Abstract
SIRT1 and orthologous sirtuins regulate a universal mechanism of ageing and thus determine lifespan across taxa; however, the precise mechanism remains vexingly polemical. They also protect against many metabolic and ageing-related diseases by dynamically integrating several processes including autophagy, proteostasis, calorie restriction, circadian rhythmicity and metabolism. These sirtuins are therefore important drug targets particularly because they also transduce allosteric signals from sirtuin-activating compounds such as resveratrol into increased healthspan in evolutionarily diverse organisms. While many of these functions are apparently regulated by deacetylation, that mechanism may not be all-encompassing. Since gonadal signals have been shown to regulate ageing/lifespan in worms and flies, the present study hypothesized that these sirtuins may act as intermediary factors for steroid hormone signal transduction. Accordingly, SIRT1 and its orthologues, Sir2 and Sir-2.1, are shown to be veritable nuclear receptor coregulators that classically coactivate the oestrogen receptor in the absence of ligand; coactivation was further increased by 17β-oestradiol. Remarkably in response to the worm steroid hormone dafachronic acid, SIRT1 reciprocally coactivates DAF-12, the steroid receptor that regulates nematode lifespan. These results suggest that steroid hormones may co-opt and modulate a phyletically conserved mechanism of sirtuin signalling through steroid receptors. Hence, it is interesting to speculate that certain sirtuin functions including prolongevity and metabolic regulation may be mechanistically linked to this endocrine signalling pathway; this may also have implications for understanding the determinative role of gonadal steroids such as oestradiol in human ageing. At its simplest, this report shows evidence for a hitherto unknown deacetylation-independent mechanism of sirtuin signalling.
Highlights
Biological ageing is a highly conserved process in all organisms and is regulated by endocrine signalling [1,2]
silent information regulator homologue 1 (SIRT1) (Wt) and SIRT1 H363Y cDNAs were subcloned into the BamHI-XbaI sites and in-frame with the Gal4 DNA-binding domain (Gal4DBD) vectors pFA-CMV (Agilent Technologies, U.K.) or pM (Clontech, France) to generate pFABD-SIRT1 and pM-SIRT1 H363Y respectively. pM-silent information regulator 2 (Sir2) was constructed by transferring the insert from p413TEF-Sir2 into the EcoR1-SalI sites of pM. pFABD-Sir2.1 was constructed by shuttling Sir-2.1 from pET28-Sir-2.1 with PmlI-HindIII into the SmaI-HindIII sites of pFA-CMV; this construct lacks the first two amino acids of Sir-2.1. pFABD-SIRT1 NRB, lacking the nuclear receptor box (NR-box), was generated by site-directed mutagenesis using the QuikChange Site-Directed Mutagenesis kit (Agilent Technologies)
In a back-to-basics approach, the N-terminal sirtuin-activating compounds (STACs)-AD of SIRT1 was tested for activation function based on the fact that that domain binds to different effector proteins involved in gene regulation
Summary
Biological ageing is a highly conserved process in all organisms and is regulated by endocrine signalling [1,2] In humans, it is the single most important risk factor for ill-health and frailty because it is invariably accompanied by complex chronic comorbidities such as cancer and heart disease [3]. Its ability to translate the health benefits of STACs [8] further qualifies SIRT1 as an ideal polypill target to treat multiple diseases combinatorially [9]. This has been stymied by a limited understanding of its complex biology as a hub protein [10]
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