Peroxisome proliferator-activated receptor-gamma ligands as cell-cycle modulators

Potential therapeutic role of peroxisome proliferator activated receptor-gamma agonists in psoriasis

Psoriasis is a chronic inflammatory condition affecting 1 - 3% of the world's population. Despite the availability of several agents, therapeutic options remain limited. With a better understanding of the pathophysiology of psoriasis, several potential therapeutic targets have been identified. Peroxisome proliferator activated receptors have been shown to play a role in cutaneous homeostasis. This review focuses on the potential therapeutic role of peroxisome proliferator activated receptor-gamma agonists in psoriasis and the possibility for the future prospects.

Peroxisome proliferator-activated receptor-gamma1 is dephosphorylated and degraded during BAY 11-7085-induced synovial fibroblast apoptosis

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) plays a central role in whole body metabolism by regulating adipocyte differentiation and energy storage. Recently, however, PPAR-gamma has also been demonstrated to affect proliferation, differentiation, and apoptosis of different cell types. As we have previously shown that BAY 11-7085-induced synovial fibroblast apoptosis is prevented by PPAR-gamma agonist 15d-PGJ2; the expression of PPAR-gamma in these cells was studied. Both PPAR-gamma1 and PPAR-gamma2 isoforms were cloned from synovial fibroblast RNA, but only PPAR-gamma1 was detected by Western blot, showing constitutive nuclear expression. Within minutes of BAY 11-7085 treatment, a PPAR-gamma1-specific band was shifted into a form of higher mobility, suggesting dephosphorylation, as confirmed by phosphatase treatment of cell extracts. Of interest, BAY 11-7085-induced PPAR-gamma1 dephosphorylation was followed by PARP and caspase-8 cleavage as well as by PPAR-gamma1 protein degradation. PPAR-gamma1 dephosphorylation was followed by the loss of PPAR-DNA binding activity ubiquitously present in synovial fibroblast nuclear extracts. Unlike the phosphorylated form, dephosphorylated PPAR-gamma1 was found in insoluble membrane cell fraction and was not ubiquitinated before degradation. PPAR-gamma1 dephosphorylation coincided with ERK1/2 phosphorylation that accompanies BAY 11-7085-induced synovial fibroblasts apoptosis. 15d-PGJ2, PGD2, and partially UO126, down-regulated ERK1/2 phosphorylation, protected cells from BAY 11-7085-induced apoptosis, and reversed both PPAR-gamma dephosphorylation and degradation. Furthermore, PPAR-gamma antagonist BADGE induced PPAR-gamma1 degradation, ERK1/2 phosphorylation, and synovial fibroblasts apoptosis. The results presented suggest an anti-apoptotic role for PPAR-gamma1 in synovial fibroblasts. Since apoptotic marker PARP is cleaved after PPAR-gamma1 dephosphorylation but before PPAR-gamma1 degradation, dephosphorylation event might be enough to mediate BAY 11-7085-induced apoptosis in synovial fibroblasts.

Thiazolidinediones ameliorate diabetic nephropathy via cell cycle-dependent mechanisms

Thiazolidinediones are ligands for peroxisome proliferator-activated receptor (PPAR)-gamma, widely used as insulin sensitizer in type 2 diabetic patients and implicated in apoptosis, cell proliferation, and cell cycle regulation. Here, the effect of thiazolidinediones on G1-phase cell cycle arrest, the hallmark in diabetic nephropathy, was investigated. Eight-week-old male Otsuka Long-Evans Tokushima fatty rats were treated with pioglitazone (1 mg x kg body wt(-1) x day(-1)) until 50 weeks of age and compared with insulin treatment. Although similar HbA(1c) levels were observed in both groups, pioglitazone significantly inhibited glomerular hypertrophy and mesangial matrix expansion and reduced urinary albumin excretion compared with the insulin-treated group. In addition, pioglitazone significantly reduced the number of glomerular p27(Kip1)-positive cells. Because prominent expression of PPAR-gamma was observed in podocytes in glomeruli and cultured cells, conditionally immortalized mouse podocyte cells were cultured under 5.5 and 25 mmol/l D-glucose supplemented with pioglitazone. Pioglitazone inhibited cell hypertrophy revealed by [(3)H]thymidine and [(3)H]proline incorporation, and pioglitazone reversed high glucose-induced G1-phase cell cycle arrest, i.e., an increase in G0/G1 phase and decrease in S and G2 phases. Pioglitazone suppressed high glucose-induced phosphorylation of p44/42 mitogen-activated protein kinase and reduced Bcl-2 and p27(Kip1) protein levels. Besides glucose-lowering action, pioglitazone ameliorates diabetic nephropathy via cell cycle-dependent mechanisms.

Expression of peroxisome proliferator activated receptor gamma and cyclo-oxygenase 2 in primary and recurrent ovarian carcinoma

AIM

Peroxisome proliferator-activated receptor gamma (PPARgamma) has emerged as a potential therapeutic target in several types of cancer. In ovarian carcinomas, limited and conflicting data on PPARgamma protein expression have been reported.

METHODS

The immunoexpression of PPARgamma and its putative target cyclo-oxygenase 2 (COX2) was investigated in tumour tissues from 80 patients with primary and corresponding recurrent ovarian serous carcinomas after conventional platinum-based chemotherapy.

RESULTS

PPARgamma expression was observed in 29% of primary and recurrent carcinomas. In the recurrent tumours, PPARgamma expression inversely correlated with COX2 overexpression in both chemosensitive (p = 0.02) and chemoresistant (p = 0.04) carcinomas.

CONCLUSIONS

The data indicate that PPARgamma may represent a potential target for second-line treatment in ovarian cancers.

Differential modulation of cell cycle, apoptosis and PPARgamma2 gene expression by PPARgamma agonists ciglitazone and 9-hydroxyoctadecadienoic acid in monocytic cells

We sought to compare the effects of the thiazolidinedione ciglitazone with the endogenous fatty acid PPARgamma agonists 9- and 13-hydroxyoctadecadienoic acid (9- and 13-HODE), in U937 monocytic cells. Ciglitazone and 9-HODE inhibited cell proliferation and all three agonists increased cellular content of C18:0 fatty acids. Ciglitazone and 13-HODE resulted in an increased percentage of cells in S phase and ciglitazone reduced the percentage of cells in G2/M phase of cell cycle, whilst 9-HODE increased the percentage of cells in G0/1 and reduced the fraction in S and G2/M phases. 9-HODE selectively induced apoptosis in U937 cells, and increased PPARgamma2 gene expression. Induction of apoptosis by 9-HODE was not abrogated by the presence of the PPARgamma antagonist GW9662. Synthetic (TZD) and endogenous fatty acid ligands for PPARgamma, ciglitazone and 9- and 13-HODE, possess differential, ligand specific actions in monocytic cells to regulate cell cycle progression, apoptosis and PPARgamma2 gene expression.

Peroxisome proliferator-activated receptor-gamma ligands for the treatment of breast cancer

Pioglitazone and rosiglitazone are thiazolidinediones used for the treatment of Type 2 diabetes mellitus. They modulate glucose and fat metabolism, mainly by binding to the nuclear hormone receptor peroxisome proliferator-activated receptor (PPAR)-gamma. PPAR-gamma signalling is involved in a number of other disease conditions including cancer. In breast cancer cells, PPAR-gamma ligands inhibit proliferation and induce apoptosis both in vitro and in vivo. PPAR-gamma ligands also inhibit tumour angiogenesis and invasion. The only published clinical trial using a PPAR-gamma ligand in patients with metastatic breast cancer failed to show any clinical benefits. The mechanism of action of the thiazolidinediones in breast cancer cells is not fully understood but involves interactions with other nuclear hormone receptors, transcriptional co-activators and repressors as well as PPAR-gamma-independent effects. A better understanding of these mechanisms will be needed before PPAR-gamma ligands may be useful in the treatment of breast cancer patients.

Lipid phosphate phosphatase-2 activity regulates S-phase entry of the cell cycle in Rat2 fibroblasts

Lipid phosphates are potent mediators of cell signaling and control processes including development, cell migration and division, blood vessel formation, wound repair, and tumor progression. Lipid phosphate phosphatases (LPPs) regulate the dephosphorylation of lipid phosphates, thus modulating their signals and producing new bioactive compounds both at the cell surface and in intracellular compartments. Knock-down of endogenous LPP2 in fibroblasts delayed cyclin A accumulation and entry into S-phase of the cell cycle. Conversely, overexpression of LPP2, but not a catalytically inactive mutant, caused premature S-phase entry, accompanied by premature cyclin A accumulation. At high passage, many LPP2 overexpressing cells arrested in G(2)/M and the rate of proliferation declined severely. This was accompanied by changes in proteins and lipids characteristic of senescence. Additionally, arrested LPP2 cells contained decreased lysophosphatidate concentrations and increased ceramide. These effects of LPP2 activity were not reproduced by overexpression or knock-down of LPP1 or LPP3. This work identifies a novel and specific role for LPP2 activity and bioactive lipids in regulating cell cycle progression.

From molecular action to physiological outputs: peroxisome proliferator-activated receptors are nuclear receptors at the crossroads of key cellular functions

Peroxisome proliferator-activated receptors (PPARs) compose a family of three nuclear receptors which act as lipid sensors to modulate gene expression. As such, PPARs are implicated in major metabolic and inflammatory regulations with far-reaching medical consequences, as well as in important processes controlling cellular fate. Throughout this review, we focus on the cellular functions of these receptors. The molecular mechanisms through which PPARs regulate transcription are thoroughly addressed with particular emphasis on the latest results on corepressor and coactivator action. Their implication in cellular metabolism and in the control of the balance between cell proliferation, differentiation and survival is then reviewed. Finally, we discuss how the integration of various intra-cellular signaling pathways allows PPARs to participate to whole-body homeostasis by mediating regulatory crosstalks between organs.

PPARgamma-dependent effects of conjugated linoleic acid on the human glioblastoma cell line (ADF)

Conjugated linoleic acid (CLA) has been shown to exert beneficial effects against carcinogenesis, atherosclerosis and diabetes. It has been demonstrated that CLA modulates lipid metabolism through the activation of peroxisome proliferator-activated receptors (PPARs). The PPAR family comprises 3 closely related gene products, PPAR alpha, beta/delta and gamma, differing for tissue distribution, developmental expression and ligand specificity. It has also been demonstrated that activated PPARgamma results in growth inhibition and differentiation of transformed cells. These observations stimulated a great interest toward PPARgamma ligands as potential anticancer drugs to be used in a differentiation therapy. Glioblastomas are the most commonly diagnosed primary tumors of the brain in humans. The prognosis of patients with high-grade gliomas is poor and only marginally improved by chemotherapy. The aim of this work was to study the effects of CLA and of a specific synthetic PPARgamma ligand on cell growth, differentiation and death of a human glioblastoma cell line as well as on parameters responsible for the metastatic behavior of this tumor. We demonstrate here that CLA and PPARgamma agonist strongly inhibit cell growth and proliferation rate and induce apoptosis. Moreover, both treatments decrease cell migration and invasiveness. The results obtained show that CLA acts, directly or indirectly, as a PPARgamma activator, strongly suggesting that this naturally occurring fatty acid may be used as brain antitumor drug and as a chemopreventive agent. Moreover, the gamma-agonist, once experimented and validated on man, may represent a useful coadjuvant in glioblastoma therapy and in the prevention of recurrences.

Proline oxidase, a proapoptotic gene, is induced by troglitazone: evidence for both peroxisome proliferator-activated receptor gamma-dependent and -independent mechanisms

Proline oxidase (POX) is a redox enzyme localized in the mitochondrial inner membrane. We and others have shown that POX is a p53-induced gene that can mediate apoptosis through generation of reactive oxygen species (ROS). The peroxisome proliferator-activated receptor gamma (PPARgamma) ligand troglitazone was found to activate the POX promoter in colon cancer cells. PPARgamma ligands have been reported to induce apoptosis in a variety of cancer cells. In HCT116 cells expressing a wild-type PPARgamma, troglitazone enhanced the binding of PPARgamma to PPAR-responsive element in the POX promoter and increased endogenous POX expression. Blocking of PPARgamma activation either by antagonist GW9662 or deletion of PPAR-responsive element in the POX promoter only partially decreased the POX promoter activation in response to troglitazone, indicating also the involvement of PPARgamma-independent mechanisms. Further, troglitazone also induced p53 protein expression in HCT116 cells, which may be the possible mechanism for PPARgamma-independent POX activation, since POX has been shown to be a downstream mediator in p53-induced apoptosis. In HCT15 cells, with both mutant p53 and mutant PPARgamma, there was no effect of troglitazone on POX activation, whereas in HT29 cells, with a mutant p53 and wild type PPARgamma, increased activation was observed by ligand stimulation, indicating that both PPARgamma-dependent and -independent mechanisms are involved in the troglitazone-induced POX expression. A time- and dose-dependent increase in POX catalytic activity was obtained in HCT116 cells treated with troglitazone with a concomitant increase in the production of intracellular ROS. Our results suggest that the induction of apoptosis by troglitazone may, at least in part, be mediated by targeting POX gene expression for generation of ROS by POX both by PPARgamma-dependent and -independent mechanisms.

Ligands for the peroxisome proliferator-activated receptor-gamma have inhibitory effects on growth of human neuroblastoma cells in vitro

The thiazolidinedione (TZD) or glitazone class of peroxisome proliferator-activated-gamma (PPAR-gamma) ligands not only induce adipocyte differentiation and increase insulin sensitivity, but also exert growth inhibitory effects on several carcinoma cell lines in vitro as well as in vivo. In the current study the in vitro effect of four PPAR-gamma agonists (ciglitazone, pioglitazone, troglitazone, rosiglitazone) on the cell growth of seven human neuroblastoma cell lines (Kelly, LAN-1, LAN-5, LS, IMR-32, SK-N-SH, SH-SY5Y) was investigated. Growth rates were assessed by a colorimetric XTT-based assay kit. Expression of PPAR-gamma protein was examined by immunohistochemistry and Western blot analysis. All glitazones inhibited in vitro growth and viability of the human neuroblastoma cell lines in a dose-dependent manner showing considerable effects only at high concentrations (10 microM and 100 microM). Effectiveness of the glitazones on neuroblastoma cell growth differed depending on the cell line and the agent. The presence of PPAR-gamma protein was demonstrated in all cell lines. Our findings indicate that ligands for PPAR-gamma may be useful therapeutic agents for the treatment of neuroblastoma. Thus the effect of glitazones on the growth of neuroblastoma should now be investigated in an in vivo animal model.

Peroxisome proliferator-activated receptors (PPARs) and ovarian function--implications for regulating steroidogenesis, differentiation, and tissue remodeling

The peroxisome proliferator-activated receptors (PPARs) are a family of transcription factors involved in varied and diverse processes such as steroidogenesis, angiogenesis, tissue remodeling, cell cycle, apoptosis, and lipid metabolism. These processes are critical for normal ovarian function, and all three PPAR family members--alpha, delta, and gamma, are expressed in the ovary. Most notably, the expression of PPARgamma is limited primarily to granulosa cells in developing follicles, and is regulated by luteinizing hormone (LH). Although much has been learned about the PPARs since their initial discovery, very little is known regarding their function in ovarian tissue. This review highlights what is known about the roles of PPARs in ovarian cells, and discusses potential mechanisms by which PPARs could influence ovarian function. Because PPARs are activated by drugs currently in clinical use (fibrates and thiazolidinediones), it is important to understand their role in the ovary, and how manipulation of their activity may impact ovarian physiology as well as ovarian pathology.

Receptor-independent actions of PPAR thiazolidinedione agonists: is mitochondrial function the key?

Agonists of the peroxisome proliferator activated receptor gamma (PPAR(gamma)) are currently used for treatment of type 2 diabetes due to their insulin sensitizing and glucose metabolism stabilizing effects. More recently some of these same agonists were shown to exert anti-inflammatory and anti-proliferative effects as well. Although PPAR(gamma) agonists can operate via receptor-mediated events occurring at the genomic level, thereby causing long lasting changes in gene expression patterns, recent studies demonstrate non-genomic as well as genomic actions, and receptor-dependent as well as receptor-independent effects of the thiazolidinedione (TZD) class of PPAR(gamma) agonists. In this review we will summarize data describing some of these novel, receptor independent actions of TZDs, review evidence that TZDs directly influence mitochondrial function, and attempt to reconcile how changes in mitochondrial function could contribute to other receptor-independent actions of these drugs.

Peroxisome proliferator activated receptor-γ ligands induced cell growth inhibition and its influence on matrix metalloproteinase activity in human myeloid leukemia cells

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is one of the best characterized nuclear hormone receptors (NHRs) in the superfamily of ligand-activated transcriptional factors. PPAR-gamma ligands have recently been demonstrated to affect proliferation, differentiation and apoptosis of different cell types. The present study was undertaken to investigate PPAR-gamma ligands induced cell growth inhibition and its influence on matrix metalloproteinase MMP-9 and MMP-2 activities on leukemia K562 and HL-60 cells in vitro. The results revealed that PPAR-gamma expression was detectable in the two kinds of leukemia cells; Both 15-deoxy-delta(12,14)-prostaglandin J2(15d-PGJ2) and troglitazone (TGZ) have significant growth inhibition effects on these two kinds of leukemia cells. These two PPAR-gamma ligands could inhibit the leukemic cell adhesion to the extracellular matrix (ECM) proteins and the invasion through matrigel matrix. The expressions of MMP-9 and MMP-2 as well as their gelatinolytic activities in both HL-60 and K562 cells were inhibited by 15d-PGJ2 and TGZ significantly. We therefore conclude that PPAR-gamma ligands 15d-PGJ2 and TGZ have significant growth inhibition effects on myeloid leukemia cells in vitro, and that PPAR-gamma ligands can inhibit K562 and HL-60 cell adhesion to and invasion through ECM as well as downregulate MMP-9 and MMP-2 expressions. The data suggest that PPAR-gamma ligands may serve as potential anti-leukemia reagents.