Genetic analysis of peroxisome proliferator-activated receptor γ1 splice variants in human colorectal cell lines

Abstract

Abstract. Peroxisome proliferator-activated receptor Á (PPARÁ) is a member of the nuclear hormone receptor family. In colon, this transcription factor is involved in differentiation of absorptive cells. PPARÁ participates also in colon carcinogenesis and cancer progression. Two isoforms, namely PPARÁ1 and PPARÁ2, have been described. Recently, new PPARÁ1 transcripts whose translation raises PPARÁ1 protein have been characterised. They differ from each other by combination of untranslated exons localised in the 5' UTR of the PPARG gene. Here, we studied whether such a diversity of PPARÁ transcripts occurs in human colon cell models. Based on bioinformatic analysis, putative untranslated exons were identified in the human PPARG gene. By RT-PCR analysis, we have demonstrated that several of these untranslated exons are included in PPARÁ transcripts from colon-derived cell lines or in those derived from other tissue. Using HT-29 cells, changes in PPARÁ1 mRNA levels were observed after treatment with PPARÁ agonists such as pioglitazone and troglitazone. These modifications correlated with particular PPARÁ transcripts excluding the untranslated exon A2. HT-29 cells treatment with actinomycin D or cycloheximide showed that the presence of PPARÁ mRNA including exon A2 was dependent on de novo protein synthesis. We concluded that diverse PPARÁ1 mRNA exist in colorectal cells. Levels of PPARÁ1 transcript varied according to the phenotype of colon cell model used. We suggest that regulation of PPARÁ1 mRNA levels could be dependent in part on the composition of untranslated exon(s) in the 5' UTR of PPARÁ1 mRNA. IntroductionPeroxisome proliferator activated receptors (PPAR) belong to the nuclear hormone receptor superfamily (1). There are three isotypes named PPAR·, PPARß/‰ and PPARÁ (2). These nuclear receptors act as heterodimers with the 9-cis retinoic acid receptor and bind to a specific DNA sequence, the peroxisome proliferator response element, a direct repeat of the 5'-AGGTCA-3' sequence spaced by one base (3,4). After ligand activation, the heterodimer complex regulates the transcription of specific genes involved in several physiological functions (2). In colon, PPARÁ is implicated in differentiation of absorptive cells (5), in carcinogenesis or tumour progression even though contradictory results have been obtained in murine and human models (6).The PPARÁ gene is localised to chromosome 3, band 3p25 (7). The open reading frame consists of exons E1-E6 which encode the different domains of the protein. In human, two PPARÁ isoforms have been identified and referred to as PPARÁ1 and PPARÁ2 (7,8). PPARÁ2 differs from PPARÁ1 by an additional 28 amino acid NH2-terminal sequence (8). The 5' UTR region of PPARÁ transcripts, raising PPARÁ1 and -2 proteins, is highly variable and three exons termed A1, A2 and B have been described (9,10). Due to different promoter usage and alternative splicing, two other PPARÁ transcripts have been identified and they have been referred to as PPARÁ3 and -Á4 (10,11). PPARÁ1 mRNA consists of A...

Effects of dietary glutamine supplementation on growth performance, antioxidant status and intestinal function in juvenile grass carp (Ctenopharyngodon idella)

“…These studies indicate that glutamine has different regulatory effects on PPARγ levels in different intestinal cell types. In our study, the expression levels of PPAR‐γ decreased significantly after the fish were fed different concentrations of Gln, which was in line with the results of the previous study on human colorectal Caco‐2 cells (Fiatte et al, ). The molecular mechanism of the inhibitory effect of Gln on PPAR‐γ expression in fish needs further study.…”

“…Recent studies have shown that glutamine can enhance the transcriptional activity of PPAR‐γ in a dose‐ and time‐dependent manner in the intestinal epithelial cell line IEC‐6 by stimulating the PPAR‐γ ligands 15‐HETE and 13‐OXO‐ODE (Ban, Sprunt, Martin, Yang, & Kozar, ). However, in human colorectal Caco‐2 cells, Fiatte et al, () reported that PPAR‐γ expression levels were decreased by medium supplementation with l ‐glutamine. These studies indicate that glutamine has different regulatory effects on PPARγ levels in different intestinal cell types.…”

HNF4α Combinatorial Isoform Heterodimers Activate Distinct Gene Targets that Differ from Their Corresponding Homodimers

“…Like HNF4a, many members of the nuclear receptor superfamily also express multiple variants. Peroxisome proliferator-activated receptor (PPAR) is encoded by 3 different gene loci (PPARA, PPARD, and PPARG), each generating multiple alternatively spliced isoforms to give rise to further 2, 3, and 2 different polypeptides, respectively, that differ at either N or C termini (Chen et al, 2006;Fiatte et al, 2006). Truncation mutants such as PPARd2 (Lundell et al, 2007) and PPARgD5 have been found to act as dominant-negative forms of the nuclear receptor, where PPARgD5 plays an important role in negative feedback to alter Peroxisome proliferator-activated receptor gamma (PPARg)dependent transcription activity and adipocyte differentiation in obesity (Aprile et al, 2018).…”

A Role for the PPARγ in Cancer Therapy

“…Rare mutations have been reported and more recently dominant negative variants of the receptor were identified although the biological impact remains to be established firmly [179]. Similarly, altered isoforms may be overexpressed in cancer [180–183]. Cytogenetic rearrangement has been identified in follicular thyroid cancer fusing the PAX-8 transcription factor to PPAR γ .…”