Second-hand smoke stimulates lipid accumulation in the liver by modulating AMPK and SREBP-1☆
Introduction
Cigarette smoke contains more than 47,000 toxic substances which significantly harm almost every organ of the body, leading to a variety of diseases and syndromes [1]. Cigarette smoke is composed of MSW (mainstream whole, “first-hand”) and SSW (side stream whole, major component of “second-hand” smoke) smokes. Smoking has been identified as one of the major risk factors for the development of atherosclerosis, the major component of cardiovascular disease [2], [3], [4] that manifests itself, among other things, by high lipid levels in the blood [5], [6]. Furthermore, increasing evidence suggests that risk for cardiovascular disease incidence is associated with non-alcoholic fatty liver diseases (NAFLD) independently of the classical risk factors and features of this metabolic syndrome [7], [8], [9], [10]. In the various stages of NAFLD, hepatic steatosis (the accumulation of lipid in the liver tissue) has become a significant public health concern because it tends to develop into more harmful hepatitis and cirrhosis. Because lipids in steatosis are stored as triglycerides in hepatocytes, understanding what causes this accumulation and how it occurs may contribute to elucidation of NAFLD [11].
Sterol regulatory element-binding proteins (SREBPs) are a family of transcription factors that control the expression of genes required for the biosynthesis of cholesterol, fatty acids, triglycerides, and phospholipids. The three isoforms of SREBP precursors located on the endoplasmic reticulum membrane, designated SREBP-1a, SREBP-1c, and SREBP-2 [12], have different functions and abundance in various animal tissues. The SREBP precursors are activated by a two-step cleavage process that releases the active form that then translocates to the nucleus of the cell to stimulate gene expression [13]. SREBP-1c preferentially controls the expression of genes involved in triglyceride synthesis and accumulation, such as fatty acid synthase (FAS) and acetyl coenzyme-A carboxylase (ACC), whereas SREBP-2 activity has been more closely linked to regulation of genes involved in cholesterol synthesis and uptake, such as low-density lipoprotein receptor (LDLR) and 3-hydroxy-3-methylglutaryl CoA reductase (HMGCR) [14], [15], [16], [17], [18]. In the liver tissue, the predominant form of SREBPs is SREBP-1c [19].
Another important modulator of lipid metabolism is 5′-AMP-activated protein kinase (AMPK). AMPK was first identified as a kinase that phosphorylates and inactivates ACC, the rate-limiting enzyme in fatty acid biosynthesis [20]. AMP binds and activates AMPK primarily by causing conformational changes that allows Thr172 phosphorylation to occur by upstream kinases. Activation of AMPK in the liver, skeletal muscle, and adipose tissue improves the status of type 2 diabetes by preventing ATP depletion, increasing fatty acid oxidation, decreasing blood glucose, etc. It has also been found that AMPK activity is inhibited in alcohol-induced fatty liver disease [21].
Although both AMPK and SREBP are related to the metabolism of the cell, the relationship between the two is not clear. We hypothesize that components of tobacco smoke cause lipid accumulation in the liver tissue of mice exposed to “second-hand” smoke by modulating the activities of AMPK and that this enzyme is important in the activation of SREBP-1, the central modulator for triglyceride synthesis. The elucidation of the mechanisms of lipid accumulation in hepatocytes caused by cigarette smoke may help understand processes involved in atherogenesis and in initiation of NAFLD, and suggest possible ways of treating both metabolic diseases.
Section snippets
Smoke solution preparation
Sidestream whole (SSW) smoke solution was prepared from 1R3F research grade cigarettes (University of Kentucky, Louisville, KY). SSW smoke was bubbled into 10 mL serum free media for the duration of 30 puffs as previously described [22] using a puffer box built by the University of Kentucky. SSW smoke was collected from the burning end of the cigarette. The pH of the smoke solutions was adjusted to 7.4. The solution is aliquoted and kept at −20 °C (stable for up to 6 weeks).
Exposure of the animals to smoke
Six- to eight-week-old
SSW causes lipid accumulation in the liver
To test our hypothesis that SSW stimulates lipid synthesis and accumulation in hepatocytes, we used ApoB100 mice on a high-fat diet and exposed them to SSW as described in Section 2. These mice were chosen because under such feeding conditions they produce levels of lipid that lead to development of atherosclerosis [27], [28], [29], which in turn has been associated with NAFLD [30], [31], [32]. To detect the effects of SSW on lipid accumulation in the hepatocytes, mice were exposed to smoke for
Discussion
Cigarette smoke has long been recognized as one of the most preventable non-hereditary factors contributing to cardiovascular disease [2], [3], [4]. Evidence is mounting that there are relationships between cardiovascular disease and metabolic diseases such as NAFLD and diabetes [7], [8], [9], [10]. These diseases all exhibit high lipid levels. Therefore, it is logical to speculate that changes in liver function, the organ where lipid synthesis takes place, affect the initiation and development
Acknowledgements
We thank M. Petreaca for discussion and critical reading of the paper, F. Sladek for the CMV-β-galactosidase plasmid, and K. Chellapa for help with the Luciferase assay.
References (62)
- et al.
Particulate and vapor phase constituents of cigarette mainstream smoke and risk of myocardial infarction
Atherosclerosis
(2001) - et al.
The SREBP pathway: regulation of cholesterol metabolism by proteolysis of a membrane-bound transcription factor
Cell
(1997) - et al.
Mutant mammalian cells as tools to delineate the sterol regulatory element-binding protein pathway for feedback regulation of lipid synthesis
Arch Biochem Biophys
(2002) - et al.
Differential stimulation of cholesterol and unsaturated fatty acid biosynthesis in cells expressing individual nuclear sterol regulatory element-binding proteins
J Biol Chem
(1998) - et al.
Topology of SREBP cleavage-activating protein, a polytopic membrane protein with a sterol-sensing domain
J Biol Chem
(1998) - et al.
Mammalian 5′-AMP-activated protein kinase non-catalytic subunits are homologs of proteins that interact with yeast Snf1 protein kinase
J Biol Chem
(1994) - et al.
Ethanol consumption impairs regulation of fatty acid metabolism by decreasing the activity of AMP-activated protein kinase in rat liver
Biochimie
(2008) - et al.
Non-alcoholic fatty liver disease (NAFLD) and cardiovascular disease: an open question
Nutr Metab Cardiovasc Dis
(2007) - et al.
Progress in human hepatocytes: isolation, culture & cryopreservation
Semin Cell Dev Biol
(2002) - et al.
Blood and urinary nicotine in non-smokers
The Lancet
(1975)
The anti-diabetic drugs rosiglitazone and metformin stimulate AMP-activated protein kinase through distinct signaling pathways
J Biol Chem
Pioglitazone treatment activates AMP-activated protein kinase in rat liver and adipose tissue in vivo
Biochem Biophys Res Commun
Paradoxical decrease of an adipose-specific protein, adiponectin, in obesity
Biochem Biophys Res Commun
Reduced adiponectin serum levels in smokers
Atherosclerosis
Prediction of coronary heart disease using risk factor categories
Circulation
Primary prevention of coronary heart disease: guidance from Framingham: a statement for healthcare professionals from the AHA Task Force on Risk Reduction. American Heart Association
Circulation
Assessment of cardiovascular risk by use of multiple-risk-factor assessment equations: a statement for healthcare professionals from the American Heart Association and the American College of Cardiology
Circulation
How Ldl receptors influence cholesterol and atherosclerosis
Sci Am
Atherosclerosis
Nature
Alanine aminotransferase as a marker of non-alcoholic fatty liver disease in relation to type 2 diabetes mellitus and cardiovascular disease
Diabetes Metab Res Rev
Endothelial dysfunction and cardiovascular risk profile in nonalcoholic fatty liver disease
Hepatology
Nonalcoholic fatty liver disease is independently associated with an increased incidence of cardiovascular events in type 2 diabetic patients
Diabetes Care
Prevalence of nonalcoholic fatty liver disease and its association with cardiovascular disease among type 2 diabetic patients
Diabetes Care
Lipid metabolism and liver inflammation. I. Hepatic fatty acid uptake: possible role in steatosis
Am J Physiol Gastrointest Liver Physiol
SREBPs: activators of the complete program of cholesterol and fatty acid synthesis in the liver
J Clin Invest
A proteolytic pathway that controls the cholesterol content of membranes, cells, and blood
Proc Natl Acad Sci USA
New perspectives in the regulation of hepatic glycolytic and lipogenic genes by insulin and glucose: a role for the transcription factor sterol regulatory element binding protein-1c
Biochem J
Differential expression of exons 1a and 1c in mRNAs for sterol regulatory element binding protein-1 in human and mouse organs and cultured cells
J Clin Invest
Effects of “second-hand” smoke on structure and function of fibroblasts, cells that are critical for tissue repair and remodeling
BMC Cell Biol
Identification of the low density lipoprotein receptor-binding site in apolipoprotein B100 and the modulation of its binding activity by the carboxyl terminus in familial defective apo-B100
J Clin Invest
The effects of second-hand smoke on biological processes important in atherogenesis
BMC Cardiovasc Disord
Cited by (98)
Lipidomic analysis reveals the effect of passive smoking on facial skin surface lipid in females
2022, Chemistry and Physics of LipidsCigarette smoking and liver diseases
2022, Journal of HepatologyUnravelling the metabolic alterations of liver damage induced by thirdhand smoke
2021, Environment International
- ☆
The underlying research reported in the study was funded by NIH (HL77448 and HL89940) and in part by the Tobacco-Related Disease Research Program TRDRP (11DT-0244). The authors who have taken part in this study declared that they do not have anything to disclose regarding funding from industry or conflict of interest with respect to this manuscript.