Date Published: June 11, 2012
Publisher: Impact Journals LLC
Author(s): Giulia Bolasco, Raffaele Calogero, Matteo Carrara, Mumna Al Banchaabouchi, Daniel Bilbao, Gianluigi Mazzoccoli, Manlio Vinciguerra.
Locally acting insulin growth factor isoform (mIGF-1) and the NAD+-dependent protein deacetylase SIRT1 are implicated in life and health span. Heart failure is associated with aging and is a major cause of death. mIGF-1 protects the heart from oxidative stresses via SIRT1. SIRT1 subcellular localization and its genomic regulation by mIGF-1 are unknown. We show here that SIRT1 is located in the nuclei of a significant fraction of cardiomyocytes. Using high throughput sequencing approaches in mIGF-1 transgenic mice, we identified new targets of the mIGF-1/SIRT1 signaling. In addition to its potent cardioprotective properties, cardiac-restricted mIGF-1 transgene induced systemic changes such as high blood pressure, leukocytosis and an enhanced fear response, in a SIRT1-dependent manner. Cardiac mIGF-1/SIRT1 signaling may thus modulate disparate systemic functions.
Cardiovascular diseases increase during aging and are first cause of mortality, representing one third of all global deaths and a major burden for the health systems. Insulin like growth factor-1 (IGF-1) and Sirtuin-1 (SIRT1) are key mediators of cell homeostasis and of cardiac stress; moreover they have been implicated in biological aging processes [1-6]. Mammals display a complex IGF-I signaling system with multiple alternative spliced isoforms that have distinct effects on cardiovascular function [1, 7, 8]. These splicing variants share a common core peptide, flanked by varying termini (Class 1 and 2 N-terminal peptides, and E peptides). IGF-I can operate both as a systemic growth factor produced by the liver in response to growth hormone and as a local growth factor acting in an autocrine/paracrine manner in organs such as the heart. The locally acting mIGF-I isoform comprises Class 1 N-terminal and Ea C-terminal peptides [1, 7, 8]. Mouse genetics studies have shown that enhancement of the mIGF-I signaling pathway is highly effective in promoting skeletal muscle regeneration, cell survival and renewal . mIGF-I transgenic overexpression throughout postnatal life does not result in perturbation of cardiac physiology and is able to recover heart function after harsh injuries that trigger heart failure , which is mediated by the inflammatory response. mIGF-I enhances antioxidative cell defenses by up-regulating genes that display anti-oxidant and anti-apoptotic properties . Also, mIGF-I repairs the heart from injury through production of specific cytokines that cross-talk with the bone marrow and recruit endothelial-primed cells for de novo vascularization of the myocardial tissue, indicating that cardiomyocyte specific overexpression of this transgene can have profound systemic effects .
Recently, using Angiotensin II and paraquat as oxidative stressors, we identified an important signaling pathway that protects cardiomyocytes and relies on the activation of SIRT1 by the locally acting mIGF-1 isoform [5, 6]. Cardiac-specific mIGF-1 Tg mice in which SIRT1 was depleted from adult cardiomyocytes confirmed that this pathway is necessary to protect the heart from paraquat-induced oxidative stress and lethality . SIRT1 deacetylates both cytoplasmic and nuclear proteins, in turn regulating several transcrip-tional factors : to understand the mechanisms of mIGF-1/SIRT1 dependent cardio-protection, it was important to assess where SIRT1 is located within the cardiomyocytes. In contrast with recent reports [21, 22], where high resolution microscopy and specific markers were not used, we have found here that SIRT1 is located in the nucleus of a fraction of cardiomyocytes, whereas it is located in the cytoplasm of non-cardiomyocyte cell types. In order to understand how SIRT1 might regulate gene expression, ChIP-Seq offered us a powerful tool to determine how SIRT1 interacts with cardiomyocyte DNA. To our knowledge this is the first SIRT1 genome-wide DNA binding data set reported, after the one obtained by Oberdoerferr et al. in embryonic stem (ES) cells with a ChIP-on-ChIP approach . In accordance with this previous report, the majority of reads for SIRT1 bound DNA accounted for repetitive regions of the genome; this pattern was not any different in WT or in mIGF-1 Tg hearts (data not shown). Oberdoerferr et al. reported a few hundreds of genes bound by SIRT1 in the promoter region in ES cells under basal conditions, and in conditions of oxidative stress (H2O2 treatment) there was a massive displacement of SIRT1 that goes to occupy other promoters . In our system, SIRT1 was found similarly bound to 302 gene promoters, but there were only few dozens of promoters exclusively bound by SIRT1 either in the WT heart or in the mIGF-1 Tg heart (Fig. 2). This distribution indicates that the mIGF-1 transgene induces only subtle alterations in SIRT1 DNA binding patterns. Subtle variations were found also in the microarrays, with only 22 genes showing a significant change in mRNA levels in the mIGF-1 Tg background. Most interestingly these variations in the mIGF-1/SIRT1 genomic and transcriptomic effects in unchallenged mouse hearts were related to genes regulating functions beyond cardiomyocyte specific homeostatic and protective mechanisms. For instance we found genes implicated in the immune response, the blood pressure control, inflammation and behavior. We thus suspected that the cardioprotective mIGF-1/SIRT1 pathway might elicit a global impact on body functions.