AMP-activated protein kinase-regulated activation of the PGC-1alpha promoter in skeletal muscle cells

Abstract

The mechanisms by which PGC-1alpha gene expression is controlled in skeletal muscle remains largely undefined. Thus, we sought to investigate the transcriptional regulation of PGC-1alpha using AICAR, an activator of AMPK, that is known to increase PGC-1alpha expression. A 2.2 kb fragment of the human PGC-1alpha promoter was cloned and sequence analysis revealed that this TATA-less sequence houses putative consensus sites including a GC-box, a CRE, several IRSs, a SRE, binding sites for GATA, MEF2, p 53, NF-kappaB, and EBox binding proteins. AMPK activation for 24 hours increased PGC-1alpha promoter activity with concomitant increases in mRNA expression. The effect of AICAR on transcriptional activation was mediated by an overlapping GATA/EBox binding site at -495 within the PGC-1alpha promoter based on gel shift analyses that revealed increases in GATA/EBox DNA binding. Mutation of the EBox within the GATA/EBox binding site in the promoter reduced basal promoter activity and completely abolished the AICAR effect. Supershift analyses identified USF-1 as a DNA binding transcription factor potentially involved in regulating PGC-1alpha promoter activity, which was confirmed in vivo by ChIP. Overexpression of either GATA-4 or USF-1 alone increased the p851 PGC-1alpha promoter activity by 1.7- and 2.0-fold respectively, while co-expression of GATA-4 and USF-1 led to an additive increase in PGC-1alpha promoter activity. The USF-1-mediated increase in PGC-1alpha promoter activation led to similar increases at the mRNA level. Our data identify a novel AMPK-mediated regulatory pathway that regulates PGC-1alpha gene expression. This could represent a potential therapeutic target to control PGC-1alpha expression in skeletal muscle.

Figure 1. The human PGC-1α promoter.

The nucleotide sequence +28 to −2190 corresponding to the proximal 2-kb hPGC-1α promoter is shown. The arrows indicate the transcription start sites, which have been previously described . Putative binding sites for transcription factors are either underlined or overlined. Also included are binding sites for transcription factors that have previously been characterized [12;13;19]. Numbers enclosed in circles represent the 5′- deletions of the PGC-1α promoter reporter constructs p 2215, p 1164, p 851, p501, p 191 as shown in Fig. 2.

Figure 2. AMPK activation induces PGC-1α mRNA expression and transcriptionally activates the PGC-1α promoter.

C₂C₁₂ cells were treated with either AICAR (1 mM) or Vehicle for 24 hrs. A. Representative Western Blot probed with a Phospho-AMPKα (Thr172), stripped and then re-probed with total AMPKα for loading control (upper panel). Summary of repeated experiments of the effect of AICAR on AMPK activation is shown (lower panel; n = 4). B. upper panel, EtBr-stained DNA gel of PGC-1α amplified by PCR from vehicle- and AICAR-treated cells. GAPDH was also amplified by PCR and used to verify equal loading. Lower panel: A summary of repeated experiments of the effects of AICAR on PGC-1α mRNA expression (n = 3). C. AICAR-induced transcriptional regulation of the PGC-1α promoter. Relative luciferase activity of the PGC-1α promoter constructs in vehicle- or AICAR-treated cells is shown (n = 4–6). D. The AICAR-responsive region (ARR) from −473 to −821 was cloned into the pGL4.23 minimal promoter vector and AICAR-induced transcriptional regulation of this region was assessed (n = 3). For all data, values are means±S.E.M, *, p<0.05 versus Vehicle-treated control; §, p<005 versus pGL4.23.

Figure 3. AMPK activation increases GATA/EBox DNA binding.

A. Representative EMSAs of nuclear extracts from Vehicle− (−AIC) and AICAR− (+AIC) treated cells that were incubated with radiolabeled oligonucleotides corresponding to the EBox, SRE and IRS sequences found within the AICAR-responsive region of the PGC-1α promoter (as shown in Fig. 1). B. A representative EMSA (Left panel) and a summary of multiple experiments (Right panel) showing the effect of AICAR (A) on GATA/EBox-DNA binding. Values are represented as means±S.E.M (n = 8) relative to vehicle-treated (V) cells. C. Representative EMSAs of nuclear extracts that were incubated with radiolabeled oligonucleotides corresponding to GATA/EBox wt. Vehicle-treated cells were incubated with radiolabeled oligonucleotides corresponding to GATA/EBox wt as well as antibodies against MyoD, USF-1 and c-Myc which are known to bind to the EBox sequence. FP: free probe, 25×, 50×, 100× CO: 25-fold, 50-fold, 100-fold molar excess of cold oligo, No Ab: No Antibody, SRF: SRF antibody, FKHR: Forkhead antibody, GATA-4: GATA-4 antibody, c-Myc: c-Myc Antibody, USF-1: USF-1 antibody. **The representative blot of IRS/DNA binding was made from parts of the same gel. D. Representative chromatin immunoprecipitation from cells treated with or without 1 mM AICAR for 24 hours. Protein/DNA complexes were immunoprecipitated with USF-1 antibody, or with non-specific IgG. Primers encompassing the region between −473 and −823 were used to analyze USF-1 binding to the PGC-1α promoter. The representative blot on the left was made from parts of the same gel. At right is a graphical summary of repeated experiments. Values are representative of means±S.E.M (n = 6).

Figure 4. Mutations to either GATA or EBox elements alters AICAR-induced increases in GATA/EBox DNA binding and affects PGC-1α promoter activity.

A. A schematic of the overlapping EBox/GATA (GATA/EBoxwt) binding sites at −495 and −486 base pairs within the PGC-1α promoter is shown. Mutations to the GATA/EBoxwt oligonucleotide that were introduced by nucleotide exchange to abolish GATA (GATAmt) or EBox (EBoxmt) binding are bolded. B. Representative EMSAs of Vehicle- (−) and AICAR− (+) treated nuclear extracts incubated with GATAmt or EBoxmt radiolabeled probes in the presence or absence of USF-1 Antibody. FP: free probe, 25× CO, and 50CO: 25-fold and 50-fold molar excess of cold oligonucleotide, No Ab: No Antibody, USF-1: USF-1 antibody. C. Relative luciferase activity of the PGC-1α promoter constructs in vehicle- or AICAR-treated cells is shown. Values are means±S.E.M, * p<0.05 versus Vehicle-treated control (n = 3). D. Western blot analysis of GATA-4 and USF-1 protein expression from total proteins extracted from vehicle− (−) and AICAR (+)-treated cells is shown. GAPDH expression was used as a loading control.

Figure 5. Effect of GATA-4 and USF-1 overexpression on PGC-1α promoter activity and mRNA expression.

A. Representative western blots of protein extracts made from C₂C₁₂ cells transfected with either 2 or 4 µg of GATA-4 or USF-1 or an empty vector (EV) control. B. USF-1 and GATA-4 were co-transfected with the pGL3 (EV; 500ng) or the p851 PGC-1α promoter reporter construct (500ng) along with the appropriate empty vector controls. Relative luciferase activities were measured 48 hours after transfection and are plotted as the fold change above empty vector. Values are means±SEM, (n = 8); * p<0.05 versus p851-EV and #, p<0.05 versus p851-USF-1 or p851-GATA-4. C. Cells were transfected with 4 µg of USF-1 or an empty vector (EV) control. EtBr-stained DNA gel of PGC-1α amplified by PCR from EV- and USF-1 transfected cells. s12rRNA was used to verify equal loading. Data are representative of one experiment with conditions repeated in duplicate (AU = arbitrary units).