Date Published: January 26, 2017
Publisher: Public Library of Science
Author(s): Matthias Rosenwald, Vissarion Efthymiou, Lennart Opitz, Christian Wolfrum, Antonio Moschetta.
Active brown adipose tissue is responsible for non-shivering thermogenesis in mammals which affects energy homeostasis. The molecular mechanisms underlying this activation as well as the formation and activation of brite adipocytes have gained increasing interest in recent years as they might be utilized to regulate systemic metabolism. We show here that the transcriptional regulators SRF and MKL1 both act as repressors of brown adipogenesis. Loss-of-function of these transcription factors leads to a significant induction of brown adipocyte differentiation, increased levels of UCP1 and other thermogenic genes as well as increased respiratory function, while SRF induction exerts the opposite effects. Interestingly, we observed that knockdown of MKL1 does not lead to a reduced expression of typical SRF target genes and that the SRF/MKL1 inhibitor CCG-1423 had no significant effects on brown adipocyte differentiation. Contrary, knockdown of MKL1 induces a significant increase in the transcriptional activity of PPARγ target genes and MKL1 interacts with PPARγ, suggesting that SRF and MKL1 independently inhibit brown adipogenesis and that MKL1 exerts its effect mainly by modulating PPARγ activity.
Brown adipocytes metabolize lipids and glucose to generate heat from the resulting proton-motive force. The responsible protein for this particular function is uncoupling protein 1 (UCP1), which uncouples the electron transport chain from ATP synthesis [1–3]. The primary biological role of brown adipose tissue (BAT) is the tight control of body temperature, however, induced metabolism in brown adipocytes can lead to enhanced energy expenditure and the protection from obesity and related metabolic complications [4–9]. The finding of functional brown adipocytes in a substantial fraction of the adult human population renewed the interest in the mechanisms regulating their formation and function [10–12]. While generally similar to the formation of white adipocytes, brown adipogenesis requires special factors driving its unique catabolic capacity . PPARγ which is a master regulator of white adipogenesis [14,15] plays a significant role in the acquisition of a brown phenotype as its chronic activation induces a thermogenic program in white adipocytes  whereas its deletion leads to loss of brown adipose tissue .
In order to detect novel regulators of brown adipocyte formation, we analyzed gene expression data of stromal-vascular fractions (SVFs) and adipocyte fractions of different adipose tissue depots of C57BL/6 mice (S1A Fig) to find transcripts preferentially expressed in either BAT SVF or mature brown adipocytes. To take into account the complex regulation of transcription factor activity on the post-transcriptional level, we identified expression signatures using a transcription factor analysis (S1B and S1C Fig). We could show that a major part of the known downstream targets of SRF were differentially regulated in BAT SVF compared to mature brown adipocytes (Fig 1A), while the mRNA levels of Srf did not differ substantially between the samples (S1D Fig). In a separate cohort of mice, we confirmed that Srf mRNA expression was similar in all analyzed cell fractions and almost identical when comparing BAT SVF and brown adipocytes (S1E Fig). We derived the subset of genes that are confirmed positive downstream targets of SRF from the microarray data. The distribution of the relative regulation of these targets showed substantially higher expression of the majority of SRF targets in BAT SVF (Fig 1B). This indicated that the modulation of SRF transcriptional activity by post-translational regulation might regulate brown adipogenesis.
SRF is a ubiquitously expressed transcription factor regulating a plethora of different cellular and developmental processes with special importance in the formation of different muscle cell types  and alterations or deletions of its function have been linked to various disease states .