Influence of Estrogen Receptors ERalpha and ERbeta on the expression of Adipose Triglycerid Lipase (ATGL)
Aims: Estrogen Receptors (ERs) are known to play an important role in the metabolic functions of adipose tissue (AT). Gender specific differences concerning lipolysis have been reported. For instance, female mice are known to mobilize energy from fat more efficiently than male littermates under exercise conditions. Also, changes in fat distribution and metabolic activity of adipose tissue during menopause are associated with an increasing risk for metabolic and cardiovascular complications in humans. The characterization of molecular mechanisms of estrogen-action in AT might help understand and treat metabolic disorders associated with adiposity, as T2D and others, in the future.
This study investigates the impact of the nuclear ERs (alpha and beta) on the expression of the Adipose Triglycerid Lipase (ATGL), the rate-limiting lipolytic enzyme catalyzing the first step in the process of triglyceride-degradation.
Methods and Results: The regulatory impact of ERalpha and ERbeta on the expression of ATGL was investigated in 3T3L1-preadipocytes on promoter- and mRNA-level. Cells were transiently transfected with ERalpha/ERbeta or PSG5 and stimulated with vehicle vs. selective ERalpha/ERbeta agonists PPT/DPN (c = 100nM) for 24h.
Promoter-activity of the sequence 3000bp upstream of the first exon of the ATGL-gene was assessed by performing luciferase assay. Transcription increased both ligand-independently under the overexpression of ERalpha/ERbeta (1.6-/2.5-fold, p < 0.05 vs. PSG5-control), as well as under receptor-overexpression and stimulation with the respective ER-agonist (2.7-/3.3-fold, p < 0.05 vs. vehicle).
In accordance, bioinformatical promoter analysis performed on the murine sequence revealed 7 putative ERbeta- and only 1 putative ERalpha-binding site.
qRT-PCR-analysis demonstrated a significant increase in the amount of ATGL-mRNA by factor 1.3 under vehicle- and factor 1.5 under PPT-stimulation for cells overexpressing ERalpha vs. PSG5-control (both p < 0.05). Similar experiments for the overexpression of ERbeta showed a significant increase in ATGL-mRNA-amount by factor 3.5 in DPN-stimulated cells vs. PSG5-control (p < 0.05). Additionally, in cells overexpressing ERbeta a significant increase by factor 2.3 could be observed between vehicle- and DPN-stimulation (p < 0.05). Most likely attributable to endogenous ERbeta, a significant 2-fold increase of ATGL-mRNA could be observed under DPN-stimulation in PSG5-control-cells (p < 0.05).
Conclusions: The present study demonstrates that both ERs exert positive regulatory actions on the expression of ATGL. The impact on ATGL promoter-activity and mRNA-expression of ERbeta appears to be stronger, when compared to ERalpha. Further experiments will be required to determine the importance of these findings for an ER isoform-specific regulation of lipolysis.