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Summary A central mechanism for controlling circadian gene amplitude remains elusive. We present evidence for a “facilitated repression (FR)” model that functions as an amplitude rheostat for circadian gene oscillation. We demonstrate that ROR and/or BMAL1 promote global chromatin decondensation during the activation phase of the circadian cycle to actively facilitate REV-ERB loading for repression of circadian gene expression. Mechanistically, we found that SRC-2 dictates global circadian chromatin remodeling through spatial and temporal recruitment of PBAF members of the SWI/SNF complex to facilitate loading of REV-ERB in the hepatic genome. Mathematical modeling highlights how the FR model sustains proper circadian rhythm despite fluctuations of REV-ERB levels. Our study not only reveals a mechanism for active communication between the positive and negative limbs of the circadian transcriptional loop but also establishes the concept that clock transcription factor binding dynamics is perhaps a central tenet for fine-tuning circadian rhythm. [ABSTRACT FROM AUTHOR]

Photoacoustic imaging (PAI) with co-registered ultrasound (US) is a hybrid non-invasive imaging modality that enables visualization and quantification of tumor hypoxia in live animals. Here, using a breast tumor xenograft model as an example, we present a stepwise protocol describing animal preparation, positioning, instrument setup, and US-PAI image acquisition procedures. This protocol also guides through detailed data analysis, explains functional readouts obtained from PAI, and discusses the potential application of the technology to study the hypoxic tumor microenvironment. For complete details on the use and execution of this protocol, please refer to Dai et al. 1 ., Competing Interests: Declaration of interests L.J.R. is currently an employee of Fujifilm-VisualSonics Corporation. S.D. holds equity in CoRegen Inc., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)

Cancer cells encounter a hostile tumor microenvironment (TME), and their adaptations to metabolic stresses determine metastatic competence. Here, we show that the metabolic enzyme 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase-4 (PFKFB4) is induced in hypoxic tumors acquiring metabolic plasticity and invasive phenotype. In mouse models of breast cancer, genetic ablation of PFKFB4 significantly delays distant organ metastasis, reducing local lymph node invasion by suppressing expression of invasive gene signature including integrin β3. Photoacoustic imaging followed by metabolomics analyses of hypoxic tumors show that PFKFB4 drives metabolic flexibility, enabling rapid detoxification of reactive oxygen species favoring survival under selective pressure. Mechanistically, hypoxic induction triggers nuclear translocation of PFKFB4 accentuating non-canonical transcriptional activation of HIF-1α, and breast cancer patients with increased nuclear PFKFB4 in their tumors are found to be significantly associated with poor prognosis. Our findings imply that PFKFB4 induction is crucial for tumor cell adaptation in the hypoxic TME that determines metastatic competence., Competing Interests: Declaration of interests S.L. serves on the Scientific Advisory Board of 20n Bio, and S.D. holds equity in Coactigon Inc., (Copyright © 2022 The Author(s). Published by Elsevier Inc. All rights reserved.)

Synchrony of the mammalian circadian clock is achieved by complex transcriptional and translational feedback loops centered on the BMAL1:CLOCK heterodimer. Modulation of circadian feedback loops is essential for maintaining rhythmicity, yet the role of transcriptional coactivators in driving BMAL1:CLOCK transcriptional networks is largely unexplored. Here, we show diurnal hepatic steroid receptor coactivator 2 (SRC-2) recruitment to the genome that extensively overlaps with the BMAL1 cistrome during the light phase, targeting genes that enrich for circadian and metabolic processes. Notably, SRC-2 ablation impairs wheel-running behavior, alters circadian gene expression in several peripheral tissues, alters the rhythmicity of the hepatic metabolome, and deregulates the synchronization of cell-autonomous metabolites. We identify SRC-2 as a potent coregulator of BMAL1:CLOCK and find that SRC-2 targets itself with BMAL1:CLOCK in a feedforward loop. Collectively, our data suggest that SRC-2 is a transcriptional coactivator of the BMAL1:CLOCK oscillators and establish SRC-2 as a critical positive regulator of the mammalian circadian clock., (Copyright © 2014 The Authors. Published by Elsevier Inc. All rights reserved.)