2.3.3. Effect of adipocyte hypertrophy on adipose tissue blood flow reduction

initialize an inflammatory responses indicated by pimonidazole hydrochloride adduction (physical evidence) as well as lactate concentration (physiological evidence) [19]. Moreover, it has also been reported that hypoxia results in inflammation in adipose tissue and insulin

The hypoxia is able to induce inflammation in adipose tissue by the induction of hypoxia-related gene expression in adipocytes and macrophages. An important and well-characterized key regulator of the adaptive response to alterations in oxygen tension is hypoxia-inducible factor-1a (HIF-1α), a transcription factor that accumulates during hypoxia and activates the nuclear factor-κβ pathways, leading to increased inflammation and stimulation of angiogenesis [19].

Of note, hypoxia-inducible factor-1a (HIF-1α), which plays a pivotal role in the response to hypoxia [20], is regarded as the master regulator of O2 homeostasis. HIF-1α has been identified in human adipose tissue and is reported to be increased in obesity [21]. In addition, several rodent studies have shown that the increased gene expression of HIF-1α, more hypoxic areas, and lower PO2 were detected in white adipose tissue of ob/ob, KKAy, and dietary-induced obese mice [22–24]. Reduced oxygen tension has been directly measured in obese fat depots in mouse models and human subjects [23]. Overexpression of HIF-1α in the adipocyte has also been proved to be more pro-fibrotic and pro-inflammatory than pro-angiogenic [25]. Adipocyte-specific deletion of HIF-1α limited high-fat diet-induced adipose tissue inflammation and insulin resistance, and the tissue was equally vascularized as wild-type controls [26]. Thereby, the augmentation of HIF-1α expression could contribute to a localized inflammation in adipose tissue that propagates an overall systemic inflammation associated with the development

Hypertrophic adipocytes are subjected to multiple cytotoxic stressors including lipotoxicity [27]. Impaired insulin action in adipocytes during the development of hypertrophy and

2.3.2. Effect of adipocyte hypertrophy on production of free fatty acids (lipotoxicity)

resistance in vitro and in animal studies [19–21].

130 Adiposity - Omics and Molecular Understanding

Figure 1. The development of unhealthy obesity and insulin resistance.

of obesity-related comorbidities [27].

The growing fat mass, in particular, the abdominal adipose tissue, is associated with unfavorable changes in adipose tissue blood flow and the development of metabolic disorders in state of obesity. The diminished blood flow shown in enlarged fat mass might mainly attribute to the development of adipocyte hypertrophy.

A decrease in adipose tissue perfusion is a common feature in obesity. West et al. [32] demonstrated that blood flow to adipose tissue, measured with radiolabeled microsphere, was reduced in Zucker obese rats. In humans, the levels of adipose tissue blood flow were measured with positron emission tomography using [15O]-labeled water [33] and the 133Xe washout method [34] and were lower in obese compared with non-obese subjects. In addition, the disturbances in the regulation of adipose tissue blood flow have been linked to obesity and insulin resistance [35]. This study demonstrated a close relationship between insulin sensitivity and the regulation of postprandial adipose tissue blood flow, independent of adiposity. Therefore, impaired regulation of adipose tissue blood flow by adipocyte hypertrophy could also be a significant and independent contributor for the development of insulin resistance in the state of obesity [35].

#### 2.3.4. Effect of adipocyte hypertrophy on adipocyte death

As mentioned above, adipocyte hypertrophy could directly and indirectly cause adipocyte hypoxia. Hypoxia may be a potential risk factor for adipocyte death in adipose tissue of obese subjects. An increase in adipocyte death was reported in adipose tissue of obese subjects and was proposed to induce macrophage infiltration [36]. The cell death may also promote lipolysis and release of FFA into blood stream under insulin resistance. This will significantly contribute to the increase in plasma FFA in obesity. Moreover, it has been demonstrated that the frequency of adipocyte death was significantly associated with the adipose gene expressions of TNF-α, IL-6, and MCP-1 in adipose tissue and the development of whole-body insulin resistance [37].

Macrophages are extremely proficient in the removal of numerous molecules, ranging from small lipids to colonies of pathogens to dead cells. Necrosis of adipocytes, driven by hypertrophy and accelerated by obesity, is a prominent phagocytic stimulus that attracts macrophage infiltration into adipose tissue [18]. Using a transgenic animal model of inducible lipoatrophy, Pajvani et al. demonstrated that massive adipocyte death can indeed drive rapid accumulation of adipose tissue macrophages (ATMs) as an integral element in the remodeling of fat pads [38]. These observations implicate an important role of adipocyte hypertrophy in the development of adipocyte death and associated inflammatory changes in AT and obesity complication.

## 2.3.5. Effect of adipocyte hypertrophy on adipokine production

Elevation of pro-inflammatory cytokines in fat and circulation such as TNF-α, IL-1, IL-6, MCP-1, and PAI-1 has been documented in obesity [23,25,39]. The increase in adipokine production in adipocyte hypertrophy and hypoxia has been suggested to underlie the development of the inflammatory response in adipose tissue, which occurs in the obese state [11,40]. It has been clearly indicated that adipocyte size is an important determinant for the secretion of several inflammatory adipokines, such as leptin, IL-6, and MCP-1, thereby providing another link between adipocyte size and inflammation in obesity [41]. In the same study, there was a tendency to reduce the release of anti-inflammatory adipokines such as IL-10 and adiponectin with increasing adipocyte size [41].

On the other hand, hypoxia has been proposed to be an inciting etiology of necrosis and macrophage infiltration into adipose tissue, which subsequently results in the dysregulation of the production of inflammation-related adipokines such as leptin, adiponectin, TNF-α, IL-6, and vascular endothelial growth factor (VEGF) [40,42]. Recently, hypoxia has also been reported to induce the production of PAI-1 and to inhibit the synthesis of adiponectin by 3T3- L1 adipocytes [39]. It is also reported to induce the expression of visfatin in these cells [43]. The expressions of other major adipokine production from murine or human adipocytes including angiopoietin-like protein 4 (Angptl4), interleukin-6 (IL-6), macrophage migration inhibitory factor (MIF), and VEGF [23,40,44] are also stimulated by hypoxia. Accordingly, Wang et al. mimicked hypoxia in human adipocytes for 24 h using cobalt chloride (CoCl2). It is shown that HIF-1α along with oxidative stress markers, inflammatory markers, and leptin was increased, but conversely adiponectin was decreased during hypoxia [42].
