Frédéric Gachon

The Complementary Strand of the Human T-Cell Leukemia Virus Type 1 RNA Genome Encodes a bZIP Transcription Factor That Down-Regulates Viral Transcription

ABSTRACT The RNA genome of the human T-cell leukemia virus type 1 (HTLV-1) codes for proteins involved in infectivity, replication, and transformation. We report in this study the characterization of a novel viral protein encoded by the complementary strand of the HTLV-1 RNA genome. This protein, designated HBZ (for HTLV-1 bZIP factor), contains a N-terminal transcriptional activation domain and a leucine zipper motif in its C terminus. We show here that HBZ is able to interact with the bZIP transcription factor CREB-2 (also called ATF-4), known to activate the HTLV-1 transcription by recruiting the viral trans-activator Tax on the Tax-responsive elements (TxREs). However, we demonstrate that the HBZ/CREB-2 heterodimers are no more able to bind to the TxRE and cyclic AMP response element sites. Taking these findings together, the functional inactivation of CREB-2 by HBZ is suggested to contribute to regulation of the HTLV-1 transcription. Moreover, the characterization of a minus-strand gene protein encoded by HTLV-1 has never been reported until now.

The mammalian circadian timing system: from gene expression to physiology

Many physiological processes in organisms from bacteria to man are rhythmic, and some of these are controlled by self-sustained oscillators that persist in the absence of external time cues. Circadian clocks are perhaps the best characterized biological oscillators and they exist in virtually all light-sensitive organisms. In mammals, they influence nearly all aspects of physiology and behavior, including sleep-wake cycles, cardiovascular activity, endocrinology, body temperature, renal activity, physiology of the gastro-intestinal tract, and hepatic metabolism. The master pacemaker is located in the suprachiasmatic nuclei, two small groups of neurons in the ventral part of the hypothalamus. However, most peripheral body cells contain self-sustained circadian oscillators with a molecular makeup similar to that of SCN (suprachiasmatic nucleus) neurons. This organization implies that the SCN must synchronize countless subsidiary oscillators in peripheral tissues, in order to coordinate cyclic physiology. In this review, we will discuss some recent studies on the structure and putative functions of the mammalian circadian timing system, but we will also point out some apparent inconsistencies in the currently publicized model for rhythm generation.

The Circadian Clock Coordinates Ribosome Biogenesis

The authors identify a new role of the circadian clock in coordinating mRNA translation during ribosome biogenesis, a key process for cell metabolism.

Circadian clock-dependent and -independent rhythmic proteomes implement distinct diurnal functions in mouse liver

Significance Circadian clocks orchestrate daily rhythms in behavior and physiology using temporal regulation of gene expression to control core clock genes and rhythmic output programs. Although transcription regulation was shown to drive extensive diurnal mRNA rhythms, less is known about the proteins. Here, we provide a proteome-wide study of rhythmic protein accumulation in mouse liver, showing that proteins preferentially accumulate in the morning and during the night. About one-half of these rhythmic proteins could not be explained by rhythmic mRNAs, suggesting that translational or posttranslational regulation plays an important role. Moreover, such rhythms involved many secreted proteins and were clock-independent. Hence, these findings indicate that feeding behavior might determine the rhythmic functions of circulating proteins in the blood. Diurnal oscillations of gene expression controlled by the circadian clock underlie rhythmic physiology across most living organisms. Although such rhythms have been extensively studied at the level of transcription and mRNA accumulation, little is known about the accumulation patterns of proteins. Here, we quantified temporal profiles in the murine hepatic proteome under physiological light–dark conditions using stable isotope labeling by amino acids quantitative MS. Our analysis identified over 5,000 proteins, of which several hundred showed robust diurnal oscillations with peak phases enriched in the morning and during the night and related to core hepatic physiological functions. Combined mathematical modeling of temporal protein and mRNA profiles indicated that proteins accumulate with reduced amplitudes and significant delays, consistent with protein half-life data. Moreover, a group comprising about one-half of the rhythmic proteins showed no corresponding rhythmic mRNAs, indicating significant translational or posttranslational diurnal control. Such rhythms were highly enriched in secreted proteins accumulating tightly during the night. Also, these rhythms persisted in clock-deficient animals subjected to rhythmic feeding, suggesting that food-related entrainment signals influence rhythms in circulating plasma factors.

Circadian Clock-Coordinated 12 Hr Period Rhythmic Activation of the IRE1α Pathway Controls Lipid Metabolism in Mouse Liver

The mammalian circadian clock plays a fundamental role in the liver by regulating fatty acid, glucose, and xenobiotic metabolism. Impairment of this rhythm has been shown to lead to diverse pathologies, including metabolic syndrome. Currently, it is supposed that the circadian clock regulates metabolism mostly by regulating expression of liver enzymes at the transcriptional level. Here, we show that the circadian clock also controls hepatic metabolism by synchronizing a secondary 12 hr period rhythm characterized by rhythmic activation of the IRE1alpha pathway in the endoplasmic reticulum. The absence of circadian clock perturbs this secondary clock and provokes deregulation of endoplasmic reticulum-localized enzymes. This leads to impaired lipid metabolism, resulting in aberrant activation of the sterol-regulated SREBP transcription factors. The resulting aberrant circadian lipid metabolism in mice devoid of the circadian clock could be involved in the appearance of the associated metabolic syndrome.

Circadian and feeding rhythms differentially affect rhythmic mRNA transcription and translation in mouse liver

Significance Rhythmic gene regulation in mouse liver results from an intertwined relationship between feeding cycles and the circadian clock. Significant efforts have been made to understand this interaction but a complete picture of the resulting diurnal transcription–translation processes is still missing. Through the simultaneous quantification of temporal transcription, accumulation, and translation of mRNA in the liver, we investigated the regulatory landscape of mice with intact or deficient circadian clock subjected to different feeding regimens. We showed that circadian clock and feeding rhythms coordinate rhythmic transcription to drive downstream rhythmic mRNA accumulation and translation. However, a subset of genes harboring 5′-Terminal Oligo Pyrimidine tract or Translation Initiator of Short 5′-UTR elements encoding proteins involved in translation and mitochondrial activity, respectively, present a transcription-independent rhythmic translation mainly regulated by feeding. Diurnal oscillations of gene expression are a hallmark of rhythmic physiology across most living organisms. Such oscillations are controlled by the interplay between the circadian clock and feeding rhythms. Although rhythmic mRNA accumulation has been extensively studied, comparatively less is known about their transcription and translation. Here, we quantified simultaneously temporal transcription, accumulation, and translation of mouse liver mRNAs under physiological light–dark conditions and ad libitum or night-restricted feeding in WT and brain and muscle Arnt-like 1 (Bmal1)-deficient animals. We found that rhythmic transcription predominantly drives rhythmic mRNA accumulation and translation for a majority of genes. Comparison of wild-type and Bmal1 KO mice shows that circadian clock and feeding rhythms have broad impact on rhythmic gene expression, Bmal1 deletion affecting surprisingly both transcriptional and posttranscriptional levels. Translation efficiency is differentially regulated during the diurnal cycle for genes with 5′-Terminal Oligo Pyrimidine tract (5′-TOP) sequences and for genes involved in mitochondrial activity, many harboring a Translation Initiator of Short 5′-UTR (TISU) motif. The increased translation efficiency of 5′-TOP and TISU genes is mainly driven by feeding rhythms but Bmal1 deletion also affects amplitude and phase of translation, including TISU genes. Together this study emphasizes the complex interconnections between circadian and feeding rhythms at several steps ultimately determining rhythmic gene expression and translation.

Molecular Interactions Involved in the Transactivation of the Human T-Cell Leukemia Virus Type 1 Promoter Mediated by Tax and CREB-2 (ATF-4)

ABSTRACT The human T-cell leukemia virus type 1 (HTLV-1) Tax protein activates viral transcription through three 21-bp repeats located in the U3 region of the HTLV-1 long terminal repeat and called Tax-responsive elements (TxREs). Each TxRE contains nucleotide sequences corresponding to imperfect cyclic AMP response elements (CRE). In this study, we demonstrate that the bZIP transcriptional factor CREB-2 is able to bind in vitro to the TxREs and that CREB-2 binding to each of the 21-bp motifs is enhanced by Tax. We also demonstrate that Tax can weakly interact with CREB-2 bound to a cellular palindromic CRE motif such as that found in the somatostatin promoter. Mutagenesis of Tax and CREB-2 demonstrates that both N- and C-terminal domains of Tax and the C-terminal region of CREB-2 are required for direct interaction between the two proteins. In addition, the Tax mutant M47, defective for HTLV-1 activation, is unable to form in vitro a ternary complex with CREB-2 and TxRE. In agreement with recent results suggesting that Tax can recruit the coactivator CREB-binding protein (CBP) on the HTLV-1 promoter, we provide evidence that Tax, CREB-2, and CBP are capable of cooperating to stimulate viral transcription. Taken together, our data highlight the major role played by CREB-2 in Tax-mediated transactivation.

Proline- and acidic amino acid-rich basic leucine zipper proteins modulate peroxisome proliferator-activated receptor α (PPARα) activity

In mammals, many aspects of metabolism are under circadian control. At least in part, this regulation is achieved by core-clock or clock-controlled transcription factors whose abundance and/or activity oscillate during the day. The clock-controlled proline- and acidic amino acid-rich domain basic leucine zipper proteins D-site-binding protein, thyrotroph embryonic factor, and hepatic leukemia factor have previously been shown to participate in the circadian control of xenobiotic detoxification in liver and other peripheral organs. Here we present genetic and biochemical evidence that the three proline- and acidic amino acid-rich basic leucine zipper proteins also play a key role in circadian lipid metabolism by influencing the rhythmic expression and activity of the nuclear receptor peroxisome proliferator-activated receptor α (PPARα). Our results suggest that, in liver, D-site-binding protein, hepatic leukemia factor, and thyrotroph embryonic factor contribute to the circadian transcription of genes specifying acyl-CoA thioesterases, leading to a cyclic release of fatty acids from thioesters. In turn, the fatty acids act as ligands for PPARα, and the activated PPARα receptor then stimulates the transcription of genes encoding proteins involved in the uptake and/or metabolism of lipids, cholesterol, and glucose metabolism.

The role of circadian timing system on drug metabolism and detoxification.

Introduction: It has been known for a long time that the efficiency and toxicity of drugs change during a 24-h period. However, the molecular mechanisms involved in these processes have started to emerge only recently. Areas covered: This review aims to highlight recent discoveries showing the direct role of the molecular circadian clock in xenobiotic metabolism at the transcriptional and post-transcriptional levels in the liver and intestine, and the different ways of elimination of these metabolized drugs via biliary and urine excretions. Most of the related literature focuses on transcriptional regulation by the circadian clock of xenobiotic metabolism in the liver; however, the role of this timing system in the excretion of metabolized drugs and the importance of the kidney in this phenomenon are generally neglected. The goal of this review is to describe the molecular mechanisms involved in rhythmic drug metabolism and excretion. Expert opinion: Chronopharmacology is used to analyze the metabolism of drugs in mammals according to the time of day. The circadian timing system plays a key role in the changes of toxicity of drugs by influencing their metabolisms in the liver and intestine in addition to their excretion via bile flow and urine.

Crosstalk between xenobiotics metabolism and circadian clock

Many aspects of physiology and behavior in organisms from bacteria to man are subjected to circadian regulation. Indeed, the major function of the circadian clock consists in the adaptation of physiology to daily environmental change and the accompanying stresses such as exposition to UV‐light and food‐contained toxic compounds. In this way, most aspects of xenobiotic detoxification are subjected to circadian regulation. These phenomena are now considered as the molecular basis for the time‐dependence of drug toxicities and efficacy. However, there is now evidences that these toxic compounds can, in turn, regulate circadian gene expression and thus influence circadian rhythms. As food seems to be the major regulator of peripheral clock, the possibility that food‐contained toxic compounds participate in the entrainment of the clock will be discussed.