Abstract
Activation of peroxisome proliferator-activated receptor gamma (PPARgamma) following exposure to PPARgamma-specific ligands resulted in growth inhibition in various carcinoma cell lines. Our aim was to elucidate the pathway of PPARgamma2 activation-mediated modulation of cyclin D1 transcription in mouse hepatocytes. To address this we utilized stable control and PPARgamma hepatocyte cell lines created via retroviral overexpression utilizing AML-12 hepatocytes. Addition of PPARgamma ligand troglitazone (TZD) activated PPARgamma2 in proliferating hepatocytes and resulted in growth arrest accompanied by a down-regulation of proliferating cell nuclear antigen, cyclin D1, and beta-catenin expression. Furthermore activation of PPARgamma2 attenuated cyclin D1 promoter activity indicating a transcriptional regulation of cyclin D1. Since beta-catenin plays a pivotal role in regulating cyclin D1 transcription, we studied whether PPARgamma2-mediated inhibition of cyclin D1 transcription involved beta-catenin. Interestingly overexpression of either wild-type or S37A mutant beta-catenin was unable to rescue PPARgamma2-mediated suppression of cyclin D1 transcription, whereas overexpression of cAMP-response element-binding protein (CREB) was capable of antagonizing this inhibitory effect of PPARgamma2. Additionally pretreatment with okadaic acid antagonized PPARgamma2-mediated inhibition of cyclin D1 transcription without any effect on beta-catenin expression. These studies also showed a TZD-mediated inhibition of total and phospho-CREB(Ser133) levels, CREB promoter activity, and cAMP-response element-mediated transcription in PPARgamma hepatocytes. Pretreatment of PPARgamma hepatocytes with okadaic acid, however, maintained higher total and phospho-CREB(Ser133) levels in the presence of TZD. These results indicated that PPARgamma2 activation inhibited cyclin D1 transcription in hepatocytes via CREB-dependent and beta-catenin-independent pathways.
Highlights
Peroxisome proliferator-activated receptors (PPARs)1 belong to the nuclear receptor superfamily and are involved in regulating various cellular processes
Luciferase assays designed with the peroxisome proliferator-activated receptor ␥ (PPAR␥)responsive reporter tk-PPREx3-Luc [38] showed a low proliferator-activated receptor response elements (PPREs) luciferase activity in the control cells, irrespective of the presence of TZD, that was highly activated in the PPAR␥ cells following exposure to TZD (Fig. 1B)
Ectopic Expression and Activation of PPAR␥2 in Proliferating Hepatocytes Arrests Growth and Inhibits Expression of proliferating cell nuclear antigen (PCNA)—To determine whether the ectopic expression and activation of PPAR␥2 in AML-12 hepatocytes can suppress growth, we analyzed the cell proliferation rate in the two cell types. These results showed that activation of PPAR␥2 following addition of TZD resulted in complete inhibition of growth in the PPAR␥ cells (Fig. 2A) and was associated with an overall decrease in PCNA expression (Fig. 2B)
Summary
Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear receptor superfamily and are involved in regulating various cellular processes. PPAR-mediated transactivation of target genes involves dimerization of PPARs with the retinoid X receptor and binding of the resulting heterodimer to specific peroxisome proliferator-activated receptor response elements (PPREs) located within the target gene promoters/enhancers [5]. In other studies expression of PPAR␥ and its downstream target genes were increased following a homozygous deletion of cyclin D1 gene [35] These cyclin D1Ϫ/Ϫ mice developed hepatic steatosis with increased neutral lipid accumulation in the liver indicative of increased PPAR␥ activity. These results clearly indicate a distinct involvement of PPAR␥ in regulating hepatocyte physiology, the mechanism and effectors involved are largely unclear
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
