The cyclic AMP-responsive element-binding protein (CREB) is phosphorylated in response to a wide variety of signals, yet target gene transcription is only increased in a subset of cases. Recent ...studies indicate that CREB functions in concert with a family of latent cytoplasmic co-activators called cAMP-regulated transcriptional co-activators (CRTCs), which are activated through dephosphorylation. A dual requirement for CREB phosphorylation and CRTC dephosphorylation is likely to explain how these activator-co-activator cognates discriminate between different stimuli. Following their activation, CREB and CRTCs mediate the effects of fasting and feeding signals on the expression of metabolic programmes in insulin-sensitive tissues.
Obesity is thought to promote insulin resistance in part via activation of the innate immune system. Increases in proinflammatory cytokine production by M1 macrophages inhibit insulin signaling in ...white adipose tissue. In contrast, M2 macrophages have been found to enhance insulin sensitivity in part by reducing adipose tissue inflammation. The paracrine hormone prostaglandin E2 (PGE2) enhances M2 polarization in part through activation of the cAMP pathway, although the underlying mechanism is unclear. Here we show that PGE2 stimulates M2 polarization via the cyclic AMP-responsive element binding (CREB)-mediated induction of Krupple-like factor 4 (KLF4). Targeted disruption of CREB or the cAMP-regulated transcriptional coactivators 2 and 3 (CRTC2/3) in macrophages downregulated M2 marker gene expression and promoted insulin resistance in the context of high-fat diet feeding. As re-expression of KLF4 rescued M2 marker gene expression in CREB-depleted cells, our results demonstrate the importance of the CREB/CRTC pathway in maintaining insulin sensitivity in white adipose tissue via its effects on the innate immune system.
3'-5'-Cyclic adenosine monophosphate (cyclic AMP or cAMP) was first described in 1957 as an intracellular second messenger mediating the effects of glucagon and epinephrine on hepatic glycogenolysis ...(Berthet et al., J Biol Chem 224(1):463-475, 1957). Since this initial characterization, cAMP has been firmly established as a versatile molecular signal involved in both central and peripheral regulation of energy homeostasis and nutrient partitioning. Many of these effects appear to be mediated at the transcriptional level, in part through the activation of the transcription factor CREB and its coactivators. Here we review current understanding of the mechanisms by which the cAMP signaling pathway triggers metabolic programs in insulin-responsive tissues.
The salt‐inducible kinase (SIK) family regulates cellular gene expression via the phosphorylation of cAMP‐regulated transcriptional coactivators (CRTCs) and class IIA histone deacetylases, which are ...sequestered in the cytoplasm by phosphorylation‐dependent 14‐3‐3 interactions. SIK activity toward these substrates is inhibited by increases in cAMP signaling, although the underlying mechanism is unclear. Here, we show that the protein kinase A (PKA)‐dependent phosphorylation of SIKs inhibits their catalytic activity by inducing 14‐3‐3 protein binding. SIK1 and SIK3 contain two functional PKA/14‐3‐3 sites, while SIK2 has four. In keeping with the dimeric nature of 14‐3‐3s, the presence of multiple binding sites within target proteins dramatically increases binding affinity. As a result, loss of a single 14‐3‐3‐binding site in SIK1 and SIK3 abolished 14‐3‐3 association and rendered them insensitive to cAMP. In contrast, mutation of three sites in SIK2 was necessary to fully block cAMP regulation. Superimposed on the effects of PKA phosphorylation and 14‐3‐3 association, an evolutionary conserved domain in SIK1 and SIK2 (the so called RK‐rich region; 595–624 in hSIK2) is also required for the inhibition of SIK2 activity. Collectively, these results point to a dual role for 14‐3‐3 proteins in repressing a family of Ser/Thr kinases as well as their substrates.
Salt‐inducible kinases (SIKs) regulate gene expression by phosphorylating cAMP‐regulated transcriptional coactivators (CRTCs) and class IIA histone deacetylases (HDACs), promoting their subsequent interaction with 14‐3‐3 proteins and cytoplasmic sequestration. Although cAMP signalling is known to inhibit SIK activity towards its substrates, the underlying mechanism was unclear. Marc Montminy and colleagues now report that protein kinase A‐mediated phosphorylation inactivates SIKs by inducing 14‐3‐3 protein binding. Their work reveals an interesting role for 14‐3‐3 proteins in inhibiting SIKs and their substrates.
The transcription factor CREB -- for 'cyclic AMP response element-binding protein' -- functions in glucose homeostasis, growth-factor-dependent cell survival, and has been implicated in learning and ...memory. CREB is phosphorylated in response to various signals, but how is specificity achieved in these signalling pathways?
A number of hormones and growth factors have been shown to stimulate target cells via second messenger pathways that in turn regulate the phosphorylation of specific nuclear factors. The second ...messenger cyclic AMP, for example, regulates a striking number of physiologic processes, including intermediary metabolism, cellular proliferation, and neuronal signaling, by altering basic patterns of gene expression. Our understanding of cyclic AMP signaling in the nucleus has expanded considerably over the past decade, owing in large part to the characterization of cyclic AMP-responsive promoter elements, transcription factors that bind them, and signal-dependent coactivators that mediate target gene induction. More importantly, these studies have revealed new insights into biological problems as diverse as biological clocks and long-term memory. The purpose of this review is to describe the components of the cyclic AMP response unit and to analyze how these components cooperate to induce target gene expression in response to hormonal stimulation.
Under fasting conditions, increases in circulating concentrations of pancreatic glucagon maintain glucose homeostasis through induction of gluconeogenic genes by the CREB coactivator CRTC2. Hepatic ...CRTC2 activity is elevated in obesity, although the extent to which this cofactor contributes to attendant increases in insulin resistance is unclear. Here we show that mice with a knockout of the CRTC2 gene have decreased circulating glucose concentrations during fasting, due to attenuation of the gluconeogenic program. CRTC2 was found to stimulate hepatic gene expression in part through an N-terminal CREB binding domain that enhanced CREB occupancy over relevant promoters in response to glucagon. Deletion of sequences encoding the CREB binding domain in CRTC2 ⁻/⁻ mice lowered circulating blood glucose concentrations and improved insulin sensitivity in the context of diet-induced obesity. Our results suggest that small molecules that attenuate the CREB-CRTC2 pathway may provide therapeutic benefit to individuals with type 2 diabetes.
In fasted mammals, circulating pancreatic glucagon stimulates hepatic gluconeogenesis in part through the CREB regulated transcription coactivator 2 (CRTC2, also referred to as TORC2). Hepatic ...glucose production is increased in obesity, reflecting chronic increases in endoplasmic reticulum (ER) stress that promote insulin resistance. Whether ER stress also modulates the gluconeogenic program directly, however, is unclear. Here we show that CRTC2 functions as a dual sensor for ER stress and fasting signals. Acute increases in ER stress triggered the dephosphorylation and nuclear entry of CRTC2, which in turn promoted the expression of ER quality control genes through an association with activating transcription factor 6 alpha (ATF6α, also known as ATF6)--an integral branch of the unfolded protein response. In addition to mediating CRTC2 recruitment to ER stress inducible promoters, ATF6α also reduced hepatic glucose output by disrupting the CREB-CRTC2 interaction and thereby inhibiting CRTC2 occupancy over gluconeogenic genes. Conversely, hepatic glucose output was upregulated when hepatic ATF6α protein amounts were reduced, either by RNA interference (RNAi)-mediated knockdown or as a result of persistent stress in obesity. Because ATF6α overexpression in the livers of obese mice reversed CRTC2 effects on the gluconeogenic program and lowered hepatic glucose output, our results demonstrate how cross-talk between ER stress and fasting pathways at the level of a transcriptional coactivator contributes to glucose homeostasis.
Activating AMPK or inactivating calcineurin slows ageing in Caenorhabditis elegans and both have been implicated as therapeutic targets for age-related pathology in mammals. However, the direct ...targets that mediate their effects on longevity remain unclear. In mammals, CREB-regulated transcriptional coactivators (CRTCs) are a family of cofactors involved in diverse physiological processes including energy homeostasis, cancer and endoplasmic reticulum stress. Here we show that both AMPK and calcineurin modulate longevity exclusively through post-translational modification of CRTC-1, the sole C. elegans CRTC. We demonstrate that CRTC-1 is a direct AMPK target, and interacts with the CREB homologue-1 (CRH-1) transcription factor in vivo. The pro-longevity effects of activating AMPK or deactivating calcineurin decrease CRTC-1 and CRH-1 activity and induce transcriptional responses similar to those of CRH-1 null worms. Downregulation of crtc-1 increases lifespan in a crh-1-dependent manner and directly reducing crh-1 expression increases longevity, substantiating a role for CRTCs and CREB in ageing. Together, these findings indicate a novel role for CRTCs and CREB in determining lifespan downstream of AMPK and calcineurin, and illustrate the molecular mechanisms by which an evolutionarily conserved pathway responds to low energy to increase longevity.
During fasting, mammals maintain normal glucose homeostasis by stimulating hepatic gluconeogenesis. Elevations in circulating glucagon and epinephrine, two hormones that activate hepatic ...gluconeogenesis, trigger the cAMP-mediated phosphorylation of cAMP response element-binding protein (Creb) and dephosphorylation of the Creb-regulated transcription coactivator-2 (Crtc2)-two key transcriptional regulators of this process. Although the underlying mechanism is unclear, hepatic gluconeogenesis is also regulated by the circadian clock, which coordinates glucose metabolism with changes in the external environment. Circadian control of gene expression is achieved by two transcriptional activators, Clock and Bmal1, which stimulate cryptochrome (Cry1 and Cry2) and Period (Per1, Per2 and Per3) repressors that feed back on Clock-Bmal1 activity. Here we show that Creb activity during fasting is modulated by Cry1 and Cry2, which are rhythmically expressed in the liver. Cry1 expression was elevated during the night-day transition, when it reduced fasting gluconeogenic gene expression by blocking glucagon-mediated increases in intracellular cAMP concentrations and in the protein kinase A-mediated phosphorylation of Creb. In biochemical reconstitution studies, we found that Cry1 inhibited accumulation of cAMP in response to G protein-coupled receptor (GPCR) activation but not to forskolin, a direct activator of adenyl cyclase. Cry proteins seemed to modulate GPCR activity directly through interaction with Gsα. As hepatic overexpression of Cry1 lowered blood glucose concentrations and improved insulin sensitivity in insulin-resistant db/db mice, our results suggest that compounds that enhance cryptochrome activity may provide therapeutic benefit to individuals with type 2 diabetes.
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