Differential Effects of Cold Exposure on Gene Expression Profiles in White Versus Brown Adipose Tissue

Mature white adipocyte plasticity during mammary gland remodelling and cancer

Mammary gland growth and differentiation predominantly rely on stromal-epithelial cellular communication. Specifically, mammary adipocytes play a crucial role in ductal morphogenesis, as well as in the proliferation and differentiation of mammary epithelial cells. The process of lactation entails a reduction in the levels of white adipose tissue associated with the MG, allowing for the expansion of milk-producing epithelial cells. Subsequently, during involution and the regression of the milk-producing unit, adipocyte layers resurface, occupying the vacated space. This dynamic phenomenon underscores the remarkable plasticity and expansion of adipose tissue. Traditionally considered terminally differentiated, adipocytes have recently been found to exhibit plasticity in certain contexts. Unraveling the significance of this cell type within the MG could pave the way for novel approaches to reduce the risk of breast cancer and enhance lactation performance. Moreover, a comprehensive understanding of adipocyte trans- and de-differentiation processes holds promise for the development of innovative therapeutic interventions targeting cancer, fibrosis, obesity, type 2 diabetes, and other related diseases. Additionally, adipocytes may find utility in the realm of regenerative medicine. This review article provides a comprehensive examination of recent advancements in our understanding of MG remodelling, with a specific focus on the tissue-specific functions of adipocytes and their role in the development of cancer. By synthesizing current knowledge in this field, it aims to consolidate our understanding of adipocyte biology within the context of mammary gland biology, thereby fostering further research and discovery in this vital area.

Transcriptome profiling of brown adipose tissue during cold exposure reveals extensive regulation of glucose metabolism

We applied digital gene expression profiling to determine the transcriptome of brown and white adipose tissues (BAT and WAT, respectively) during cold exposure. Male C57BL/6J mice were exposed to cold for 2 or 4 days. A notable induction of genes related to glucose uptake, glycolysis, glycogen metabolism, and the pentose phosphate pathway was observed in BAT from cold-exposed animals. In addition, glycerol-3-phosphate dehydrogenase 1 expression was induced in BAT from cold-challenged mice, suggesting increased synthesis of glycerol from glucose. Similarly, expression of lactate dehydrogenases was induced by cold in BAT. Pyruvate dehydrogenase kinase 2 (Pdk2) and Pdk4 were expressed at significantly higher levels in BAT than in WAT, and Pdk2 was induced in BAT by cold. Of notice, only a subset of the changes detected in BAT was observed in WAT. Based on changes in gene expression during cold exposure, we propose a model for the intermediary glucose metabolism in activated BAT: 1) fluxes through glycolysis and the pentose phosphate pathway are induced, the latter providing reducing equivalents for de novo fatty acid synthesis; 2) glycerol synthesis from glucose is increased, facilitating triacylglycerol synthesis/fatty acid re-esterification; 3) glycogen turnover and lactate production are increased; and 4) entry of glucose carbon into the tricarboxylic acid cycle is restricted by PDK2 and PDK4. In summary, our results demonstrate extensive and diverse gene expression changes related to glucose handling in activated BAT.

Cold-induced changes in gene expression in brown adipose tissue, white adipose tissue and liver

Cold exposure imposes a metabolic challenge to mammals that is met by a coordinated response in different tissues to prevent hypothermia. This study reports a transcriptomic analysis in brown adipose tissue (BAT), white adipose (WAT) and liver of mice in response to 24 h cold exposure at 8°C. Expression of 1895 genes were significantly (P<0.05) up- or down-regulated more than two fold by cold exposure in all tissues but only 5 of these genes were shared by all three tissues, and only 19, 14 and 134 genes were common between WAT and BAT, WAT and liver, and BAT and liver, respectively. We confirmed using qRT-PCR, the increased expression of a number of characteristic BAT genes during cold exposure. In both BAT and the liver, the most common direction of change in gene expression was suppression (496 genes in BAT and 590 genes in liver). Gene ontology analysis revealed for the first time significant (P<0.05) down regulation in response to cold, of genes involved in oxidoreductase activity, lipid metabolic processes and protease inhibitor activity, in both BAT and liver, but not WAT. The results reveal an unexpected importance of down regulation of cytochrome P450 gene expression and apolipoprotein, in both BAT and liver, but not WAT, in response to cold exposure. Pathway analysis suggests a model in which down regulation of the nuclear transcription factors HNF4α and PPARα in both BAT and liver may orchestrate the down regulation of genes involved in lipoprotein and steroid metabolism as well as Phase I enzymes belonging to the cytochrome P450 group in response to cold stress in mice. We propose that the response to cold stress involves decreased gene expression in a range of cellular processes in order to maximise pathways involved in heat production.