Endocrine Abstracts

Multiple aspects of physiology and pathophysiology, including endocrine function, are influenced by endogenous circadian timing mechanisms. In mammals, cell autonomous clocks are present in all major tissues throughout the body. These clocks are synchronised via complex signalling pathways and are believed to drive local aspects of physiology.

Recently published data have revealed 24-h rhythms in up to 20% of the adipose transcriptome, suggesting profound circadian regulation of adipose biology. However, adipose tissue is extremely heterogeneous and it is therefore unclear whether published rhythms represent an endogenous clock within the key endocrine cells present within adipose tissue, namely the adipocytes. Moreover, all studies to date have analysed tissue biopsies, which have been under the control of multiple external neuronal and humoral signalling pathways.

In this study, we have tested the hypothesis that adipocyte cells contain a circadian clock. We achieved this by in vitro analysis of 3T3-L1 adipocytes, in order to measure circadian rhythmicity in an isolated cell population. In an initial experiment, we extracted total RNA from both pre-adipocytes and differentiated 3T3-L1 cells for analysis of clock gene expression by PCR. Next, pre-adipocytes were grown to confluence and differentiated by a standard protocol. Eight days following the onset of differentiation, cells were treated with a serum pulse to synchronise cellular rhythms. Samples were collected every 4-h over a 28-h period for analysis by PCR.

Our data revealed the presence of multiple clock genes in both pre-adipocytes and differentiated 3T3-L1 cells. Moreover, clear 24-h variation of gene expression was observed in adipocytes following a serum pulse. These data therefore reveal a circadian clock within adipocyte cells. Current work is extending the existing data by employing quantitative analysis of longer time course experiments and focussing on the synthesis and secretion of adipokines, such as leptin and adiponectin.