Inhibition of testicular embryonal carcinoma cell tumorigenicity by peroxisome proliferator-activated receptor-β/δ- and retinoic acid receptor-dependent mechanisms

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.

PPAR Beta/Delta and the Hallmarks of Cancer

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone receptor family. Three different isoforms, PPAR alpha, PPAR beta/delta and PPAR gamma have been identified. They all form heterodimers with retinoic X receptors to activate or repress downstream target genes dependent on the presence/absence of ligands and coactivators or corepressors. PPARs differ in their tissue expression profile, ligands and specific agonists and antagonists. PPARs attract attention as potential therapeutic targets for a variety of diseases. PPAR alpha and gamma agonists are in clinical use for the treatment of dyslipidemias and diabetes. For both receptors, several clinical trials as potential therapeutic targets for cancer are ongoing. In contrast, PPAR beta/delta has been suggested as a therapeutic target for metabolic syndrome. However, potential risks in the settings of cancer are less clear. A variety of studies have investigated PPAR beta/delta expression or activation/inhibition in different cancer cell models in vitro, but the relevance for cancer growth in vivo is less well documented and controversial. In this review, we summarize critically the knowledge of PPAR beta/delta functions for the different hallmarks of cancer biological capabilities, which interplay to determine cancer growth.

Co-expression of carcinoembryonic antigen-related cell adhesion molecule 6 and 8 inhibits proliferation and invasiveness of breast carcinoma cells

The carcinoembryonic antigen-related cell adhesion molecule (CEACAM) 6 and CEACAM8 form heterodimers and exert their effects. Therefore, we examined the effects of CEACAM6 and CEACAM8 co-expression in breast cancer. We first studied CEACAM6/8 expression using immunohistochemistry in 109 patients with breast cancer. We then established MCF-7 cells that were stably transfected with CEACAM8 and studied cell proliferation, invasion and adhesion. The number of CEACAM6 and CEACAM8 double-positive breast carcinoma cells significantly increased in patients with low histopathological grade and stage. Proximity ligation assay (PLA) confirmed high CEACAM6/8 expression in MCF-7 cells. CEACAM6/8 expression promoted the adhesion of MCF-7 cells to endothelial cell monolayers but inhibited their invasion and proliferation. Furthermore, CEACAM6 status in carcinoma cells was significantly higher in bone than in lung metastases. CEACAM6/8 expression is associated with the inhibition of vascular invasion and cell proliferation. CEACAM6 expression was also considered to be involved in bone metastases of breast cancer. This is the first study to demonstrate the possible role of CEACAM6/8 heterodimer and CEACAM6 expression in breast cancer patients.

Editor's Highlight: PPARβ/δ and PPARγ Inhibit Melanoma Tumorigenicity by Modulating Inflammation and Apoptosis

Skin tumorigenesis results from DNA damage, increased inflammation, and evasion of apoptosis. The peroxisome proliferator-activated receptors (PPARs) can modulate these mechanisms in non-melanoma skin cancer. However, limited data exists regarding the role of PPARs in melanoma. This study examined the effect of proliferator-activated receptor-β/δ (PPARβ/δ) and PPARγ on cell proliferation, anchorage-dependent clonogenicity, and ectopic xenografts in the UACC903 human melanoma cell line. Stable overexpression of either PPARβ/δ or PPARγ enhanced ligand-induced expression of a PPARβ/δ/PPARγ target gene in UACC903 cell lines as compared with controls. The induction of target gene expression by ligand activation of PPARγ was not altered by overexpression of PPARβ/δ, or vice versa. Stable overexpression of either PPARβ/δ or PPARγ reduced the percentage of cells in the G1 and S phase of the cell cycle, and increased the percentage of cells in the G2/M phase of the cell cycle in UACC903 cell lines as compared with controls. Ligand activation of PPARβ/δ did not further alter the distribution of cells within each phase of the cell cycle. By contrast, ligand activation of PPARγ enhanced these changes in stable UACC903 cells overexpressing PPARγ compared with controls. Stable overexpression of either PPARβ/δ or PPARγ and/or ligand activation of either PPARβ/δ or PPARγ inhibited cell proliferation, and anchorage-dependent clonogenicity of UACC903 cell lines as compared with controls. Further, overexpression of either PPARβ/δ or PPARγ and/or ligand activation of either PPARβ/δ or PPARγ inhibited ectopic xenograft tumorigenicity derived from UACC903 melanoma cells as compared with controls, and this was likely due in part to induction of apoptosis. Results from these studies demonstrate the antitumorigenic effects of both PPARβ/δ and PPARγ and suggest that targeting these receptors may be useful for primary or secondary melanoma chemoprevention.

Inhibition of tumorigenesis by peroxisome proliferator-activated receptor (PPAR)-dependent cell cycle blocks in human skin carcinoma cells

To examine the functional role of peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) and PPARγ in skin cancer, stable cell lines were created in the A431 human squamous cell carcinoma cell line. Expression of PPAR target genes was greatly enhanced in response to ligand activation of PPARβ/δ or PPARγ in A431 cells expressing these receptors. PPARβ/δ expression blocked the cell cycle at the G2/M phase, and this effect was increased by ligand activation. Ligand activation of PPARβ/δ markedly inhibited clonogenicity as compared to vehicle-treated controls. Similarly, ligand activation of PPARγ in A431 cells expressing PPARγ resulted in reduced clonogenicity. Expression of either PPARβ/δ or PPARγ markedly reduced tumor volume in ectopic xenografts, while ligand activation of these receptors had little further influence on tumor volume. Collectively, these studies demonstrate that stable expression and activation of PPARβ/δ or PPARγ in A431 cells led to reduced tumorigenicity. Importantly, PPAR expression or ligand activation had major impacts on clonogenicity and/or tumor volume. Thus, PPARβ/δ or PPARγ could be therapeutically targeted for the treatment of squamous cell carcinomas.

Peroxisome proliferator-activated receptor-β/δ modulates mast cell phenotype

The peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) is known to have multiple anti-inflammatory effects, typically observed in endothelial cells, macrophages, T cells and B cells. Despite the fact that mast cells are important mediators of inflammation, to date, the role of PPARβ/δ in mast cells has not been examined. Hence, the present study examined the hypothesis that PPARβ/δ modulates mast cell phenotype. Bone-marrow-derived mast cells (BMMCs) and peritoneal mast cells from Pparβ/δ^+/+ mice expressed higher levels of high-affinity IgE receptor (FcεRI) compared with Pparβ/δ^-/- mice. BMMCs from Pparβ/δ^+/+ mice also exhibited dense granules, associated with higher expression of enzymes and proteases compared with Pparβ/δ^-/- mice. Resting BMMCs from Pparβ/δ^+/+ mice secreted lower levels of inflammatory cytokines, associated with the altered activation of phospholipase Cγ1 and extracellular signal-regulated kinases compared with Pparβ/δ^-/- mice. Moreover, the production of cytokines by mast cells induced by various stimuli was highly dependent on PPARβ/δ expression. This study demonstrates that PPARβ/δ is an important regulator of mast cell phenotype.

Peroxisome proliferator-activated receptor-β/δ inhibits human neuroblastoma cell tumorigenesis by inducing p53- and SOX2-mediated cell differentiation

Neuroblastoma is a common childhood cancer typically treated by inducing differentiation with retinoic acid (RA). Peroxisome proliferator-activated receptor-β/δ, (PPARβ/δ) is known to promote terminal differentiation of many cell types. In the present study, PPARβ/δ was over-expressed in three human neuroblastoma cell lines, NGP, SK-N-BE(2), and IMR-32, that exhibit high, medium, and low sensitivity, respectively, to retinoic acid-induced differentiation to determine if PPARβ/δ and retinoic acid receptors (RARs) could be jointly targeted to increase the efficacy of treatment. All-trans-RA (atRA) decreased expression of SRY (sex determining region Y)-box 2 (SOX2), a stem cell regulator and marker of de-differentiation, in NGP and SK-N-BE(2) cells with inactive or mutant tumor suppressor p53, respectively. However, atRA did not suppress SOX2 expression in IMR-32 cells carrying wild-type p53. Over-expression and/or ligand activation of PPARβ/δ reduced the average volume and weight of ectopic tumor xenografts from NGP, SK-N-BE(2), or IMR-32 cells compared to controls. Compared with that found with atRA, PPARβ/δ suppressed SOX2 expression in NGP and SK-N-BE(2) cells and ectopic xenografts, and was also effective in suppressing SOX2 expression in IMR-32 cells that exhibit higher p53 expression compared to the former cell lines. Combined, these observations demonstrate that activating or over-expressing PPARβ/δ induces cell differentiation through p53- and SOX2-dependent signaling pathways in neuroblastoma cells and tumors. This suggests that combinatorial activation of both RARα and PPARβ/δ may be suitable as an alternative therapeutic approach for RA-resistant neuroblastoma patients.

Regulation of Cytochrome P450 2B10 (CYP2B10) Expression in Liver by Peroxisome Proliferator-activated Receptor-β/δ Modulation of SP1 Promoter Occupancy

Alcoholic liver disease is a pathological condition caused by overconsumption of alcohol. Because of the high morbidity and mortality associated with this disease, there remains a need to elucidate the molecular mechanisms underlying its etiology and to develop new treatments. Because peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) modulates ethanol-induced hepatic effects, the present study examined alterations in gene expression that may contribute to this disease. Chronic ethanol treatment causes increased hepatic CYP2B10 expression inPparβ/δ^+/+ mice but not in Pparβ/δ^-/- mice. Nuclear and cytosolic localization of the constitutive androstane receptor (CAR), a transcription factor known to regulate Cyp2b10 expression, was not different between genotypes. PPARγ co-activator 1α, a co-activator of both CAR and PPARβ/δ, was up-regulated in Pparβ/δ^+/+ liver following ethanol exposure, but not in Pparβ/δ^-/- liver. Functional mapping of the Cyp2b10 promoter and ChIP assays revealed that PPARβ/δ-dependent modulation of SP1 promoter occupancy up-regulated Cyp2b10 expression in response to ethanol. These results suggest that PPARβ/δ regulates Cyp2b10 expression indirectly by modulating SP1 and PPARγ co-activator 1α expression and/or activity independent of CAR activity. Ligand activation of PPARβ/δ attenuates ethanol-induced Cyp2b10 expression in Pparβ/δ^+/+ liver but not in Pparβ/δ^-/- liver. Strikingly, Cyp2b10 suppression by ligand activation of PPARβ/δ following ethanol treatment occurred in hepatocytes and was mediated by paracrine signaling from Kupffer cells. Combined, results from the present study demonstrate a novel regulatory role of PPARβ/δ in modulating CYP2B10 that may contribute to the etiology of alcoholic liver disease.