EDITORIAL
Cardiovascular Research (2012) 94, 6–7 doi:10.1093/cvr/cvs101
Regulating a uniter: control of mitofusin 2 expression Anne A. Knowlton 1,2,3* and Le Chen 1 1 Molecular and Cellular Cardiology, Department of Medicine, University of California, Davis, CA, USA; 2Department of Pharmacology, University of California, Davis, CA, USA; and 3Northern California, VA, Sacramento, CA, USA
Online publish-ahead-of-print 23 February 2012
This editorial refers to ‘The promoter activity of human Mfn2 depends on Sp1 in vascular smooth muscle cells’ by E. Sorianello et al., pp. 38 –47, this issue.
Sorianello et al. 1 provide important new information on the regulation on mitofusin (Mfn)2 expression. Mitochondrial fission and fusion were first described in yeast in the 1990s and found to be processes that are essential to maintain healthy mitochondria. Key findings from this work include that mitochondria continuously divide and fuse (fission and fusion), that fission and fusion are essential to maintain mitochondrial health (although the exact reason for this is a subject of debate), and that mitochondria exist as interconnected networks.2 Fission and fusion occur as frequently as every 2 min in yeast, but are thought to occur much more slowly in mammalian cells, taking hours or longer.3,4 There initially had been scepticism that the mitochondria in the highly organized and densely packed cardiac myocyte would be able to undergo fission and fusion, but it is becoming clear that fission and fusion are important processes in all cells.5,6 It is increasingly evident that abnormal fission and fusion contribute to cardiovascular disease.7 OPA1 has been found to be decreased in ischaemic cardiomyopathy, while Mfn1/2 were increased in both ischaemic and non-ischaemic heart failure.8 Deletion of Mfn 2 alone resulted in mild cardiac hypertrophy with very minor changes in function.9 However, the absence of Mfn2 delayed the mitochondrial permeability transition, protecting cardiac myocytes from injury and improving the recovery of the heart after ischaemia/reperfusion injury.9 Interestingly, these hearts had increased expression of Bcl2. In contrast, knockout of Mfn1 and Mfn2 in cardiac myocytes was lethal in embryos, while induction of knockdown of these two proteins in the adult mouse heart resulted in mitochondrial fragmentation, depressed mitochondrial respiration, and a dilated cardiomyopathy, which was fatal.6 Inhibition of mitochondrial fission protected the heart against ischaemia/reperfusion injury.10 Fission and fusion proteins also have a role in apoptosis.11 Thus, these proteins have functions in the cell beyond mitochondrial fission and fusion.
Mfn2 was previously known also as hyperplasia suppressor gene and has been shown to prevent vascular smooth muscle cell (VSMC) proliferation both in culture and following balloon injury in the rat carotid.12 Overexpression of Mfn2 leads to increased expression of p21 and p27 and cell cycle arrest, inhibiting VSMC proliferation without an increase in apoptosis.12 Similarly, overexpression of Mfn2 blocked LDL-induced VSMC proliferation and decreased atherogenesis in a rabbit model.13 Mfn2 expression is decreased in the skeletal muscle in both diabetes and obesity, and this can be at least partially reversed by exercise.14,15 Thus, there are a number of findings that suggest an important link between Mfn2 and vascular disease. In addition, work on Mfn2 supports a role for Mfn2 in metabolic control, a position that has been championed by the Zorzano group.16 Although recent work has begun to investigate fusion and fission in mammalian mitochondria, there has been little work on the regulation of the expression of the involved genes. Mitochondrial fission and fusion have become a growing focus of cardiovascular research. Sorianello et al. 1 demonstrate that there are common predicted elements in the human, rat, and mouse Mfn2 promoters, including the absence of a TATA box along with binding elements for NFkB, ERRa, C/EBP, and GATA-1. In addition, the authors identify six Sp1 sites in all three promoters. A high level of CpG islands is present in the promoters, and, as is typical for mammalian cells, these are not methylated, leading to greater activity. Neither actively proliferating cells nor quiescent cells had evidence of methylation of the CpG islands, indicating regulation of activity by other means. Six transcription start sites are present, and a core region for promoter activity was identified between 2229 and 254. The multiple Sp1 sites are consistent with the theory that Sp1 sites direct basal activity in promoters with CpG islands, and in fact, the authors show that progressive deletion of the Sp1 sites reduces and finally eliminates basal activity of the promoter. Chromatin immunoprecipitation assays were used to confirm binding of Sp1 to the Mfn2 promoter in VSMCs and L6E9 cells. The importance of Sp1 to Mfn2 activity is further confirmed by the inhibition of Mfn2 promoter activity in L6E9 cells by WP631, an inhibitor of Sp1. Knockdown of Sp1 in VSMCs by shRNA also
The opinions expressed in this article are not necessarily those of the Editors of Cardiovascular Research or of the European Society of Cardiology.
* Corresponding author: Molecular and Cellular Cardiology, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA. Tel: +1 530 752 5461; fax +1 530 754 7167, Email:
[email protected] Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2012. For permissions please email:
[email protected].
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Editorial
greatly reduced Mfn2 promoter activity, consistent with other findings. Most interesting is one final set of experiments investigating the role of Mfn2 in the pre-atherosclerotic artery. The investigators placed ApoE-knockout mice on a high-fat diet and found a 50% decrease in Sp1 mRNA levels and a 60% decrease in Mfn2 mRNA levels in the aorta after 1 week. Interestingly, these effects were transient and disappeared with prolonged treatment. However, these findings offer tantalizing new insights into early atherosclerosis, linking the intake of a high-fat diet to a decrease in Mfn2 expression and VSMC proliferation. These findings have significant implications for cardiovascular disease. There is increasing evidence of a link between decreased Mfn2 levels and vascular disease. Both diabetes and obesity were associated with decreased Mfn2. Sorianello et al. found that exposure to a high-fat diet reduced Mfn2, but that this decrease disappeared after prolonged exposure to this diet; however, several investigators have shown that diabetics and obese subjects have decreased Mfn2 in their skeletal muscle.14,15 It may be that humans have repetitive exposure to high-fat meals, rather than sustained exposure, and this may prevent some type of adaptive response. One study showed improved Mfn2 levels with exercise, so there are behavioural approaches to ameliorating the drop in Mfn2 in diabetics and the obese.15 The current study, with its detailed analysis of the Mfn2 promoter and its regulation, provides key information for the eventual development of pharmacological therapies to increase expression of Mfn2, a protein with important vascular effects, and potentially slow the progression of atherosclerosis in diabetics and the obese. Conflict of interest: none declared.
Funding Supported by the National Institutes of Health (grant numbers HL077281 and HL079071 both to A.A.K.) and a VA Merit Award (A.A.K.)
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