Fig. 1.

Engaging the mammalian clock regulatory network in signal transduction. Interlocked systems are represented by independent panels (i.e., core clock, cell division, nicotinamide adenine dinucleotide [NAD] metabolism, clock-controlled genes, and output metabolic processes) where major players and regulatory relationships are connected by arrows (dashed arrows indicate that other intermediaries or processes might exist). As summarized in the center-left panel, the core components of the mammalian clock consist of casein kinase 1ε/δ (CK1); chryptochrome proteins (CRY); period proteins (PER); circadian locomotor output cycles kaput protein (CLOCK); and brain-muscle-aryl hydrocarbon receptor nuclear translocator-like 1 protein (BMAL1). Briefly, CLOCK/BMAL1 heterodimers selectively bind to E-box enhancers and drive the expression of PER, CRY, and REV-ERBα genes. REV-ERBα proteins then repress BMAL1 transcription through Rev-Erbα/retinoic acid-related orphan receptor (ROR) elements (RORE) in its promoter. Thus, BMAL1 RNA falls while PER and CRY RNA levels peak. As the day progresses, PER proteins accumulate in the cytoplasm, become phosphorylated by CK1, ubiquitinated by E3-ligases (e.g., F-box/LRR-repeat protein 3 [Fbxl3] and F-box/WD repeat-containing protein 1 [Fbw1]), and targeted for degradation by the proteasome system. Later in the day, CRY accumulates, associates with PER/CK1, and the trimeric complex translocates to the nucleus where CRY disrupts the CLOCK/BMAL1-associated transcriptional complex, resulting in CRY, PER, and REV-ERBα transcriptional inhibition and de-repression of BMAL1 transcription. Thus, both transcriptional feedback loops are co-regulated by CLOCK/BMAL1. In addition, core clock components modulate the expression of clock-controlled genes (ccgs) that encode for intermediaries in processes that relate to cell growth, division, death, and maintenance, cell communication and metabolite transport, redox state, detoxification and stress response, carbohydrate, nucleobase, and amino acid metabolisms, extracellular adhesion and communication, protein turnover, hormone synthesis and secretion, and lipid synthesis and accumulation, among other clock-controlled responses (lower panels [34]). Points of intersection between circadian components and the cell division machinery exist and are relevant to the timely progression of the cell cycle (for review see Hunt and Sassone-Corsi [35] and Antoch and Kondratov [95]). For example, PER/CRY modulates CCND1, c-MYC, and the cyclin-dependent inhibitor p21 expression, therefore influencing G1 initiation and progression. Furthermore, CK1 activity is implicated in progression through the S and G2 phases by targeting cyclin A/Cdk complexes and expression of WEE1, a dual-specificity kinase that phosphorylates cyclin B/Cdc2 complex for inhibition and G2 arrest. CRY (through its interaction with Hausp) and PER (by stabilizing p53) influence checkpoint activation in response to genotoxic stress [74, 75]. An additional intersecting loop worth mentioning in the context of clock functioning refers to the role of the adenosine monophosphate-activated protein kinase (AMPK) as a metabolic sensor [76]. When activated, AMPK signals back to the clock core by phosphorylating PER and CRY and promoting their degradation, creating a reciprocal loop between the clock and metabolism. AMPK AMP-activated protein kinase, ATM ataxia-telangectasia mutated, ATR ataxia telangiectasia and Rad3-related protein, ATRIP ATR interacting protein, ccg clock-controlled gene, Cdc25A cell division cycle 25A phosphatase, Cdk2 cyclin-dependent kinase 2, Chk1 checkpoint kinase 1, Chk2 checkpoint kinase 2, cycA, B cyclins A and B, cycD cyclin D, cycE cyclin E, DBP D site of albumin promoter (albumin D-box) binding protein, GI gastrointestinal, G2 gap 2 phase, GSK3 glycogen synthase kinase-3, Hausp herpesvirus-associated ubiquitin-specific protease, M mitosis phase, NMNAT mononucleotide adenylyltransferase 1, NAMPT nicotinamide phosphoribosyl-transferase, P phosphate, PPARs peroxisome proliferator-activated receptors, REV-ERB (NR1D1) nuclear receptor subfamily 1, group D, member 1, S DNA synthesis phase, SIRT1 NAD-dependent deacetylase sirtuin-1, TIM timeless, WEE1 Mitosis inhibitor protein kinase