title: Adipocyte hyperplasia and hypertrophy in the development of Type 2 Diabetes Mellitus
Creators: name: Gao, Xuan
Publisher: name: University of Glasgow
Year: 2023

description: Increasingly obesity and obesity-induced type 2 diabetes mellitus (T2DM) are becoming significant societal issues affecting health and longevity in today's world. When the human body takes in excess energy it is stored in adipocytes. Adipocytes store the excess energy via the processes of hypertrophy and/or hyperplasia. Storing energy through hypertrophy leads to the formation of large adipocytes, which are considered unhealthy due to their increased propensity to induce insulin resistance and inflammation. Consequently, these large adipocytes increase the risk of developing T2DM. Therefore, understanding how adipocyte hypertrophy and hyperplasia are regulated is essential for investigating the etiology of T2DM. South Asians are a susceptible population for T2DM and develop the disease at a younger age and lower BMI compared to European Caucasians. Notably, when South Asians migrate to Europe or America, their probability of developing T2DM is two to four times higher than Caucasians with the same BMI. Thus, understanding the causes of South Asians' susceptibility to T2DM is also crucial for comprehending the disease. This thesis first investigated the relationship between adipocyte hypertrophy and hyperplasia, obesity and T2DM by undertaking a cross-sectional analysis of subcutaneous adipose tissue (SAT) samples obtained through needle biopsy from human cohorts of varying age, ethnicity (South Asian and Caucasian), exercise status and insulin resistance. Adipocytes were isolated from human SAT, and their diameter and gene expression were measured. It was found that adipocyte diameter increased with age and declined with metabolic fitness, changes which were related to changes in expression of genes involved in adipocyte differentiation capacity and adipokine secretion. Long-term exercise mitigated the effect of aging, whereas insulin-resistant individuals exhibited exacerbated age-related effects. Insulin-resistant individuals had a significantly higher proportion of very large adipocytes than age and BMI-matched healthy controls. Interestingly, healthy, young, lean South Asians had adipocyte size distribution and gene expression more akin to older and higher BMI Caucasians. In particular, young South Asians had a proportion of very large adipocytes similar to that seen in insulin-resistant Caucasians. Based on these findings, this thesis then conducted a longitudinal analysis of adipocytes from the GlasVEGAs study, a study where South Asians and European Caucasians gained 6.4% body weight through overfeeding. The results showed that at baseline, and after weight gain, South Asians had a higher proportion of large and very large adipocytes and a lower proportion of small and medium-sized adipocytes. Compared to Caucasians, South Asian adipocytes exhibited lower expression of genes involved in insulin signalling and lipid oxidation and higher expression of genes involved in lipid storage and inflammation, potentially explaining the increased susceptibility of South Asian to T2DM. The work in this thesis demonstrated successful stimulation of preadipocyte differentiation in vitro, however possibly due to low sample size, no significant differences were discerned. Gene expression patterns suggested that South Asians may have lower adipogenesis capability than Europeans at baseline and after weight gain, though the effect lacked statistical significance. These findings underscore the importance of considering adipogenesis in future research exploring the link between adipose tissue function and T2DM in South Asians, Europeans, and potentially other populations. This thesis presented the first exploration of circulating extracellular vesicle (EV) concentration and size in response to acute feeding. A peak in both EV concentration and size was observed two hours post-feeding, returning to fasting levels six hours later. This pattern was consistent across healthy males and females, suggesting that future studies of circulating EV should consider the effects of feeding and that fasted samples may need to be collected as controls. A positive correlation was found between circulating EV size and age in both sexes, indicating the importance of age adjustment in future studies. The EV miRNA profile revealed the MIRLET7 family and MIR125B2 as the highest copy number of miRNA in circulating EV in both fasting and fed states. However, statistical analysis was not possible due to unsuitability of the data for normalisation procedures due to low copy number and lack of complexity of miRNA species. This study successfully established a protocol for collecting, isolating, and measuring EV from blood, and extracting RNA from circulating EV. Future work should include Taqman RT-qPCR to validate the expression of miRNA, thereby validating these initial findings. The studies included in this thesis have confirmed that aging and South Asian ethnicity can induce adipocyte hypertrophy and potentially reduce hyperplasia, leading to the formation of metabolically unhealthy adipocytes, thereby increasing the risk of T2DM. The research has also explored the possibility of a lower adipogenesis capability in South Asians, and the impact of acute feeding on circulating extracellular vesicle concentration and size, revealing potential implications for future research. Furthermore, it has been demonstrated that long-term exercise can alter adipocyte gene expression, attenuating the adverse impacts of aging and ethnicity. These findings collectively underscore the complex interplay between ethnicity, aging, adipocyte function, and lifestyle factors in T2DM development, highlighting the need for further comprehensive studies.