Obesity can be characterised as a mild chronic inflammatory condition. Observed inflammation can be driven by different factors, including infiltration of macrophages into adipose tissue and by hypoxia. As white adipose tissue is heterogenous at the cellular level, it provides an ideal environment for cell-to-cell communication, and in particular for crosstalk between adipocytes, preadipocytes and macrophages. Over the past two decades there have been several studies that have examined the role of macrophage-driven inflammation in adipose tissue and how this impacts the inflammatory state of both adipocytes and preadipocytes. An area that has yet to be fully explored involves the crosstalk between adipocytes and preadipocytes, as well as their influence on macrophages. This pilot study seeks to investigate this area of crosstalk through a candidate gene approach.
SGBS (human) preadipocytes were cultured (20,000 cells/well) and maintained at 37°C in a humidified atmosphere of 5% CO2/ 95% air. Once confluent, the media was collected to be used as preadipocyte conditioned medium (PACM). The preadipocytes were then differentiated into adipocytes, and 10 days post differentiation media was collected to be used as adipocyte conditioned media (ACM). U937 monocytes were plated at 2 x 105 cells/ml at 37°C in a humidified atmosphere of 5% CO2/95% air, differentiation was induced by the addition of 30 nM phorbol myristic acid. Cells were treated with 150 µl of the relevant conditioned media for 24 h, after which mRNA was extracted and reverse transcribed. Expression of candidate genes was carried out using qPCR (n = 6) and fold-changes determined using the 2-ΔΔCT method, results expressed as ± standard error of the mean and a Student’s t-test applied to the results to determine statistical significance (p<0.05).
Treating adipocytes with PACM led to changes in mRNA expression of key adipokines. A 16-fold increase in IL-6 mRNA was observed (p<0.001 compared to control), and an 8-fold increase was seen in MCP-1 mRNA level (p<0.001 compared to control). When it came to adiponectin expression, the treatment led to no significant changes in mRNA level. Macrophages treated with PACM demonstrated a 1.8-fold increase in IL-6 expression (p<0.05 compared to control), 2.1-fold increase in TNFα (p<0.01 compared to control), and a 3.2-fold increase in MCP-1 mRNA levels (p<0.05 compared to control). When treated with ACM, preadipocytes demonstrated no measurable changes in IL-6, MCP-1 and adiponectin mRNA levels. However, when U937 cells were treated with ACM, no significant change in TNFα expression was seen, but a statistically significant decrease in IL-6 (1.7-fold) and MCP-1 (2.5-fold) mRNA level (p<0.05 compared to control) was evident.
These experiments show that preadipocytes can exert an inflammatory response in adipocytes, and a very mild effect on macrophages. In contrast, adipocytes do not appear to induce a significant change in inflammation-related genes in preadipocytes but a slight decrease in inflammation related genes in macrophages is apparent. These data suggest a possible pro-inflammatory role for preadipocytes in adipose tissue, with adipocytes possibly exerting an anti-inflammatory effect on macrophages.