Effects of cigarette smoking in fat and muscle contributing to the metabolic syndrome

Metabolic syndrome (MetS) is a cluster of metabolic disorders of central obesity, dyslipidaemia, insulin resistance and hyperglycaemia which underlies type 2 diabetes mellitus (T2D) and cardiovascular disease. It is believed that central obesity and associated ectopic lipid accumulation are central to the development of metabolic syndrome which is heavily influenced by lifestyle factors. Among the lifestyle factors, the effects of nutrient compositions, such as a high-fat diet have been extensively studied. However, the effects of smoking on MetS are less clear despite it being considered as an important risk factor for T2D. Therefore, the overall aim of this thesis was to investigate the mechanism underlying the metabolic effects of cigarette smoking (CS) with different diets. This is highly relevant to public health in humans because the number of smokers is around 1.2 billion worldwide and some smokers over consume diets high in fat. The overall hypothesis of this thesis was that CS may divert lipids from adipose tissue to the circulating muscle to cause hyperlipidaemia and muscle insulin resistance independent of diet. At the same time, the well documented muscle wasting by CS may also contribute to the development of MetS by diminishing the metabolic capacity and property of muscle. A review of relevant literature described in Chapter 1 found evidence indicating that chronic CS can contribute to the development of MetS or T2D. However, the effects of CS in combination with different diets and the mechanisms involved are not clear. To test the hypothesis described above, studies were conducted in mice exposed to chronic CS followed by either a chow (CH) diet or a high-fat (HF) diet with a major focus of analyses on adipose tissue and skeletal muscle because these two tissues are critical to the pathogenesis of MetS. These are described in Chapter 3. The results showed that CS increased triglyceride (TG) levels in both plasma and muscle despite reductions in body weight gain and adiposity. CS led to glucose intolerance in CH-fed mice and it retained the glucose intolerance induced by the HF diet. In adipose tissue, CS increased macrophage infiltration and the expression of TNFalpha mRNA but suppressed the protein expression of adipose triglyceride lipase and PPARgamma. While CS increased hormone-sensitive lipase and suppressed the expression of leptin mRNA, these effects were mitigated in HF-fed mice. These results imply that CS impairs insulin signalling in skeletal muscle via accumulation of intramuscular lipids from lipolysis and lipodystrophy of adipose tissues. These findings collectively support the proposed hypothesis of the diversion of lipids from adipose tissue to muscle as a mechanism of CS-induced MetS. In studies described in Chapter 4 the impact of CS on muscle wasting in relation to the MetS was further investigated. The results showed that CS caused atrophy in red muscle but hypertrophy in white muscle irrespective of diet. These findings suggest that CS can cause muscle wasting as hypothesised but this only occurs in red muscle type. As red muscle is the major type responsible for the oxidation of glucose and fat, its wasting is likely to contribute to CS-induced MetS. Although white muscle mass is increased, this may not be sufficient to compensate for the impact of lost red muscle tissue on MetS. The mechanism of red muscle wasting is likely to result from inhibited myogenesis (as indicated by HDAC2, MEF2A and MEF2D expression) and increased autophagy (indicated by LC3II/LC3I expression). In contrast, the hypertrophy of white muscle may be due to increased myogenesis. The mechanisms involved in the different responses of red and white muscle types to CS were explored by examining the pathways including mTOR, AMPK, inflammatory cytokines and oxidative stress markers. Studies reported in Chapter 5 tested the hypothesis that cigarette smoke extract (CSE) can directly affect adipocytes (3T3L1 cells) and muscle cells (C2C12 cells) without influence from other tissues such as the lung. The results showed that CSE inhibited the differentiation of cultured 3T3L1 adipocytes and lipid content but increased lipid release to the culture medium. At the same time, cell viability was reduced. In C2C12 myotubes, CSE increased lipid accumulation, reduced cell viability and inhibited insulin signalling. These results are consistent with the interpretation of data presented in Chapter 3 that CSE causes a lipid overabundance to muscle due to increased lipolysis and lipodystrophy. Also consistent with the findings in Chapter 4, these results indicate that smoking is able to directly reduce the viability of muscle cells, contributing to MetS. In summary, the findings from the studies reported in this thesis indicate that CS can cause whole-body insulin resistance irrespective of diets. The underlying mechanism involves diverting lipids from dystrophic adipose tissue and muscle to cause insulin resistance. In addition, muscle wasting in red muscle fibers was the major muscular type responsible for insulin-stimulated glucose disposal. These direct effects of smoking in adipose tissue (or adipocytes) and muscle (or muscle cells) are mainly responsible for smoking-induced metabolic syndrome phenotypes at the whole-body level.