Date Published: February 2, 2017
Publisher: Public Library of Science
Author(s): Li Fen Liu, Colleen M. Craig, Lorna L. Tolentino, Okmi Choi, John Morton, Homero Rivas, Samuel W. Cushman, Edgar G. Engleman, Tracey McLaughlin, Francesco Giorgino.
The physiologic mechanisms underlying the relationship between obesity and insulin resistance are not fully understood. Impaired adipocyte differentiation and localized inflammation characterize adipose tissue from obese, insulin-resistant humans. The directionality of this relationship is not known, however. The aim of the current study was to investigate whether adipose tissue inflammation is causally-related to impaired adipocyte differentiation.
Abdominal subcutaneous(SAT) and visceral(VAT) adipose tissue was obtained from 20 human participants undergoing bariatric surgery. Preadipocytes were isolated, and cultured in the presence or absence of CD14+ macrophages obtained from the same adipose tissue sample. Adipocyte differentiation was quantified after 14 days via immunofluorescence, Oil-Red O, and adipogenic gene expression. Cytokine secretion by mature adipocytes cultured with or without CD14+macrophages was quantified.
Adipocyte differentiation was significantly lower in VAT than SAT by all measures (p<0.001). With macrophage removal, SAT preadipocyte differentiation increased significantly as measured by immunofluorescence and gene expression, whereas VAT preadipocyte differentiation was unchanged. Adipocyte-secreted proinflammatory cytokines were higher and adiponectin lower in media from VAT vs SAT: macrophage removal reduced inflammatory cytokine and increased adiponectin secretion from both SAT and VAT adipocytes. Differentiation of preadipocytes from SAT but not VAT correlated inversely with systemic insulin resistance. The current results reveal that proinflammatory immune cells in human SAT are causally-related to impaired preadipocyte differentiation, which in turn is associated with systemic insulin resistance. In VAT, preadipocyte differentiation is poor even in the absence of tissue macrophages, pointing to inherent differences in fat storage potential between the two depots.
While the link between obesity and insulin resistance in humans has been clearly established, the causal basis for this association is unclear. Not all obese are metabolically unhealthy and even normal-weight individuals can be insulin resistant: thus there are likely qualitative differences in adipose tissue that contribute to metabolic risk independent of adiposity per se. We and others have hypothesized that in the setting of excess body weight, impaired differentiation of new adipocytes leads to hypertrophy of existing adipocytes and spillover of fat into non-adipose tissues, thereby contributing to insulin resistance via lipotoxicity[1–4]. Furthermore, hypertrophic adipocytes in subcutaneous adipose tissue (SAT) appear to attract inflammatory cells, as histologic sections reveal rings of macrophages surrounding single enlarged necrotic adipocytes . Increased subcutaneous adipocyte diameter has also been associated with insulin resistance and type 2 diabetes [6,7]. Finally, increasing evidence suggests that preferential deposition of fat in visceral adipose tissue (VAT) is associated with metabolic abnormalities such as insulin resistance, dyslipidemia, fatty liver, and type 2 diabetes [8–10]. While VAT comprises only approximately 10% of total adipose tissue mass , it is characterized by increased inflammation relative to SAT , which might explain its link to systemic insulin resistance .
The results of this study extend a small but growing body of literature suggesting that inflammation in adipose tissue plays a causal role in adipocyte dysfunction and insulin resistance. While multiple studies in rodents have shown that diet-induced obesity leads to adipocyte hypertrophy [5,15], systemic and localized inflammation , and the development of insulin resistance, the directionality of these relationships is unclear. Hypertrophic (versus hyperplastic) obesity, for example, which is presumed to reflect impaired differentiation of new adipocytes to “offload” triglyceride storage demands, correlates with macrophage density in human adipose tissue, and macrophages tend to cluster geographically around single large necrotic adipocytes . Thus, adipocyte hypertrophy may initiate inflammation in adipose tissue. Alternatively, 3T3-L1 adipocytes, when cultured with inflammatory cytokines, exhibit defective terminal differentiation and markedly reduced lipid uptake , implying that inflammation may impair adipocyte differentiation and thus, lead to hypertrophy of non-inflamed adipocytes. Inflammatory cytokines also have been shown to directly antagonize insulin action in human adipocytes [14, 20–22], thereby inhibiting fat storage and promoting lipotoxicity.