As the proliferation promoter noradrenaline induces expression of ICER (induced cAMP early repressor) in proliferative brown adipocytes, ICER may not be a universal tumour suppressor

Lipolysis regulates major transcriptional programs in brown adipocytes

β-Adrenergic signaling is a core regulator of brown adipocyte function stimulating both lipolysis and transcription of thermogenic genes, thereby expanding the capacity for oxidative metabolism. We have used pharmacological inhibitors and a direct activator of lipolysis to acutely modulate the activity of lipases, thereby enabling us to uncover lipolysis-dependent signaling pathways downstream of β-adrenergic signaling in cultured brown adipocytes. Here we show that induction of lipolysis leads to acute induction of several gene programs and is required for transcriptional regulation by β-adrenergic signals. Using machine-learning algorithms to infer causal transcription factors, we show that PPARs are key mediators of lipolysis-induced activation of genes involved in lipid metabolism and thermogenesis. Importantly, however, lipolysis also activates the unfolded protein response and regulates the core circadian transcriptional machinery independently of PPARs. Our results demonstrate that lipolysis generates important metabolic signals that exert profound pleiotropic effects on transcription and function of cultured brown adipocytes.

ComBATing aging-does increased brown adipose tissue activity confer longevity?

Brown and its related beige adipose tissue (BAT) play a definitive role in maintaining body temperature by producing heat through uncoupling protein 1 (UCP1), which acts by dissociating oxidative phosphorylation from ATP production, resulting in the release of heat. Therefore, in order to maintain high thermogenic capacity, BAT must act as a metabolic sink by taking up vast amounts of circulating glucose and lipids for oxidation. This, along with the rediscovery of BAT in adult humans, has fueled the study of BAT as a putative therapeutic approach to manage the growing rates of obesity and metabolic syndromes. Notably, many of the beneficial consequences of BAT activity overlap with metabolic biomarkers of extended lifespan and healthspan. In this review, we provide background about BAT including the thermogenic program, BAT's role as a secretory organ, and differences between BAT in mice and humans. We also provide details on BAT during aging, and perspectives on the potential of targeting BAT to promote lifespan and healthspan.

From White to Brown - Adipose Tissue Is Critical to the Extended Lifespan and Healthspan of Growth Hormone Mutant Mice

Growth hormone (GH) is a metabolic hormone that has major functions in the liver, muscle, and adipose tissue (AT). In the past 20 years, numerous studies have demonstrated that decreased growth hormone (GH) action is clearly linked to alterations in longevity. Therefore, it is not surprising that mechanisms underlying the extended longevity of GH-mutant animals include alterations in AT function. This Review aims to describe the basics of AT biology, GH secretion and action, and the effects of altered GH signaling in mice and humans. Lastly, this Review discusses the intersection of GH and AT, and how the influence of GH on AT may play a critical role in determining lifespan and healthspan.

The neuropeptide calcitonin gene-related peptide causes repression of tumor necrosis factor-alpha transcription and suppression of ATF-2 promoter recruitment in Toll-like receptor-stimulated dendritic cells

Sensory nerves may dampen inflammatory processes through the release of the neuropeptide calcitonin gene-related peptide (CGRP). CGRP mediates immunosuppressive activities through up-regulation of interleukin-10 or, alternatively, through an interleukin-10-independent pathway that is associated with rapid induction of the transcriptional inducible cAMP early repressor (ICER). In this work, we further investigated the molecular mechanisms of immune modulation by CGRP. Using TLR2-stimulated dendritic cells, we show that inhibition of tumor necrosis factor-alpha production by CGRP is dependent on up-regulation of endogenous ICER. Dendritic cell expression of ICER was selectively induced by CGRP and elevation of cellular cAMP levels but not by numerous pro- and anti-inflammatory cytokines. Treatment of dendritic cells with CGRP did not interfere with the induction of Tnfa gene expression but caused premature repression of TLR2-induced transcriptional activity. ATF-2 was rapidly phosphorylated and recruited to the Tnfa promoter following ligation of TLR2. Concomitant administration of CGRP completely prevented binding of ATF-2 to the Tnfa promoter, whereas recruitment of ICER was markedly elevated. In contrast, CGRP did not influence TLR2-stimulated binding of NF-kappaB p65. Together, these results are consistent with a model suggesting that CGRP causes rapid up-regulation of ICER, which in turn competes with ATF-2 for binding to the Tnfa promoter, leading to repression of gene expression.

Osteopenia in transgenic mice with osteoblast-targeted expression of the inducible cAMP early repressor

ICER is a member of the CREM family of basic leucine zipper transcription factors that acts as a dominant negative regulator of gene transcription. Four different isoforms of ICER (I, Igamma, II and IIgamma) are transcribed from the P2 promoter of the Crem gene. We previously found that each of the ICER isoforms is induced by parathyroid hormone in osteoblasts. The goal of the present study was to assess the function of ICER in bone by overexpressing ICER in osteoblasts of transgenic mice. ICER I and ICER II cDNAs, each containing an N-terminal FLAG epitope tag, were cloned downstream of a fragment containing 3.6 kb of the rat Col1a1 promoter and most of the rat Col1a1 first intron to produce pOBCol3.6-ICER I and pOBCol3.6-ICER II transgenes, respectively. Multiple lines of mice were generated bearing the ICER I and ICER II transgenes. At 8 weeks of age, ICER I and ICER II transgenic mice had lower body weights and decreased bone mineral density of femurs and vertebrae. Further studies were done with ICER I transgenic mice, which had greatly reduced trabecular bone volume and a markedly decreased bone formation rate in femurs. Osteoblast differentiation and osteocalcin expression were reduced in ex vivo bone marrow cultures from ICER I transgenic mice. ICER I antagonized the activity of ATF4 at its consensus DNA binding site in the osteocalcin promoter in vitro. Thus, transgenic mice with osteoblast-targeted overexpression of ICER exhibited osteopenia caused primarily by reduced bone formation. We speculate that ICER regulates the activity and/or expression of ATF/CREB factors required for normal bone formation.

Alpha1- and beta1-adrenoceptor signaling fully compensates for beta3-adrenoceptor deficiency in brown adipocyte norepinephrine-stimulated glucose uptake

To assess the relative roles and potential contribution of adrenergic receptor subtypes other than the beta3-adrenergic receptor in norepinephrine-mediated glucose uptake in brown adipocytes, we have here analyzed adrenergic activation of glucose uptake in primary cultures of brown adipocytes from wild-type and beta3-adrenergic receptor knockout (KO) mice. In control cells in addition to high levels of beta3-adrenergic receptor mRNA, there were relatively low alpha1A-, alpha1D-, and moderate beta1-adrenergic receptor mRNA levels with no apparent expression of other adrenergic receptors. The levels of alpha1A-, alpha1D-, and beta1-adrenergic receptor mRNA were not changed in the beta3-KO brown adipocytes, indicating that the beta3-adrenergic receptor ablation does not influence adrenergic gene expression in brown adipocytes in culture. As expected, the beta3-adrenergic receptor agonists BRL-37344 and CL-316 243 did not induce 2-deoxy-d-glucose uptake in beta3-KO brown adipocytes. Surprisingly, the endogenous adrenergic neurotransmitter norepinephrine induced the same concentration-dependent 2-deoxy-D-glucose uptake in wild-type and beta3-KO brown adipocytes. This study demonstrates that beta1-adrenergic receptors, and to a smaller degree alpha1-adrenergic receptors, functionally compensate for the lack of beta3-adrenergic receptors in glucose uptake. Beta1-adrenergic receptors activate glucose uptake through a cAMP/protein kinase A/phosphatidylinositol 3-kinase pathway, stimulating conventional and novel protein kinase Cs. The alpha1-adrenergic receptor component (that is not evident in wild-type cells) stimulates glucose uptake through a phosphatidylinositol 3-kinase and protein kinase C pathway in the beta3-KO cells.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Interferon-gamma stimulates the expression of the inducible cAMP early repressor in macrophages through the activation of casein kinase 2. A potentially novel pathway for interferon-gamma-mediated inhibition of gene transcription

Interferon-gamma (IFN-gamma) is a pleiotropic cytokine that modulates the immune function, cell proliferation, apoptosis, macrophage activation, and numerous other cellular responses. These biological actions of IFN-gamma are characterized by both the activation and the inhibition of gene transcription. Unfortunately, in contrast to gene activation, the mechanisms through which the cytokine suppresses gene transcription remain largely unclear. We show here for the first time that exposure of macrophages to IFN-gamma leads to a dramatic induction in the expression of the inducible cAMP early repressor (ICER), a potent inhibitor of gene transcription. In addition, a synergistic action of IFN-gamma and calcium in the activation of ICER expression was identified. The IFN-gamma-mediated activation of ICER expression was not blocked by H89, bisindoylmaleimide, SB202190, PD98059, W7, and AG490, which inhibit protein kinase A, protein kinase C, p38 mitogen-activated protein kinase, extracellular signal-regulated kinase, calcium-calmodulin-dependent protein kinase, and Janus kinase-2, respectively. In contrast, apigenin, a selective casein kinase 2 (CK2) inhibitor, was found to inhibit response. Consistent with this finding, IFN-gamma stimulated CK2 activity and the level of phosphorylated cAMP response element-binding protein, which is known to induce ICER gene transcription, and this response was inhibited in the presence of apigenin. These studies, therefore, identify a previously uncharacterized pathway, involving the IFN-gamma-mediated stimulation of CK2 activity, activation of cAMP response element-binding protein, and increased production of ICER, which may then play an important role in the inhibition of macrophage gene transcription by this cytokine.