2.3.1. Effect of adipocyte hypertrophy on adipose tissue hypoxia

Obesity as excess of adipose tissue is attributed to hypertrophy and hyperplasia of adipocytes. Adipocytes have a limited capacity for hypertrophy. One explanation is considered the diffusion limit of oxygen, which is at most 100 μm [17]. Therefore, it is possible that hypertrophic adipocytes might endure less than adequate oxygen supply. On the other hand, it has been demonstrated that hypoxia of obese mice may occur in the areas within adipose tissue as a result of adipocyte hypertrophy compromising effective O2 supply from the vascular and then

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

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

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 of obesity-related comorbidities [27].

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

Hypertrophic adipocytes are subjected to multiple cytotoxic stressors including lipotoxicity [27]. Impaired insulin action in adipocytes during the development of hypertrophy and hypoxia is associated with elevated lipolysis and increased release of free fatty acids leading to ectopic fat deposition in liver and skeletal muscle. Subsequently, it will cause the progress of systemic insulin resistance [27]. Chronic hypoxia has been suggested to be part of the pathological mechanisms causing dysfunction of adipocytes [20,25]. For instance, chronic hypoxia leads to derangements in lipid metabolism and reduced lipoprotein clearance by decreasing lipoprotein lipase activity in mice [28] and diminished subcutaneous adipose tissue lipolysis by decreased efficiency of lipolytic signaling driving by the lipolytic signaling of beta-adrenergic nervous system, growth hormone, and parathyroid hormone in humans [29]. Moreover, the increment of circulating free fatty acids (FFA) could be triggered by acute hypoxia in an ischemia model of adult rats [24]. Acute hypoxia was also shown to increase lipolysis by activation of adipocyte protein kinase A via increased epinephrine and norepinephrine release following sympathetic nervous system stimulation [30]. In addition, the study conducted with ob/ob mice showed that adipose tissue hypoxia caused free fatty acid release and inhibited glucose uptake in adipocytes by inhibition of the insulin-signaling pathway [24].

Nevertheless, the increase in fatty acid flux into the fat cells also results in greater synthesis of the FFA into triglycerides, which would lead to endoplasmic reticulum (ER) stress activating the JNK pathway and thus further increasing insulin resistance in the fat cells [31].
