Down-regulation of PPARgamma1 suppresses cell growth and induces apoptosis in MCF-7 breast cancer cells

Knockdown of AMIGO2 suppresses proliferation and migration through regulating PPAR-γ in bladder cancer

PURPOSE

This study aims to reveal the relationship between AMIGO2 and proliferation, migration and tumorigenicity of bladder cancer, and explore the potential molecular mechanisms.

METHODS

The expression level of AMIGO2 is measured by qRT-PCR and immunohistochemistry (IHC). Stable AMIGO2 knockdown cell lines T24 and 5637 were established by lentivirus transfection. Cell Counting Kit (CCK-8 assay) was produced to determine cell proliferation, flow cytometry analysis was utilized to detect cell cycle, and wound healing assay was proceeded to test migration ability of bladder cancer cells. Xenograft mouse model was established for investigating the effect of AMIGO2 on tumor formation in vivo. The RNA Sequencing technology was applied to explore the underlying mechanisms. The expression level of PPAR-γ was measured by Western Blot.

RESULTS

AMIGO2 was upregulated in bladder cancer cells and tissues. Inhibited expression of AMIGO2 suppresses cell proliferation and migration. Low AMIGO2 expression inhibited tumorigenicity of 5637 in nude mice. According to RNA-Seq and bioinformatics analysis, 917 DEGs were identified. The DEGs were mainly enriched in cell-cell adhesion, peroxisome proliferators-activated receptors (PPARs) signaling pathway and some other pathways. PPAR-γ is highly expressed in bladder cancer cell lines T24 and 5637, but when AMIGO2 is knocked down in T24 and 5637, the expression level of PPAR-γ is also decreased, and overexpression of PPAR-γ could reverse the suppression effect of cell proliferation and migration caused by the inhibition of AMIGO2.

CONCLUSION

AMIGO2 is overexpressed in bladder cancer cells and tissues. Knockdown of AMIGO2 suppresses bladder cancer cell proliferation and migration. These processes might be regulated by PPAR-γ signaling pathway.

Peroxisome Proliferator-Activated Receptors and the Hallmarks of Cancer

Peroxisome proliferator-activated receptors (PPARs) function as nuclear transcription factors upon the binding of physiological or pharmacological ligands and heterodimerization with retinoic X receptors. Physiological ligands include fatty acids and fatty-acid-derived compounds with low specificity for the different PPAR subtypes (alpha, beta/delta, and gamma). For each of the PPAR subtypes, specific pharmacological agonists and antagonists, as well as pan-agonists, are available. In agreement with their natural ligands, PPARs are mainly focused on as targets for the treatment of metabolic syndrome and its associated complications. Nevertheless, many publications are available that implicate PPARs in malignancies. In several instances, they are controversial for very similar models. Thus, to better predict the potential use of PPAR modulators for personalized medicine in therapies against malignancies, it seems necessary and timely to review the three PPARs in relation to the didactic concept of cancer hallmark capabilities. We previously described the functions of PPAR beta/delta with respect to the cancer hallmarks and reviewed the implications of all PPARs in angiogenesis. Thus, the current review updates our knowledge on PPAR beta and the hallmarks of cancer and extends the concept to PPAR alpha and PPAR gamma.

Roles of Myc-associated zinc finger protein in malignant tumors

As an important transcription factor that is widely expressed in most tissues of the human body, Myc-associated zinc finger protein (MAZ) has been reported highly expressed in many malignant tumors and thought to be a promising therapeutic target for cancer treatment. In this review, we aim to offer a comprehensive understanding of MAZ regulation in malignant tumors. The carboxy terminal of MAZ protein contains six C2H2 zinc fingers, and its regulation of transcription is based on the interaction between the GC-rich DNA binding sites of target genes and its carboxy-terminal zinc finger motifs. MAZ protein has been found to activate or inhibit the transcriptional initiation process of many target genes, as well as play an important role in the transcriptional termination process of some target genes, so MAZ poses dual regulatory functions in the initiation and termination process of gene transcription. Through the transcriptional regulation of c-myc and Ras gene family, MAZ poses an important role in the occurrence and development of breast cancer, pancreatic cancer, prostate cancer, glioblastoma, neuroblastoma, and other malignant tumors. Our review shows a vital role of MAZ in many malignant tumors and provides novel insight for cancer diagnosis and treatment.

PPARα-Selective Antagonist GW6471 Inhibits Cell Growth in Breast Cancer Stem Cells Inducing Energy Imbalance and Metabolic Stress

Breast cancer is the most frequent cancer and the second leading cause of death among women. Triple-negative breast cancer is the most aggressive subtype of breast cancer and is characterized by the absence of hormone receptors and human epithelial growth factor receptor 2. Cancer stem cells (CSCs) represent a small population of tumor cells showing a crucial role in tumor progression, metastasis, recurrence, and drug resistance. The presence of CSCs can explain the failure of conventional therapies to completely eradicate cancer. Thus, to overcome this limit, targeting CSCs may constitute a promising approach for breast cancer treatment, especially in the triple-negative form. To this purpose, we isolated and characterized breast cancer stem cells from a triple-negative breast cancer cell line, MDA-MB-231. The obtained mammospheres were then treated with the specific PPARα antagonist GW6471, after which, glucose, lipid metabolism, and invasiveness were analyzed. Notably, GW6471 reduced cancer stem cell viability, proliferation, and spheroid formation, leading to apoptosis and metabolic impairment. Overall, our findings suggest that GW6471 may be used as a potent adjuvant for gold standard therapies for triple-negative breast cancer, opening the possibility for preclinical and clinical trials for this class of compounds.

New insights into acquired endocrine resistance of breast cancer

The translational research strategy of targeting estrogen receptor α (ERα) positive breast cancer and then using long term anti-hormone adjuvant therapy (5-10 years) has reduced recurrences and mortality. However, resistance continues to occur and improvements are required to build on the success of tamoxifen and aromatase inhibitors (AIs) established over the past 40 years. Further translational research has described the evolution of acquired resistance of breast cancer cell lines to long term estrogen deprivation that parallels clinical experience over years. Additionally, recent reports have identified mutations in the ERα obtained from the recurrences of AI treated patients. These mutations allow the ERα to activate without ligands and auto stimulate metastatic tumor growth. Furthermore, the new biology of estrogen-induced apoptosis in acquired resistant models in vitro and in vivo has been interrogated and applied to clinical trials. Inflammation and stress are emerging concepts occurring in the process of acquired resistance and estrogen-induced apoptosis with different mechanisms. In this review, we will present progress in the understanding of acquired resistance, focus on stress and inflammatory responses in the development of acquired resistance, and consider approaches to create new treatments to improve the treatment of breast cancer with endocrine resistance.

Targeting Peroxisome Proliferator-Activated Receptor γ to Increase Estrogen-Induced Apoptosis in Estrogen-Deprived Breast Cancer Cells

Peroxisome proliferator-activated receptor γ (PPARγ) is an important transcription factor that modulates lipid metabolism and inflammation. However, it remains unclear whether PPARγ is involved in modulation of estrogen (E₂)-induced inflammation, thus affecting apoptosis of E₂-deprived breast cancer cells, MCF-7:5C and MCF-7:2A. Here, we demonstrated that E₂ treatment suppressed the function of PPARγ in both cell lines, although the suppressive effect in MCF-7:2A cells was delayed owing to high PPARγ expression. Activation of PPARγ by a specific agonist, pioglitazone, selectively blocked the induction of TNFα expression by E₂, but did not affect other adipose inflammatory genes, such as fatty acid desaturase 1 and IL6. This suppression of TNFα expression by pioglitazone was mainly mediated by transrepression of nuclear factor-κB (NF-κB) DNA-binding activity. A novel finding was that NF-κB functions as an oxidative stress inducer in MCF-7:5C cells but an antioxidant in MCF-7:2A cells. Therefore, the NF-κB inhibitor JSH-23 displayed effects equivalent to those of pioglitazone, with complete inhibition of apoptosis in MCF-7:5C cells, but it increased E₂-induced apoptosis in MCF-7:2A cells. Depletion of PPARγ by siRNA or the PPARγ antagonist T0070907 accelerated E₂-induced apoptosis, with activation of NF-κB-dependent TNFα and oxidative stress. For the first time, we demonstrated that PPARγ is a growth signal and has potential to modulate NF-κB activity and oxidative stress in E₂-deprived breast cancer cell lines. All of these findings suggest that anti-PPARγ therapy is a novel strategy to improve the therapeutic effects of E₂-induced apoptosis in E₂-deprived breast cancer.

Elevated expression of ZNF217 promotes prostate cancer growth by restraining ferroportin-conducted iron egress

Although we and other studies indicated ZNF217 expression was increased in prostate cancer (PCa), the factors mediating its misregulated expression and their oncogenic activity remain largely unexplored. Recent evidence demonstrated that ferroportin (FPN) reduction lead to decreased iron export and increased intercellular iron that consequently aggravates the oncogenic effects of iron. In the present study, ZNF217 was identified as a transcriptional repressor that inhibits FPN expression. Increased of ZNF217 expression led to decreased FPN concentration, coupled with resultant intracellular iron retention, increased iron-related cellular activities and enhanced tumor cell growth. In contrast, decreased of ZNF217 expression restrained tumor cell growth by promoting FPN-driven iron egress. Mechanistic investigation manifested that ZNF217 facilitated the H3K27me3 levels of FPN promoter by interacting with EZH2. Besides, we also found that MAZ increased the transcription level of ZNF217, and subsequently inhibited the FPN expression and their iron–related activities. Strikingly, the expression of MAZ, EZH2 and ZNF217 were concurrently upregulated in PCa, leading to decreased expression of FPN, which induce disordered iron metabolism. Collectively, this study underscored that elevated expression of ZNF217 promotes prostate cancer growth by restraining FPN-conducted iron egress.

Peroxisome Proliferator-Activated Receptor γ and PGC-1α in Cancer: Dual Actions as Tumor Promoter and Suppressor

Peroxisome proliferator-activated receptor γ (PPARγ) is part of a nuclear receptor superfamily that regulates gene expression involved in cell differentiation, proliferation, immune/inflammation response, and lipid metabolism. PPARγ coactivator-1α (PGC-1α), initially identified as a PPARγ-interacting protein, is an important regulator of diverse metabolic pathways, such as oxidative metabolism and energy homeostasis. The role of PGC-1α in diabetes, neurodegeneration, and cardiovascular disease is particularly well known. PGC-1α is also now known to play important roles in cancer, independent of the role of PPARγ in cancer. Though many researchers have studied the expression and clinical implications of PPARγ and PGC-1α in cancer, there are still many controversies about the role of PPARγ and PGC-1α in cancer. This review examines and summarizes some recent data on the role and action mechanisms of PPARγ and PGC-1α in cancer, respectively, particularly the recent progress in understanding the role of PPARγ in several cancers since our review was published in 2012.

The Anticancer Potential of Peroxisome Proliferator-Activated Receptor Antagonists

The effects on cancer-cell proliferation and differentiation mediated by peroxisome proliferator-activated receptors (PPARs) have been widely studied, and pleiotropic outcomes in different cancer models and under different experimental conditions have been obtained. Interestingly, few studies report and little preclinical evidence supports the potential antitumor activity of PPAR antagonists. This review focuses on recent findings on the antitumor in vitro and in vivo effects observed for compounds able to inhibit the three PPAR subtypes in different tumor models, providing a rationale for the use of PPAR antagonists in the treatment of tumors expressing the corresponding receptors.

A novel interaction of PAK4 with PPARγ to regulate Nox1 and radiation-induced epithelial-to-mesenchymal transition in glioma

Tumor recurrence in glioblastoma (GBM) is, in part, attributed to increased epithelial-to-mesenchymal transition (EMT) and enhanced tumor cell dissemination in adjacent brain parenchyma after ionizing radiation (IR). EMT is associated with aggressive behavior, increased stem-like characteristics and treatment resistance in malignancies; however, the underlying signaling mechanisms that regulate EMT are poorly understood. We identified grade-dependent PAK4 upregulation in gliomas and further determined its role in mesenchymal transition and radioresistance. IR treatment significantly elevated expression and nuclear localization of PAK4 in correlation with induction of reactive oxygen species (ROS) and mesenchymal transition in GBM cells. Stable PAK4 overexpression promoted mesenchymal transition by elevating EMT marker expression in these cells. Of note, transcription factor-DNA binding arrays and chromatin immunoprecipitation experiments identified the formation of a novel nuclear PAK4/PPARγ complex which was recruited to the promoter of Nox1, a PPARγ target gene. In addition, IR further elevated PAK4/PPARγ complex co-recruitment to Nox1 promoter, and increased Nox1 expression and ROS levels associated with mesenchymal transition in these cells. Conversely, specific PAK4 downregulation decreased PPARγ-mediated Nox1 expression and suppressed EMT in IR-treated cells. In vivo orthotopic tumor experiments showed inhibition of growth and suppression of IR-induced PPARγ and Nox1 expression by PAK4 downregulation in tumors. Our results provide the first evidence of a novel role for PAK4 in IR-induced EMT and suggest potential therapeutic efficacy of targeting PAK4 to overcome radioresistance in gliomas.

The Transcription Factor IRF6 Co-Represses PPARγ-Mediated Cytoprotection in Ischemic Cerebrovascular Endothelial Cells

Activation of peroxisome proliferator-activated receptor gamma (PPARγ) in the cerebrovascular endothelium is a key suppressor of post-stroke brain damage. However, the role of PPARγ’s co-regulators during cerebral ischemia remains largely unknown. Here, we show that the transcription factor IRF6 is a novel PPARγ co-regulator that directly binds to and suppresses PPARγ activity in murine cerebrovascular endothelial cells. Moreover, IRF6 was also revealed to be a transcriptional target of PPARγ suppression, with PPARγ silencing significantly promoting IRF6 expression in cerebrovascular endothelial cells. In addition, IRF6 silencing significantly promoted pioglitazone’s cytoprotective effects in ischemic murine cerebrovascular endothelial cells. Mechanistically, IRF6 significantly suppressed PPARγ’s transcriptional inhibition of the ischemia-induced, pro-apoptotic microRNA miR-106a. In conclusion, we identified IRF6 as a novel PPARγ co-suppressor that serves a key role in suppressing PPARγ-mediated cerebrovascular endothelial cytoprotection following ischemia. Further investigation into IRF6 and other PPARγ co-regulators should provide additional insights into PPARγ’s cytoprotective role in the cerebrovascular endothelium following stroke.

Overexpression and clinical significance of MYC-associated zinc finger protein in pancreatic carcinoma

This study aimed to investigate the expression and clinical significance of MYC-associated zinc finger protein (MAZ) in pancreatic carcinoma (PC), and the biological functions of MAZ in PC cells. MAZ expression was detected in 57 PC tissues and 41 paired adjacent nontumor tissues by immunohistochemistry. Compared to the expression in adjacent nontumor tissues, MAZ was significantly higher expressed in PC tissues (P<0.0001). In addition, MAZ expression had a significant correlation with certain clinical characteristics of PC patients, such as age, tumor diameter, tumor number, and the serum level of CA199 (P<0.05). The survival analysis showed that the survival time of PC patients with high expression of MAZ was significantly lower than patients with low expression of MAZ (P=0.0365). After MAZ was knocked down in PANC-1 cells by RNA interference, the cells’ ability to proliferate, invade, and migrate was decreased significantly (P<0.01). Moreover, MAZ expression was found to be associated with Ki-67, a cell proliferation marker, in PC tissues, further supporting the idea that MAZ promotes PC cell proliferation. Our study clarifies an oncogenic role of MAZ in pathogenesis of PC and provides MAZ as a biomarker in the treatment and prognosis of PC.

The histone code reader SPIN1 controls RET signaling in liposarcoma

The histone code reader Spindlin1 (SPIN1) has been implicated in tumorigenesis and tumor growth, but the underlying molecular mechanisms remain poorly understood. Here, we show that reducing SPIN1 levels strongly impairs proliferation and increases apoptosis of liposarcoma cells in vitro and in xenograft mouse models. Combining signaling pathway, genome-wide chromatin binding, and transcriptome analyses, we found that SPIN1 directly enhances expression of GDNF, an activator of the RET signaling pathway, in cooperation with the transcription factor MAZ. Accordingly, knockdown of SPIN1 or MAZ results in reduced levels of GDNF and activated RET explaining diminished liposarcoma cell proliferation and survival. In line with these observations, levels of SPIN1, GDNF, activated RET, and MAZ are increased in human liposarcoma compared to normal adipose tissue or lipoma. Importantly, a mutation of SPIN1 within the reader domain interfering with chromatin binding reduces liposarcoma cell proliferation and survival. Together, our data describe a molecular mechanism for SPIN1 function in liposarcoma and suggest that targeting SPIN1 chromatin association with small molecule inhibitors may represent a novel therapeutic strategy.

MiR-449a exerts tumor-suppressive functions in human glioblastoma by targeting Myc-associated zinc-finger protein

Glioblastoma (GBM) is one of the most common and aggressive primary brain tumors in adults. Deregulated expression of microRNAs (miRNAs) has been associated with GBM progression through alterations in either oncogenic or tumor suppressor targets. Here, we elucidated the function and the possible molecular mechanisms of miR-449a in human GBM cell lines and tumor specimens-derived glioblastoma stem cells (GSCs). Quantitative real-time PCR demonstrated that miR-449a was down-regulated in human GBM cell lines and GSCs. Functionally, miR-449a acted as a tumor suppressor by reducing cell proliferation, migration and invasion as well as inducing apoptosis in human GBM cell lines and GSCs. Myc-associated zinc-finger protein (MAZ) was identified as a direct target of miR-449a, mediating these tumor-suppressive effects, demonstrated by Western blot assay and luciferase assays. Moreover, over-expression of miR-449a inhibited the expression of Podoplanin (PDPN) by down-regulating MAZ which could positively control the promoter activities via binding to the promoter of PDPN, demonstrated by luciferase assays and chromatin immunoprecipitation assays. Further, the PI3K/AKT pathway was blocked when MAZ was down-regulated by miR-449a. This process was coincided with the up-regulation of apoptotic proteins and the down-regulation of anti-apoptotic proteins, MMP2 and MMP9. Furthermore, nude mice carrying over-expressed miR-449a combined with knockdown MAZ tumors produced the smallest tumors and the highest survival. These results elucidated a novel molecular mechanism of GBM progression, and may thus suggest a promising application for GBM treatment.

X-box binding protein 1 is a novel key regulator of peroxisome proliferator-activated receptor γ2

X-box binding protein 1 (XBP1), a transcription factor of the unfolded protein response, plays various roles in many biological processes. We examined its pro-adipogenic activity and target genes during adipogenic differentiation in wild-type and genetically modified 3T3-L1 cells. Signalling pathways that contribute to Xbp1 mRNA splicing, and the correlation of the transcriptionally active XBP1 isoform (XBP1s) level with body mass index and the level of peroxisome proliferator-activated receptor γ2 (PPARγ2) in human adipose tissues were also examined. The mRNA and nuclear protein expression levels of XBP1s increased immediately following hormonal induction of adipogenesis, reaching a peak at 6 h. Results from cDNA microarray and gene expression analyses using genetically modified cells indicated that PPARγ2 was a principal target of XBP1s. The XBP1s-specific binding motif, which is distinct from the CCAAT/enhancer-binding protein α binding site, was identified in the PPARγ2 promoter by site-directed mutagenesis. Fetal bovine serum, insulin, 3-isobutyl-1-methylxanthine and dexamethasone contributed independently to Xbp1 mRNA splicing. In human subcutaneous adipose tissues, the levels of both Xbp1s and Pparγ2 mRNA increased proportionally with body mass index, and there was a significant positive correlation between the two genes. These data suggest for the first time that positive regulation of PPARγ2 is a principal mechanism of XBP1s-mediated adipogenesis in 3T3-L1 cells.

Synergistic Antiproliferative Effects of Combined γ -Tocotrienol and PPAR γ Antagonist Treatment Are Mediated through PPAR γ -Independent Mechanisms in Breast Cancer Cells

Previous findings showed that the anticancer effects of combined γ-tocotrienol and peroxisome proliferator activated receptor γ (PPARγ) antagonist treatment caused a large reduction in PPARγ expression. However, other studies suggest that the antiproliferative effects of γ-tocotrienol and/or PPARγ antagonists are mediated, at least in part, through PPARγ-independent mechanism(s). Studies were conducted to characterize the role of PPARγ in mediating the effects of combined treatment of γ-tocotrienol with PPARγ agonists or antagonists on the growth of PPARγ negative +SA mammary cells and PPARγ-positive and PPARγ-silenced MCF-7 and MDA-MB-231 breast cancer cells. Combined treatment of γ-tocotrienol with PPARγ antagonist decreased, while combined treatment of γ-tocotrienol with PPARγ agonist increased, growth of all cancer cells. However, treatment with high doses of 15d-PGJ₂, an endogenous natural ligand for PPARγ, had no effect on cancer cell growth. Western blot and qRT-PCR studies showed that the growth inhibitory effects of combined γ-tocotrienol and PPARγ antagonist treatment decreased cyclooxygenase (COX-2), prostaglandin synthase (PGDS), and prostaglandin D₂ (PGD₂) synthesis. In conclusion, the anticancer effects of combined γ-tocotrienol and PPARγ antagonists treatment in PPARγ negative/silenced breast cancer cells are mediated through PPARγ-independent mechanisms that are associated with a downregulation in COX-2, PGDS, and PGD₂ synthesis.

The prostate cancer-up-regulated Myc-associated zinc-finger protein (MAZ) modulates proliferation and metastasis through reciprocal regulation of androgen receptor

Prostate cancer (PCa) is one of the most commonly diagnosed malignancies in men and the second leading cause of male cancer mortality. MAZ (Myc-associated zinc-finger protein) is a transcription factor that regulates the transcription of oncogenes, and the deregulated MAZ expression is closely related to the development and progression of a variety of cancers. In the present study, the role of MAZ in PCa tumorigenesis and its interaction with androgen receptor (AR), which is essential to PCa development in humans, were investigated. MAZ expression was found to be higher in clinical PCa specimens than in benign prostatic hyperplasia (BPH) and adjacent normal tissues, and MAZ expression was positively correlated with AR expression, which was also observed in PCa cell lines. After knockdown of MAZ by siRNA, cell proliferation was notably inhibited, colony formation declined, the cell cycle was arrested at G0/G1 phase, and the number of cells in S phase decreased (p < 0.05). MAZ knockdown resulted in a significant decline in the migration and invasion capacity of the LNCaP-AD cell line. siRNA knockdown of AR significantly decreased MAZ expression, and knockdown of MAZ significantly increased the expression of AR and DHT-induced androgen response element (ARE). These results suggest that MAZ and AR are interrelated and that MAZ plays an important role in PCa pathogenesis, which could be a potential therapeutic target.

Resolution of inflammation: mechanisms and opportunity for drug development

Inflammation is a beneficial host reaction to tissue damage and has the essential primary purpose of restoring tissue homeostasis. Inflammation plays a major role in containing and resolving infection and may also occur under sterile conditions. The cardinal signs of inflammation dolor, calor, tumor and rubor are intrinsically associated with events including vasodilatation, edema and leukocyte trafficking into the site of inflammation. If uncontrolled or unresolved, inflammation itself can lead to further tissue damage and give rise to chronic inflammatory diseases and autoimmunity with eventual loss of organ function. It is now evident that the resolution of inflammation is an active continuous process that occurs during an acute inflammatory episode. Successful resolution requires activation of endogenous programs with switch from production of pro-inflammatory towards pro-resolving molecules, such as specific lipid mediators and annexin A1, and the non-phlogistic elimination of granulocytes by apoptosis with subsequent removal by surrounding macrophages. These processes ensure rapid restoration of tissue homeostasis. Here, we review recent advances in the understanding of resolution of inflammation, highlighting the pharmacological strategies that may interfere with the molecular pathways which control leukocyte survival and clearance. Such strategies have proved beneficial in several pre-clinical models of inflammatory diseases, suggesting that pharmacological modulation of the resolution process may be useful for the treatment of chronic inflammatory diseases in humans.

Mechanisms Mediating the Effects of γ-Tocotrienol When Used in Combination with PPARγ Agonists or Antagonists on MCF-7 and MDA-MB-231 Breast Cancer Cells

γ-Tocotrienol is a natural vitamin E that displays potent anticancer activity, and previous studies suggest that these effects involve alterations in PPARγ activity. Treatment with 0.5–6 μM  γ-tocotrienol, 0.4–50 μM PPARγ agonists (rosiglitazone or troglitazone), or 0.4–25 μM PPARγ antagonists (GW9662 or T0070907) alone resulted in a dose-responsive inhibition of MCF-7 and MDA-MB-231 breast cancer proliferation. However, combined treatment of 1–4 μM  γ-tocotrienol with PPARγ agonists reversed the growth inhibitory effects of γ-tocotrienol, whereas combined treatment of 1–4 μM  γ-tocotrienol with PPARγ antagonists synergistically inhibited MCF-7 and MDA-MB-231 cell growth. Combined treatment of γ-tocotrienol and PPARγ agonists caused an increase in transcription activity of PPARγ along with increased expression of PPARγ and RXR, and decrease in PPARγ coactivators, CBP p/300, CBP C-20, and SRC-1, in both breast cancer cell lines. In contrast, combined treatment of γ-tocotrienol with PPARγ antagonists resulted in a decrease in transcription activity of PPARγ, along with decreased expression of PPARγ and RXR, increase in PPARγ coactivators, and corresponding decrease in PI3K/Akt mitogenic signaling in these cells. These findings suggest that elevations in PPARγ are correlated with increased breast cancer growth and survival, and treatment that decreases PPARγ expression may provide benefit in the treatment of breast cancer.

Crosstalk between the peroxisome proliferator-activated receptor γ (PPARγ) and the vitamin D receptor (VDR) in human breast cancer cells: PPARγ binds to VDR and inhibits 1α,25-dihydroxyvitamin D3 mediated transactivation

Heterodimerization and cross-talk between nuclear hormone receptors often occurs. For example, estrogen receptor alpha (ERα) physically binds to peroxisome proliferator-activated receptor gamma (PPARγ) and inhibits its transcriptional activity. The interaction between PPARγ and the vitamin D receptor (VDR) however, is unknown. Here, we elucidate the molecular mechanisms linking PPARγ and VDR signaling, and for the first time we show that PPARγ physically associates with VDR in human breast cancer cells. We found that overexpression of PPARγ decreased 1α,25-dihydroxyvitamin D(3) (1,25D(3)) mediated transcriptional activity of the vitamin D target gene, CYP24A1, by 49% and the activity of VDRE-luc, a vitamin D responsive reporter, by 75% in T47D human breast cancer cells. Deletion mutation experiments illustrated that helices 1 and 4 of PPARγ's hinge and ligand binding domains, respectively, governed this suppressive function. Additionally, abrogation of PPARγ's AF2 domain attenuated its repressive action on 1,25D(3) transactivation, indicating that this domain is integral in inhibiting VDR signaling. PPARγ was also found to compete with VDR for their binding partner retinoid X receptor alpha (RXRα). Overexpression of RXRα blocked PPARγ's suppressive effect on 1,25D(3) action, enhancing VDR signaling. In conclusion, these observations uncover molecular mechanisms connecting the PPARγ and VDR pathways.

PPARs: Interference with Warburg' Effect and Clinical Anticancer Trials

The metabolic/cell signaling basis of Warburg's effect ("aerobic glycolysis") and the general metabolic phenotype adopted by cancer cells are first reviewed. Several bypasses are adopted to provide a panoramic integrated view of tumoral metabolism, by attributing a central signaling role to hypoxia-induced factor (HIF-1) in the expression of aerobic glycolysis. The cancer metabolic phenotype also results from alterations of other routes involving ras, myc, p53, and Akt signaling and the propensity of cancer cells to develop signaling aberrances (notably aberrant surface receptor expression) which, when present, offer unique opportunities for therapeutic interventions. The rationale for various emerging strategies for cancer treatment is presented along with mechanisms by which PPAR ligands might interfere directly with tumoral metabolism and promote anticancer activity. Clinical trials using PPAR ligands are reviewed and followed by concluding remarks and perspectives for future studies. A therapeutic need to associate PPAR ligands with other anticancer agents is perhaps an important lesson to be learned from the results of the clinical trials conducted to date.

Myc-associated zinc finger protein (MAZ) is regulated by miR-125b and mediates VEGF-induced angiogenesis in glioblastoma

In patients with glioblastomas, vascular endothelial growth factor (VEGF) is a key mediator of tumor-associated angiogenesis. Glioblastomas are notorious for their capacity to induce neovascularization, driving continued tumor growth. Here we report that miR-125b is down-regulated in glioblastoma-associated endothelial cells, resulting in increased expression of its target, myc-associated zinc finger protein (MAZ), a transcription factor that regulates VEGF. The down-regulation of miR-125b was also observed on exposure of endothelial cells to glioblastoma-conditioned medium or VEGF, resulting in increased MAZ expression. Further analysis revealed that inhibition of MAZ accumulation by miR-125b, or by MAZ-specific shRNAs, attenuated primary human brain endothelial cell migration and tubule formation in vitro, phenomena considered to mimick angiogenic processes in vitro. Moreover, MAZ expression was elevated in brain blood vessels of glioblastoma patients. Altogether these results demonstrate a functional feed-forward loop in glioblastoma-related angiogenesis, in which VEGF inhibits the expression of miR-125b, resulting in increased expression of MAZ, which in its turn causes transcriptional activation of VEGF. This loop is functionally impeded by the VEGF receptor inhibitor vandetanib, and our results may contribute to the further development of inhibitors of tumor-angiogenesis.

Skp2 is a promising therapeutic target in breast cancer

Breast cancer is the most common type of cancer among American women, and remains the second leading cause of cancer-related death for female in the United States. It has been known that several signaling pathways and various factors play critical roles in the development and progression of breast cancer, such as estrogen receptor, Notch, PTEN, human epidermal growth factor receptor 2, PI3K/Akt, BRCA1, and BRCA2. Emerging evidence has shown that the F-box protein S-phase kinase associated protein 2 (Skp2) also plays an important role in the pathogenesis of breast cancer. Therefore, in this brief review, we summarize the novel functions of Skp2 in the pathogenesis of breast cancer. Moreover, we provide further evidence regarding the state of our knowledge toward the development of novel Skp2 inhibitors especially natural "chemopreventive agents" as targeted approach for the prevention and/or treatment of breast cancer.

MiR-34a expression has an effect for lower risk of metastasis and associates with expression patterns predicting clinical outcome in breast cancer

MiR-34a acts as a candidate tumour suppressor gene, and its expression is reduced in several cancer types. We aimed to study miR-34a expression in breast cancer and its correlation with tumour characteristics and clinical outcome, and regulatory links with other genes. We analysed miR-34a expression in 1,172 breast tumours on TMAs. 25% of the tumours showed high, 43% medium and 32% low expression of miR-34a. High miR-34a expression associated with poor prognostic factors for breast cancer: positive nodal status (p = 0.006), high tumour grade (p<0.0001), ER-negativity (p = 0.0002), HER2-positivity (p = 0.0002), high proliferation rate (p<0.0001), p53-positivity (p<0.0001), high cyclin E (p<0.0001) and γH2AX (p<0.0001). However, multivariate analysis adjusting for conventional prognostic factors indicated that high miR-34a expression in fact associated with a lower risk of recurrence or death from breast cancer (HR = 0.63, 95% CI = 0.41–0.96, p = 0.031). Gene expression analysis by differential miR-34a expression revealed an expression signature with an effect on both the 5-year and 10-year survival of the patients (p<0.001). Functional genomic analysis highlighted a novel regulatory role of the transcription factor MAZ, apart from the known control by p53, on the expression of miR-34a and a number of miR-34a targets. Our findings suggest that while miR-34a expression activation is a marker of aggressive breast tumour phenotype it exerts an independent effect for a lower risk of recurrence or death from breast cancer. We also present an expression signature of 190 genes associated with miR-34a expression. Our analysis for regulatory loops suggest that MAZ and p53 transcription factors co-operate in modulating miR-34a, as well as miR-34a targets involved in several cellular pathways. Taken together, these results suggest that the network of genes co-regulated with and targeted by miR-34a form a group of down-stream effectors that maybe of use in predicting clinical outcome, and that highlight novel regulatory mechanisms in breast cancer.

CARM1 is an important determinant of ERα-dependent breast cancer cell differentiation and proliferation in breast cancer cells

Breast cancers with estrogen receptor α (ERα) expression are often more differentiated histologically than ERα-negative tumors, but the reasons for this difference are poorly understood. One possible explanation is that transcriptional cofactors associated with ERα determine the expression of genes which promote a more differentiated phenotype. In this study, we identify one such cofactor as coactivator-associated arginine methyltransferase 1 (CARM1), a unique coactivator of ERα that can simultaneously block cell proliferation and induce differentiation through global regulation of ERα-regulated genes. CARM1 was evidenced as an ERα coactivator in cell-based assays, gene expression microarrays, and mouse xenograft models. In human breast tumors, CARM1 expression positively correlated with ERα levels in ER-positive tumors but was inversely correlated with tumor grade. Our findings suggest that coexpression of CARM1 and ERα may provide a better biomarker of well-differentiated breast cancer. Furthermore, our findings define an important functional role of this histone arginine methyltransferase in reprogramming ERα-regulated cellular processes, implicating CARM1 as a putative epigenetic target in ER-positive breast cancers.

Specific thiazolidinediones inhibit ovarian cancer cell line proliferation and cause cell cycle arrest in a PPARγ independent manner

BACKGROUND

Peroxisome Proliferator Activated Receptor gamma (PPARγ) agonists, such as the thiazolinediones (TZDs), have been studied for their potential use as cancer therapeutic agents. We investigated the effect of four TZDs--Rosiglitazone (Rosi), Ciglitazone (CGZ), Troglitazone (TGZ), and Pioglitazone (Pio)--on ovarian cancer cell proliferation, PPARγ expression and PPAR luciferase reporter activity. We explored whether TZDs act in a PPARγ dependent or independent manner by utilizing molecular approaches to inhibit or overexpress PPARγ activity.

PRINCIPAL FINDINGS

Treatment with CGZ or TGZ for 24 hours decreased proliferation in three ovarian cancer cell lines, Ovcar3, CaOv3, and Skov3, whereas Rosi and Pio had no effect. This decrease in Ovcar3 cell proliferation was due to a higher fraction of cells in the G(0)/G(1) stage of the cell cycle. CGZ and TGZ treatment increased apoptosis after 4 hours of treatment but not after 8 or 12 hours. Treatment with TGZ or CGZ increased PPARγ mRNA expression in Ovcar3 cells; however, protein levels were unchanged. Surprisingly, luciferase promoter assays revealed that none of the TZDs increased PPARγ activity. Overexpression of wild type PPARγ increased reporter activity. This was further augmented by TGZ, Rosi, and Pio indicating that these cells have the endogenous capacity to mediate PPARγ transactivation. To determine whether PPARγ mediates the TZD-induced decrease in proliferation, cells were treated with CGZ or TGZ in the absence or presence of a dominant negative (DN) or wild type overexpression PPARγ construct. Neither vector changed the TZD-mediated cell proliferation suggesting this effect of TZDs on ovarian cancer cells may be PPARγ independent.

CONCLUSIONS

CGZ and TGZ cause a decrease in ovarian cancer cell proliferation that is PPARγ independent. This concept is supported by the finding that a DN or overexpression of the wild type PPARγ did not affect the changes in cell proliferation and cell cycle.

Pigment epithelium-derived factor induces interleukin-10 expression in human macrophages by induction of PPAR gamma

AIM

In search for the anti-inflammation mechanism of PEDF, we investigate whether pigment epithelium-derived factor (PEDF) induces the gene expression of interleukin (IL)-10 in human macrophages and determine the molecular basis of this induction.

MAIN METHODS

Human macrophages derived from a monocytic cell line, THP-1, and peripheral monocytes were treated with PEDF. IL-10 expression was assessed by quantitative real-time PCR, enzyme-linked immunosorbent assay, semi-quantitative reverse transcriptase (RT)-PCR, and promoter-reporter assay. Activity of extracellular signal-regulated kinase 2 (ERK2) and p38 mitogen-activated protein kinase (MAPK) was assessed by immunoblotting using antibodies targeting phosphorylated kinases forms. Elk-1 and ATF-2 phosphorylation was determined as well. Pharmacological inhibitors were used to examine the involvement of ERK, p38 MAPK, and peroxisome proliferator-activated receptor gamma (PPARgamma) on the IL-10 expression induced by PEDF.

KEY FINDINGS

PEDF increased the levels of IL-10 mRNA and protein in THP-1 cells and human macrophages derived from peripheral monocytes. Blockade of activity of ERK or p38 MAPK attenuated PEDF effects on induction of PPARgamma and IL-10. PEDF increased the transcriptional activity of IL-10 promoter. The effect was synergistically augmented by PPARgamma agonist, but attenuated by inhibitors of PPARgamma, ERK or p38 MAPK. These results showed that PEDF promotes IL-10 expression at transcriptional level, and that this is achieved through the ERK2/p38MAPK-dependent PPARgamma expression.

SIGNIFICANCE

The anti-inflammatory property of PEDF may in part through the induction of IL-10 in macrophages. Our study supports the therapeutic potential of PEDF and PPARgamma agonists in inflammatory diseases.

Signal transducer and activator of transcription 3 (STAT3) regulates adipocyte differentiation via peroxisome-proliferator-activated receptor gamma (PPARgamma)

BACKGROUND INFORMATION

STAT3 (signal transducer and activator of transcription 3) is an important transcription factor involved in many biological events, including apoptosis, tumorigenesis, angiogenesis and epithelial-to-mesenchymal transition. However, no direct evidence for a role of STAT3 in 3T3-L1 adipocyte differentiation has been reported.

RESULTS

In the present study, we found that rapid activation of STAT3, lasting for more than 48 h, was elicited upon induction of adipogenesis. Both the STAT3-selective inhibitor stattic and the JAK2 (Janus kinase 2)/STAT3-selective inhibitors AG490 and Gö6976 inhibited STAT3 activation, leading to the suppression of adipocyte differentiation. Adipocyte differentiation was also suppressed by STAT3 siRNA (small interfering RNA) or dominant-negative STAT3. Interestingly, the PPARgamma (peroxisome-proliferator-activated receptor gamma) agonist TAZ (troglitazone) abolished the STAT3-inhibitor- and RNAi (RNA interference)-mediated suppression of adipogenesis. However, TAZ treatment had no effect on the stattic- and AG490-mediated down-regulation of STAT3 activation, suggesting that STAT3 regulates adipocyte differentiation through signalling that occurs upstream of PPARgamma.

CONCLUSION

These data indicate that STAT3 functions as a critical factor for adipogenesis via a mechanism involving the PPARgamma activation pathway.