Cycling through metabolism

Abstract

Since the discovery of cyclins, the role of cell cycle regulators in the control of cell proliferation has been extensively studied. It is clear that proliferation requires an adapted metabolic response of the cells; hence the regulation of cell cycle must be linked to metabolic control. While at a much slower pace, the impact that the activities of cell cycle regulators such as cyclins, cyclin dependent kinases or E2F factor, transcription factor have on cell metabolism are also being uncovered. Here we will focus on recent data implicating cell cycle regulators in metabolic control, with particular attention to studies performed using mouse models. Furthermore, we will discuss the possible relevance of these findings in the context of metabolic disorders such as obesity or diabetes.

Figure 1. Molecular Regulation of cell cycle

Mitogen-induced signal transduction pathways activates the cyclin D-CDK complex early in G1 phase by: (i) induction of cyclin D transcription, translation, stability, (ii) assembly with CDK partners and (iii) import of the holoenzyme to the nucleus. All these process induce an accumulation of active cyclin D-CDK complex during G1 phase. Activation of these complexes leads to partial inactivation by phosphorylation of the pocket proteins, comprising Rb, p107 and p130. Inactivated pocket proteins will release E2Fs transcription factor activity and thus, allow the expression of E-type cyclins, necessary for G1/S transition. CDKs are further activated during cell cycle by A-type cyclins to drive transition form S phase to mitosis, a period known as G2 phase. The activities and functions of cyclin/Cdk complexes are regulated by CKIs both under normal as well as extreme conditions, such as stress, DNA damage and others. There are two families of CKIs. The first includes the INK4 proteins that specifically bind and inhibit the catalytic subunits of CDK4 and CDK6. The INK4 family includes four members p16INK4a, p15INK4b, p18INK4c and p19INK4d. The locus that encodes p16INK4a also directs the expression of a second unrelated protein designated as p19ARF. The second family of CKIs is termed Cip/Kip and has a broad action of inhibition including the activities of both cyclins and CDKs. This class includes p21Cip1, p27Kip1 and p57Kip2.

Figure 2. Schematic representation of the participation of cell cycle regulators in the function of four main metabolic tissues

In pancreas, E2F1 regulates the expression of genes, such as Kir6.2 implicated in insulin secretion, as described in the text. In addition, E2F1, cdk4 and pRB participate in the control of β-cell growth and replication. Impairment of the function of these cell cycle regulators in pancreas often results in diabetes in mice. White adipose tissue (WAT) is another important metabolic tissue that controls whole body lipids and glucose homeostasis. Cyclins D, cdk4, E2F1 and pRB, as well as CKI have been directly implicated in adipose tissue differentiation and function. This is described in the text in the section concerning obesity. Finally, recent literature implicates E2F1, cdk4 and pRB in the oxidative metabolism of muscle. Participation of these factors in liver is likely, although not yet demonstrated.

Figure 3. General overview of the dual role of the cdk-pRB-E2F complex in the regulation of both metabolism and cancer

Under the proper stimuli cell cycle regulators trigger proliferation of the cells. This is accompanied by an adapted metabolic response that includes inhibition of oxidative metabolism, increased glycolysis and lipid synthesis. This is particularly relevant for cancer cells, which require these precise changes in metabolism. Under particular conditions, however, when cells are not ‘primed’ to proliferate, cell cycle regulators are pure metabolic regulators, for instance in response to fasting or hormonal induction. Deregulation of this system can therefore result in the development or progression of metabolic pathologies such as obesity or diabetes.

Figure 4. Participation of cell cycle regulators in energy homeostasis

E2F1 has two distinct roles. First, it regulates the expression of PDK4, which inhibits the activity of PDH, and therefore blocks the conversion of pyruvate in to acetyl CoA. Second, associated to pRB E2F1 modulates the expression of key genes implicated in mitochondrial biogenesis or oxidative phosphorylation (OXPHOS), such as Top1MT or PGC-1α. Overall, E2F1 negatively regulates mitochondrial function. Cyclin D1 has an E2F1-independent role in the control of oxidative metabolism. It directly modulates the activity of the transcription factor NRF-1 thereby inhibiting OXPHOS. In addition cyclin D1 facilitates the expression of HKII facilitating glycolysis. Concomitant increase in glycolysis and blockade of OXPHOS may result in the accumulation of TCA intermediates that leave the TCA cycle and the mitochondria in order to provide substrates for biosynthetic processes, such as lipid synthesis. In red and blue color are indicated the factors or processes that are, respectively, inhibited or activated by cell cycle regulators. NRF-1, nuclear respiratory factor-1; PGC-1α, PPAR gamma coactivator-1, Top1MT, mitochondrial topoisomerase 1; PDK4, pyruvate dehydrogenase kinase-4, PDH, pyruvate dehydrogenase; HKII, hexokinase II; TCA, tricarboxylic cycle.

Figure 5. Schematic representation of the dual role of cell cycle regulators in pancreatic growth and function

Proliferation of β-cells or precursors is regulated by the classical cdk-pRB-E2F pathway. In this way, proliferative stimuli activates cdk4 that phosphorylates the retinoblastoma protein pRB, thereby releasing E2F activity which regulates the expression of genes, such as TK or DHFR implicated in cell division. In this context, CKIs, such as p27 inhibit cdk4 action and have a negative role in β-cell expansion. In addition to the control of β-cell proliferation cell cycle regulators participate in the insulin secretion process of β-cells. Under glucose-stimulated insulin signalling the cyclin D/cdk holoenzyme phosphorylates pRB and facilitates the expression of Kir6.2 gene under the control of E2F1. Expression of the ATP-dependent Kir6.2 potassium channel is required for insulin secretion.

Figure 6. Participation of cell cycle regulators in the different stages of adipogenesis

During the clonal expansion phase of adipocyte differentiation E2F1 regulates the expression of genes implicated in the entry of the cells into cell cycle. In addition, E2F1 regulates, at this stage the expression of the master regulator of adipogenesis, PPARγ. pRB represses E2F1 activity, whereas cdk4 represses pRB, and therefore activates E2F1. In terminal differentiation cyclins and cyclin-dependent kinases still participate in the biology of adipocytes through regulation of the activity of PPARγ in an E2F-independent manner. pRB can also directly repress PPARγ activity at this stage.