ob/ob mice treated with rosiglitazone; combined, our results suggest that an important effect of rosiglitazone in adipose tissue is based on activation of PPAR-gamma in preexisting preadipocytes found among the mature adipocytes, resulting in subsequent adipocyte differentiation.

Ligands of peroxisome proliferator-activated receptor-gamma induce apoptosis in AR42J cells

INTRODUCTION

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a ligand-activated transcription factor that controls growth, differentiation, and inflammation in different tissues. Roles of PPAR-gamma activation in pancreatic acinar cells are poorly characterized.

AIMS

To examine the effects of PPAR-gamma activation on the induction of apoptosis in rat pancreatic AR42J cells.

METHODOLOGY

AR42J cells were treated with ligands of PPAR-gamma, and induction of apoptosis was evaluated by cell viability, DNA-fragmentation, and flow cytometry.

RESULTS

Treatment of the cells with ligands of PPAR-gamma (15-deoxy-open triangle12,14-prostaglandin J2 or troglitazone) induced apoptosis in a dose-dependent manner. Troglitazone-induced apoptosis was not blocked by inhibitors of caspases (acetyl-DEVD-aldehyde and benzoyloxycarbonyl-VAD-fluoromethylketone). Troglitazone induced the expression of pancreatitis-associated protein-1 and clusterin mRNAs. Troglitazone activated c-Jun NH2-terminal kinase/stress-activated protein kinase, but inhibited the activation of extracellular signal-regulated kinases 1/2. Troglitazone did not activate NF-kappaB, suggesting a role of NF-kappaB-independent pathways. In AR42J cells and isolated pancreatic acini, PPAR-gamma gene and protein were detected. In addition, troglitazone increased the PPAR-dependent transcriptional activity, suggesting that PPAR-gamma is functional in AR42J cells.

CONCLUSION

These results indicate that activation of PPAR-gamma induces apoptosis in AR42J cells and imply that PPAR-gamma may be a potential therapeutic target of pancreatic inflammation, because of its anti-inflammatory effects in addition to its proapoptotic effects.

Peroxisome proliferator-activated receptor-gamma agonist 15-deoxy-Delta(12,14)-prostaglandin J(2) ameliorates experimental autoimmune encephalomyelitis

Peroxisome proliferator-activated receptors (PPAR) are members of a nuclear hormone receptor superfamily that includes receptors for steroids, retinoids, and thyroid hormone, all of which are known to affect the immune response. Previous studies dealing with PPAR-gamma expression in the immune system have been limited. Recently, PPAR-gamma was identified in monocyte/macrophage cells. In this study we examined the role of PPAR-gamma in experimental autoimmune encephalomyelitis (EAE), an animal model for the human disease multiple sclerosis. The hypothesis we are testing is whether PPAR-gamma plays an important role in EAE pathogenesis and whether PPAR-gamma ligands can inhibit the clinical expression of EAE. Initial studies have shown that the presence of the PPAR-gamma ligand 15-deoxy-Delta(12,14)-PGJ(2) (15d-PGJ2) inhibits the proliferation of Ag-specific T cells from the spleen of myelin basic protein Ac(1-11) TCR-transgenic mice. 15d-PGJ2 suppressed IFN-gamma, IL-10, and IL-4 production by both Con A- and myelin basic protein Ac(1-11) peptide-stimulated lymphocytes as determined by ELISA and ELISPOT assay. Culture of encephalitogenic T cells with 15d-PGJ2 in the presence of Ag reduced the ability of these cells to adoptively transfer EAE. Examination of the target organ, the CNS, during the course of EAE revealed expression of PPAR-gamma in the spinal cord inflammatory infiltrate. Administration of 15d-PGJ2 before and at the onset of clinical signs of EAE significantly reduced the severity of disease. These results suggest that PPAR-gamma ligands may be a novel therapeutic agent for diseases such as multiple sclerosis.

Ligands of peroxisome proliferator-activated receptor-gamma block activation of pancreatic stellate cells

Activated pancreatic stellate cells (PSCs) have recently been implicated in the pathogenesis of pancreatic fibrosis and inflammation. Peroxisome proliferator-activated receptor gamma (PPAR-gamma) is a ligand-activated transcription factor which controls growth, differentiation, and inflammation in different tissues. Roles of PPAR-gamma activation in PSCs are poorly characterized. Here we examined the effects of PPAR-gamma ligands on the key parameters of PSC activation. PSCs were isolated from rat pancreas tissue, and used in their culture-activated, myofibroblast-like phenotype. Activation of PPAR-gamma was induced with 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) or with troglitazone. Expression of PPAR-gamma was predominantly localized in the nuclei, and PPAR-gamma was transcriptionally active after ligand stimulation. PPAR-gamma ligands inhibited platelet-derived growth factor-induced proliferation. This effect was associated with inhibition of cell cycle progression beyond the G1 phase. PPAR-gamma ligands decreased alpha-smooth muscle actin protein expression and alpha1(I) procollagen and prolyl 4-hydroxylase(alpha) mRNA levels. Activation of PPAR-gamma also resulted in the inhibition of inducible monocyte chemoattractant protein-1 expression. 15d-PGJ2, but not troglitazone, inhibited the degradation of IkappaB-alpha and consequent NF-kappaB activation. In conclusion, activation of PPAR-gamma inhibited profibrogenic and proinflammatory actions in activated PSCs, suggesting a potential application of PPAR-gamma ligands in the treatment of pancreatic fibrosis and inflammation.

Putative activation of the peroxisome proliferator-activated receptor gamma impairs androgen and enhances progesterone biosynthesis in primary cultures of porcine theca cells

Ovarian theca cells are the predominant source of gonadotropin-stimulated androgen biosynthesis in vivo. Troglitazone (TG), a synthetic agonist of the peroxisome proliferator-activated receptor gamma (PPARgamma) and a thiazolidinedione used to treat insulin resistance, decreases serum androgen concentrations in women with hyperthecosis and/or polycystic ovary syndrome. Using reverse transcription-polymerase chain reaction (RT-PCR), we demonstrated the presence of PPARgamma mRNA in the porcine ovary. Since activation of ovarian PPARgamma may alter hormone-stimulated steroidogenesis in vitro, we cultured porcine theca cells for 48 h in the presence of two different PPARgamma ligands, TG and 15-deoxy-Delta(12,14)-prostaglandin J2 (15d-PGJ2). Putative TG-mediated activation of PPARgamma resulted in a 53%-69% decrease in LH- and/or insulin-stimulated androstenedione and testosterone accumulation. Although TG reduced 3-isobutylmethylxanthine-enhanced LH-stimulated cAMP accumulation by 74%-78%, it did not alter basal cAMP concentrations. Exposure to 8Br-cAMP did not overcome the TG-induced inhibition of androgen accumulation. In contrast, TG administration amplified basal and hormone-stimulated progesterone accumulation, particularly in the presence of insulin, without altering levels of 17alpha-hydroxyprogesterone. The putative natural PPARgamma ligand, 15d-PGJ2, inhibited androgen biosynthesis and stimulated progesterone production. RT-PCR-based amplification of cytochrome P450 cholesterol side-chain cleavage (CYP11A) and cytochrome P450 17alpha-hydroxylase/C-17,20-lyase (CYP17) transcripts indicated that TG moderately enhanced expression of these genes. However, TG did not affect CYP17 protein expression. We conclude that putative ligand-mediated activation of PPARgamma decreases LH- and/or insulin-driven theca cell androgen production by impairing the ability of CYP17 to synthesize androstenedione from available progestins. The corresponding augmentation of progesterone production could suggest that PPARgamma activation induces theca cell differentiation toward a progestin-synthesizing phenotype.

Sebaceous epithelial cell differentiation requires cyclic adenosine monophosphate generation

The cyclic adenosine monophosphate (cAMP) generator choleratoxin is known to promote the growth of sebaceous epithelial cells (sebocytes) in monolayer culture in classical serum-containing media. Now that sebocytes can be grown in serum-free medium, we have examined whether choleratoxin or other cAMP generators are required for differentiation of rat preputial sebocytes in response to specific ligand activators of peroxisome proliferator-activated receptors (PPARs). Unexpectedly, choleratoxin reduced sebocyte proliferation. However, sebocyte differentiation in response to specific PPARalpha and PPARgamma agonists required a cAMP generator such as choleratoxin, and this response was suppressed by a protein kinase A inhibitor. In contrast, the stable prostacyclin analog, carbaprostacyclin (cPGI2), a PPARalpha,delta agonist that also generates cAMP, stimulated differentiation independently of choleratoxin. Furthermore, unlike the selective PPARalpha and PPARgamma agonists, cPGI2 stimulated both sebocyte DNA synthesis and proliferation. These data are compatible with the evidence that prostacyclin has the additional effect of generating cAMP. In addition, we addressed the possibility that choleratoxin may act as a surrogate for beta-adrenergic catecholamines in generating cAMP. In contrast with choleratoxin, both alpha- and beta-adrenergic catecholamines stimulated sebocyte growth and interfered with the choleratoxin effect on differentiation. These data suggest ligand-dependent, complex interactions between cAMP and the other signal transduction pathways involved in sebocyte growth and development.

Sibling rivalry in the E2F family

The E2F transcription factor family determines whether or not a cell will divide by controlling the expression of key cell-cycle regulators. The individual E2Fs can be divided into distinct subgroups that act in direct opposition to one another to promote either cellular proliferation or cell-cycle exit and terminal differentiation. What is the underlying molecular basis of this 'push-me-pull-you' regulation, and what are its biological consequences?

Prostacyclin-dependent apoptosis mediated by PPAR delta

Prostacyclin (PGI(2)) plays important roles in hemostasis both as a vasodilator and an endogenous inhibitor of platelet aggregation. PGI(2) functions in these roles through a specific IP receptor, a G protein-coupled receptor linked to G(s) and increases in cAMP. Here, we report that intracellular prostacyclin formed by expressing prostacyclin synthase in human embryonic kidney 293 cells promotes apoptosis by activating endogenous peroxisome proliferator-activated receptor delta (PPAR delta). In contrast, treatment of cells with extracellular prostacyclin or dibutyryl cAMP actually reduced apoptosis. On the contrary, treatment of the cells with RpcAMP (adenosine 3',5'-cyclic monophosphothioate, Rp-isomer), an antagonist of cAMP, enhanced prostacyclin-mediated apoptosis. The expression of an L431A/G434A mutant of PPAR delta completely blocked prostacyclin-mediated PPAR delta activation and apoptosis. These observations indicate that prostacyclin can act through endogenous PPAR delta as a second signaling pathway that controls cell fate.

PPARgamma agonists inhibit cell growth and suppress the expression of cyclin D1 and EGF-like growth factors in ras-transformed rat intestinal epithelial cells

Peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits the growth of several types of cancer cells. However, the mechanisms by which this occurs are poorly understood. The goal of the present study was to investigate the effects of PPARgamma on mutated ras-induced cell growth, activation of transcription factors and expression of genes associated with cellular transformation in rat intestinal epithelial cells. A human PPARgamma cDNA was introduced to the activated H-ras-transfected IEC-6 cells (IECras) and 1 clone (IECrasPR82) that stably expresses both activated ras and PPARgamma was obtained. Thiazolidinedione derivatives such as troglitazone and rosiglitazone, selective ligands for PPARgamma, inhibited the cellular growth of IECrasPR82 cells in a time-dependent manner and induced G1 cell cycle arrest. Treatment with troglitazone (20 microM) decreased the expression of cyclin D1, heparin-binding epidermal growth factor-like growth factor (HB-EGF) and amphiregulin and suppressed the promoter activities of cyclin D1 and HB-EGF. Furthermore, a luciferase assay and an electrophoretic mobility shift assay showed that thiazolidinedione derivatives suppressed the transcriptional activities of AP-1 and Ets, both of which play crucial roles in the expression of cyclin D1 and HB-EGF. These findings suggest that reduction of EGF-like growth factors and cyclin D1 through the suppression of AP-1 and Ets may be 1 mechanism whereby PPARgamma inhibits their growth.

Regulation of adipocyte differentiation

Once multipotent mesenchymal cells become committed to the adipoblast lineage, adipogenesis, the process of preadipocytes differentiation into adipocytes is initiated. This process starts with a phase of exponential growth of adipoblasts. Following confluence of these adipoblasts, the cells enter into a cell cycle arrest, they re-enter the cell cycle and pass through a limited number of cell divisions, and finally differentiate into fully mature adipocytes. Adipogenesis is controlled by a complex cross-talk between positive and negative regulators, such as hormonal and nutritional stimuli, that change the activity of a selected set of transcription factors. Regulation of adipogenesis is crucial to keep the body energy balance because a limited amount of adipose tissue, lipodystrophy, or an excess of adipose tissue, such as occurs in obesity, lead to profound metabolic dysfunctions and disease.

Regulation of peroxisome proliferator-activated receptor gamma expression in human asthmatic airways: relationship with proliferation, apoptosis, and airway remodeling

Airway inflammation and alterations in cellular turnover are histopathologic features of asthma. We show that the expression of peroxisome proliferator-activated receptor gamma (PPAR gamma), a nuclear hormone receptor involved in cell activation, differentiation, proliferation, and/or apoptosis, is augmented in the bronchial submucosa, the airway epithelium, and the smooth muscle of steroid-untreated asthmatics, as compared with control subjects. This is associated with enhanced proliferation and apoptosis of airway epithelial and submucosal cells, as assessed by the immunodetection of the nuclear antigen Ki67, and of the cleaved form of caspase-3, respectively, and with signs of airway remodeling, including thickness of the subepithelial membrane (SBM) and collagen deposition. PPAR gamma expression in the epithelium correlates positively with SBM thickening and collagen deposition, whereas PPAR gamma expressing cells in the submucosa relate both to SBM thickening and to the number of proliferating cells. The intensity of PPAR gamma expression in the bronchial submucosa, the airway epithelium, and the smooth muscle is negatively related to FEV(1) values. Inhaled steroids alone, or associated with oral steroids, downregulate PPAR gamma expression in all the compartments, cell proliferation, SBM thickness, and collagen deposition, whereas they increase apoptotic cell numbers in the epithelium and the submucosa. Our findings have demonstrated that PPAR gamma (1) is a new indicator of airway inflammation and remodeling in asthma; (2) may be involved in extracellular matrix remodeling and submucosal cell proliferation; (3) is a target for steroid therapy.

Expression of peroxisome proliferator-activated receptor gamma in renal cell carcinoma and growth inhibition by its agonists

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a ligand-activated transcriptional factor belonging to the steroid receptor superfamily. It plays a role in both adipocyte differentiation and tumorgenesis. Up-date, the up-regulation of PPAR-gamma expression is a frequent occurrence in a variety of different malignant tumors. In this study, we investigated the expression of PPAR-gamma in human renal cell carcinoma (RCC) tissues, and the role of PPAR-gamma in cell growth in human RCC-derived cell lines. Immunohistochemistry showed a strong immunoreactive expression of PPAR-gamma in all slides from cancer specimens. RT-PCR and Western blot analysis showed 3 RCC cell lines expressed PPAR-gamma mRNA and its protein. MTT assay in 3 RCC cells showed that the synthetic PPAR-gamma agonists thiazolidinedione compounds (pioglitazone and troglitazone) and the endogeneous PPAR-gamma ligand, 15-deoxy-Delta12,14-prostaglandin J(2) (15dPGJ(2)) inhibited the growth of the RCC cells. These results suggest that PPAR-gamma may become a new target in the treatment of RCC.

E2F1-mediated transcriptional inhibition of the plasminogen activator inhibitor type 1 gene

Gene expression of the plasminogen activation system is cell-cycle dependent. Previously, we showed that ectopic expression of E2F1 repressed the plasminogen activator inhibitor type 1 (PAI-1) promoter in a manner dependent on the presence of DNA-binding and transactivation domains of E2F1 but independent of binding to pocket-binding proteins, suggesting a novel mechanism for E2F-mediated negative gene regulation [Koziczak, M., Krek, W. & Nagamine, Y. (2000) Mol. Cell. Biol. 20, 2014-2022]. However, it remains to be seen whether endogenous E2F can exert a similar effect. We report here that down-regulation of PAI-1 gene expression correlates with an increase in endogenous E2F activity. When cells were treated with a cdk2/4-specific inhibitor, which maintains E2F in an inactive state, the decline of serum-induced PAI-1 mRNA levels was suppressed. In mutant U2OS cells expressing a temperature-sensitive retinoblastoma protein (pRB), a shift to a permissive temperature induced PAI-1 mRNA expression. In U2OS cells stably expressing an E2F1-estrogen receptor chimeric protein that could be activated by tamoxifen, PAI-1 gene transcription was markedly reduced by tamoxifen even in the presence of cycloheximide. These results all indicate that endogenous E2F can directly repress the PAI-1 gene. DNase I hypersensitive-site analysis of the PAI-1 promoter suggested the involvement of conformation changes in chromatin structure of the PAI-1 promoter. 5' deletion analysis of the PAI-1 promoter showed that multiple sites were responsible for the E2F negative regulation, some of which were promoter dependent. Interestingly, one of these sites is a p53-binding element.

The roles of PPARs in adipocyte differentiation

Adipose tissue development takes place primarily around birth but adipose cell number can increase throughout life in response to nutritional changes. At the molecular level, adipogenesis is the result of transcriptional remodeling that leads to activation of a considerable number of genes. Several transcription factors act cooperatively and sequentially in this process. This article attempts to review the roles of peroxisome proliferator-activated receptors gamma and delta in the control of preadipocyte proliferation and differentiation during adipose tissue development or during the adaptive response of adipose tissue mass to high-fat feeding.

Peroxisome proliferator-activated receptors (PPARs): novel therapeutic targets in renal disease

Peroxisome proliferator-activated receptors (PPARs): Novel therapeutic targets in renal disease. Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily of ligand-dependent transcription factors. PPARs play an important role in the general transcriptional control of numerous cellular processes, including lipid metabolism, glucose homeostasis, cell cycle progression, cell differentiation, inflammation and extracellular matrix remodeling. Three PPAR isoforms, designated PPARalpha, PPARbeta and PPARgamma, have been cloned and are differentially expressed in several tissues including the kidney. PPARalpha primary regulates lipid metabolism and modulates inflammation. PPARalpha is the molecular target of the hypolipidemic fibrates including bezafibrate and clofibrate. PPARbeta participates in embryonic development, implantation and bone formation. PPARgamma is a key factor in adipogenesis and also plays an important role in insulin sensitivity, cell cycle regulation and cell differentiation. Antidiabetic thiazolidinediones (TZDs) such as troglitazone and rosiglitazone are specific ligands of PPARgamma, and this interaction is responsible for the insulin-sensitizing and hypoglycemic effect of these drugs. The kidney has been shown to differentially express all PPAR isoforms. PPARalpha is predominantly expressed in proximal tubules and medullary thick ascending limbs, while PPARgamma is expressed in medullary collecting ducts, pelvic urothelium and glomerular mesangial cells. PPARbeta is ubiquitously expressed at low levels in all segments of nephron. Accumulating data has begun to emerge suggesting physiological and pathophysiological roles of PPARs in several tissues including the kidney. The availability of PPAR-selective agonists and antagonists may provide a new approach to modulate the renal response to diseases including glomerulonephritis, glomerulosclerosis and diabetic nephropathy.

Functional Interaction of bZIP Proteins and the Large Subunit of Replication Factor C in Liver and Adipose Cells

The transcription factor CCAAT/enhancer-binding protein-alpha (C/EBPalpha) has a vital role in cell growth and differentiation. To delineate further a mechanism for C/EBPalpha-mediated differentiation, we screened C/EBPalpha-interacting proteins through far-Western screening. One of the strongest interactions was with RFC140, the large subunit of the replication factor C complex. C/EBPalpha specifically interacted with RFC140 from rat liver nuclear extract as determined by a combination of affinity chromatography and co-immunoprecipitation. Subsequent far-Western blotting showed that the bZIP domain of C/EBPalpha interacted with the DNA-binding region of RFC140. Overexpression of RFC140 in mammalian cells increased the transactivation activity of C/EBPalpha on both minimal and native promoters. Consistent with the enhanced transactivation, a complex of C/EBPalpha and RFC140 proteins with the cognate DNA element was detected in vitro. The specific interaction between C/EBPalpha and RFC140 was detected in the terminal differentiation of 3T3-L1 preadipocytes to adipocytes. The synergistic transcription effect of these two proteins increased the promoter activity and protein expression of peroxisome proliferator-activated receptor-gamma, which is a main regulator of adipocyte differentiation. Our results demonstrate that the specific transcription factor C/EBPalpha and the general DNA replication factor RFC140 interact functionally and physically. This observation highlights a unique mechanism by which the levels of the general replication factor can strongly modulate the functional activity of the specific transcription factor as a coactivator.

Inhibition of peroxisome-proliferator-activated receptor (PPAR)alpha by MK886

Although MK886 was originally identified as an inhibitor of 5-lipoxygenase activating protein (FLAP), recent data demonstrate that this activity does not underlie its ability to induce apoptosis [Datta, Biswal and Kehrer (1999) Biochem. J. 340, 371--375]. Since FLAP is a fatty-acid binding protein, it is conceivable that MK886 may affect other such proteins. A family of nuclear receptors that are activated by fatty acids and their metabolites, the peroxisome-proliferator-activated receptors (PPARs), have been implicated in apoptosis and may represent a target for MK886. The ability of MK886 to inhibit PPAR-alpha, -beta and -gamma activity was assessed using reporter assay systems (peroxisome-proliferator response element--luciferase). Using a transient transfection system in monkey kidney fibroblast CV-1 cells, mouse keratinocyte 308 cells and human lung adenocarcinoma A549 cells, 10--20 microM MK886 inhibited Wy14,643 activation of PPAR alpha by approximately 80%. Similar inhibition of PPAR alpha by MK886 was observed with a stable transfection reporter system in CV-1 cells. Only minimal inhibitory effects were seen on PPAR beta and PPAR gamma. MK886 inhibited PPAR alpha by a non-competitive mechanism as shown by its effects on the binding of arachidonic acid to PPAR alpha protein, and a dose-response study using a transient transfection reporter assay in COS-1 cells. An assay assessing PPAR ligand-receptor interactions showed that MK886 prevents the conformational change necessary for active-complex formation. The expression of keratin-1, a protein encoded by a PPAR alpha-responsive gene, was reduced by MK886 in a culture of mouse primary keratinocytes, suggesting that PPAR inhibition has functional consequences in normal cells. Although Jurkat cells express all PPAR isoforms, various PPAR alpha and PPAR gamma agonists were unable to prevent MK886-induced apoptosis. This is consistent with MK886 functioning as a non-competitive inhibitor of PPAR alpha, but may also indicate that PPAR alpha is not directly involved in MK886-induced apoptosis. Although numerous PPAR activators have been identified, the results show that MK886 can inhibit PPAR alpha, making it the first compound identified to have such an effect.

Inhibition of cellular proliferation through IkappaB kinase-independent and peroxisome proliferator-activated receptor gamma-dependent repression of cyclin D1

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-regulated nuclear receptor superfamily member. Liganded PPARgamma exerts diverse biological effects, promoting adipocyte differentiation, inhibiting tumor cellular proliferation, and regulating monocyte/macrophage and anti-inflammatory activities in vitro. In vivo studies with PPARgamma ligands showed enhancement of tumor growth, raising the possibility that reduced immune function and tumor surveillance may outweigh the direct inhibitory effects of PPARgamma ligands on cellular proliferation. Recent findings that PPARgamma ligands convey PPARgamma-independent activities through IkappaB kinase (IKK) raises important questions about the specific mechanisms through which PPARgamma ligands inhibit cellular proliferation. We investigated the mechanisms regulating the antiproliferative effect of PPARgamma. Herein PPARgamma, liganded by either natural (15d-PGJ(2) and PGD(2)) or synthetic ligands (BRL49653 and troglitazone), selectively inhibited expression of the cyclin D1 gene. The inhibition of S-phase entry and activity of the cyclin D1-dependent serine-threonine kinase (Cdk) by 15d-PGJ(2) was not observed in PPARgamma-deficient cells. Cyclin D1 overexpression reversed the S-phase inhibition by 15d-PGJ(2). Cyclin D1 repression was independent of IKK, as prostaglandins (PGs) which bound PPARgamma but lacked the IKK interactive cyclopentone ring carbonyl group repressed cyclin D1. Cyclin D1 repression by PPARgamma involved competition for limiting abundance of p300, directed through a c-Fos binding site of the cyclin D1 promoter. 15d-PGJ(2) enhanced recruitment of p300 to PPARgamma but reduced binding to c-Fos. The identification of distinct pathways through which eicosanoids regulate anti-inflammatory and antiproliferative effects may improve the utility of COX2 inhibitors.

Troglitazone, a ligand for peroxisome proliferator-activated receptor gamma, inhibits chemically-induced aberrant crypt foci in rats

The biological roles of peroxisome proliferator-activated receptors (PPARs) in various diseases, including inflammation and cancer, have been highlighted recently. Although PPARgamma ligand is suspected to play an important role in carcinogenesis, its effects on colon tumorigenesis remain undetermined. The present time-course study was conducted to investigate possible modifying effects of a PPARgamma ligand, troglitazone, on the development and growth of aberrant crypt foci (ACF), putative precursor lesions for colon carcinoma, induced by azoxymethane (AOM) or dextran sodium sulfate (DSS) in male F344 rats. Oral troglitazone (10 or 30 mg / kg body weight (b.w.)) significantly reduced AOM (two weekly subcutaneous injections, 20 mg / kg b.w.)-induced ACF. Treatment with troglitazone increased apoptosis and decreased polyamine content and ornithine decarboxylase (ODC) activity in the colonic mucosa of rats treated with AOM. Gastric gavage of troglitazone also inhibited colitis and ACF induced by DSS (1% in drinking water), in conjunction with increased apoptosis and reduced colonic mucosal polyamine level and ODC activity. Our results suggest that troglitazone, a synthetic PPARgamma ligand, can inhibit the early stage of colon tumorigenesis with or without colitis.