The nuclear phosphoinositide 3-kinase/AKT pathway: a new second messenger system

Lipid second messengers, particularly those derived from the polyphosphoinositide cycle, play a pivotal role in several cell signaling networks. Phosphoinositide 3-kinases (PI3Ks) generate specific inositol lipids that have been implicated in a plethora of cell functions. One of the best-characterized targets of PI3K lipid products is the serine/threonine protein kinase Akt. Recent findings have implicated Akt in cancer progression because it stimulates cell proliferation and suppresses apoptosis. Evidence accumulated over the past 15 years has highlighted the presence of an autonomous nuclear inositol lipid metabolism, and suggests that lipid molecules are important components of signaling pathways operating within the nucleus. PI3Ks, their lipid products, and Akt have also been identified at the nuclear level. In this review, we shall summarize the most updated findings about these molecules in relationship with the nuclear compartment and provide an overview of the possible mechanisms by which they regulate important cell functions.     

Intranuclear 3′-phosphoinositide metabolism and Akt signaling: New mechanisms for tumorigenesis and protection against apoptosis?

Lipid second messengers, particularly those derived from the polyphosphoinositide metabolism, play a pivotal role in multiple cell signaling networks. Phosphoinositide 3-kinase (PI3K) generate 3'-phosphorylated inositol lipids that are key players in a multitude of cell functions. One of the best characterized targets of PI3K lipid products is the serine/threonine protein kinase Akt (protein kinase B, PKB). Recent findings have implicated the PI3K/Akt pathway in tumorigenesis because it stimulates cell proliferation and suppresses apoptosis. However, it was thought that this signal transduction network would exert its carcinogenetic effects mainly by operating in the cytoplasm. Evidence accumulated over the past 15 years has highlighted the presence of an autonomous nuclear inositol lipid cycle, and strongly suggests that lipid molecules are important components of signaling pathways operating at the nuclear level. PI3K, its lipid product phosphatidylinositol (3,4,5) trisphosphate (PtdIns(3,4,5)P3), and Akt have been identified within the nucleus and recent data suggest that they counteract apoptosis also by operating in this cell compartment through a block of caspase-activated DNase and inhibition of chromatin condensation. In this review, we shall summarize the most updated and intriguing findings about nuclear PI3K/PtdIns(3,4,5)P3/Akt in relationship with tumorigenesis and suppression of apoptotic stimuli.     

Thematic Review Series: Bile Acids: Bile acids as regulatory molecules

In the past, bile acids were considered to be just detergent molecules derived from cholesterol in the liver. They were known to be important for the solubilization of cholesterol in the gallbladder and for stimulating the absorption of cholesterol, fat-soluble vitamins, and lipids from the intestines. However, during the last two decades, it has been discovered that bile acids are regulatory molecules. Bile acids have been discovered to activate specific nuclear receptors (farnesoid X receptor, preganane X receptor, and vitamin D receptor), G protein coupled receptor TGR5 (TGR5), and cell signaling pathways (c-jun N-terminal kinase 1/2, AKT, and ERK 1/2) in cells in the liver and gastrointestinal tract. Activation of nuclear receptors and cell signaling pathways alter the expression of numerous genes encoding enzyme/proteins involved in the regulation of bile acid, glucose, fatty acid, lipoprotein synthesis, metabolism, transport, and energy metabolism. They also play a role in the regulation of serum triglyceride levels in humans and rodents. Bile acids appear to function as nutrient signaling molecules primarily during the feed/fast cycle as there is a flux of these molecules returning from the intestines to the liver following a meal. In this review, we will summarize the current knowledge of how bile acids regulate hepatic lipid and glucose metabolism through the activation of specific nuclear receptors and cell signaling pathways.     

Protein kinase C isoforms and lipid second messengers: a critical nuclear partnership?

A growing body of evidence, accumulated over the past 15 years, has highlighted that the protein kinase C family of isozymes is capable of translocating to the nucleus or is resident within the nucleus. The comprehension of protein kinase C isoform regulation within this organelle is under development. At present, it is emerging that lipid second messengers may play at least two roles in the control of nuclear protein kinase C: on one side they serve as chemical attractants, on the other they directly modulate the activity of specific isoforms. One of the best characterized lipid second messenger that could be involved in the regulation of nuclear PKC activity is DAG. The existence of two separate pools of nuclear DAG suggests that this lipid second messenger might be involved in distinct pathways that lead to different cell responses. Nuclear phosphatidylglycerol, D-3 phosphorylated inositol lipids and nuclear fatty acids are involved in a striking variety of critical biological functions which may act by specific PKC activation. The fine tuning of PKC regulation in cells subjected to proliferating or differentiating stimuli, might prove to be of great interest also for cancer therapy, given the fact that PKC-dependent signaling pathways are increasingly being seen as possible pharmacological target in some forms of neoplastic diseases. In this article, we review the current knowledge about lipid second messengers that are involved in regulating the translocation and/or the activity of different protein kinase C isoforms identified at the nuclear level.     

Nuclear phosphatidylinositol 3,4,5-trisphosphate, phosphatidylinositol 3-kinase, Akt, and PTen: emerging key regulators of anti-apoptotic signaling and carcinogenesis

Inositol lipid-derived second messengers have long been known to have an important regulatory role in cell physiology. Phosphatidylinositol 3-kinase (PI3K) synthesizes the second messenger 3,4,5'-phosphatidylinositol trisphosphate (Ptdlns 3,4,5P3) which controls a multitude of cell functions. Down-stream of PI3K/PtdIns 3,4,5P3 is the serine/threonine protein kinase Akt (protein kinase B, PKB). Since the PI3K/ PtdIns 3,4,5P3 /Akt pathway stimulates cell proliferation and suppresses apoptosis, it has been implicated in carcinogenesis. The lipid phosphatase PTEN is a negative regulator of this signaling network. Until recently, it was thought that this signal transduction cascade would promote its anti-apoptotic effects when activated in the cytoplasm. Several lines of evidence gathered over the past 20 years, have highlighted the existence of an autonomous nuclear inositol lipid cycle, strongly suggesting that lipids are important components of signaling pathways operating at the nuclear level. PI3K, PtdIns(3,4,5)P3, Akt, and PTEN have been identified within the nucleus and recent findings suggest that they are involved in cell survival also by operating in this organelle, through a block of caspase-activated DNase and inhibition of chromatin condensation. Here, we shall summarize the most updated and intriguing findings about nuclear PI3K/ PtdIns(3,4,5)P3/Akt/PTEN in relationship with carcinogenesis and suppression of apoptosis.     

Transit of hormonal and EGF receptor-dependent signals through cholesterol-rich membranes

The functional consequences of changes in membrane lipid composition that coincide with malignant growth are poorly understood. Sufficient data have been acquired from studies of lipid binding proteins, post-translational modifications of signaling proteins, and biochemical inhibition of lipidogenic pathways to indicate that growth and survival pathways might be substantially re-directed by alterations in the lipid content of membranes. Cholesterol and glycosphingolipids segregate into membrane patches that exhibit a liquid-ordered state in comparison to membrane domains containing relatively lower amounts of these classes of lipids. These "lipid raft" structures, which may vary in size and stability in different cell types, both accumulate and exclude signaling proteins and have been implicated in signal transduction through a number of cancer-relevant pathways. In prostate cancer cells, signaling from epidermal growth factor receptor (EGFR) to the serine-threonine kinase Akt1, as well as from IL-6 to STAT3, have been demonstrated to be influenced by experimental interventions that target cholesterol homeostasis. The recent finding that classical steroid hormone receptors also reside in these microdomains, and thus may function within these structures in a signaling capacity independent of their role as nuclear factors, suggests a novel means of cross-talk between receptor tyrosine kinase-derived and steroidogenic signals. Potential points of intersection between components of the EGFR family of receptor tyrosine kinases and androgen receptor signaling pathways, which may be sensitive to disruptions in cholesterol metabolism, are discussed. Understanding the manner in which these pathways converge within cholesterol-rich membranes may present new avenues for therapeutic intervention in hormone-dependent cancers.     

Nuclear inositide signaling: An appraisal of phospholipase C β1 behavior in myelodysplastic and leukemia cells

Location impinges on function of some of the main players of nuclear inositol lipid cycle. Here we have discussed the behavior of PI-PLCβ1 in myelodysplastic and cultured leukemia cells.The presence of a cryptic deletion of PI-PLCβ1 gene in high-risk MDS patients is accompanied by altered expression of its mRNA in that the overall decrease of mRNA is characterized by a dramatic decrease of the splicing variant 1a, which is cytosolic and partially nuclear, whilst the splicing variant 1b is still highly represented. This suggests that altered expression of nuclear PI-PLCβ1 could be involved in a disregulation of the cell cycle and have also important effects on cell apoptotic pathways.Moreover, in cultured leukemia cells (Felc) it has been reported by means of a proteomic approach that the splicing factor SRp20 interacts with nuclear PI-PLCβ1 and its expression is modulated by this signaling molecule.All in all, it appears more and more evident that nuclear signaling elicited by PI-PLCβ1 is a key event in the control of cell cycle progression.     

Roles for inositol polyphosphate kinases in the regulation of nuclear processes and developmental biology

Our laboratory studies the biology and enzyme regulation of inositol signal transduction pathways, which are activated in response to a wide range of stimuli. As a six-carbon cyclitol, inositol and its numerous phosphorylated derivatives efficiently generate combinatorial ensembles of signaling molecules. Through the cloning and characterization of inositol polyphosphate kinases (IPK), novel roles for inositol tetrakisphosphate (IP₄), inositol pentakisphosphate (IP₅), and inositol hexakisphosphate (IP₆) and inositol pyrophosphates (PP-IPs), have been identified. Studies have linked the IPKs and their inositide products to the regulation of nuclear processes including gene expression, chromatin remodeling, mRNA export, DNA repair and telomere maintenance. Analysis of IPK knockout animals has revealed a role for production of IPs in regulation of embryogenesis and organism development.The discoveries of the IPK proteins and their connection to nuclear signaling have generated significant interest in the field. Furthermore, they have provided interesting clues into the evolution of inositide-signaling pathways. Ipk2/IPMK and IPS/IP6K family members are conserved from yeast to man. In contrast, the IP₃ 3-kinase (ITPK) branch is observed in selected metazoans and not in plant or fungi. This may imply that Ipk2 and IPS activities evolved first among the group. The promiscuity of the Ipk2 protein further supports this notion and may provide the cell with a means to generate many IP species in a genetically economical fashion. Studies of yeast inositide signaling reveal that these simple eukaryotes do not have an IP₃ receptor in their genome and do not utilize diacylglycerol to activate protein kinase C. Thus, it appears that the canonical “text book” aspects of inositide-signaling pathways are not conserved throughout eukaryotic evolution. In light of the conservation of Ipk2/IPMK, Ipk1 and IPS/IP6K pathways from yeast to man it is interesting to speculate that a primordial role of phospholipase C-induced, IPK-dependent inositide signaling was to regulate nuclear processes. As calcium and PKC signaling evolved in metazoans, these may have greatly enhanced signaling capabilities. Recent studies demonstrating an essential role for IP₅, IP₆ and possibly PP-IP production in metazoan development highlight the importance of IPK signaling in cellular responses in metazoans. With these thoughts in mind, we eagerly await future studies aimed at further elucidating how these signaling codes participate in developmental processes and the control of gene expression, mRNA export, and DNA metabolism.     

“New”‐clear functions of PDK1: Beyond a master kinase?

Activation of cytosolic phosphoinositide-3 kinase (PI-3K) signaling pathway has been well established to regulate gene expression, cell cycle, and survival by feeding signals to the nucleus. In addition, strong evidences accumulated over the past few years indicate the presence of an autonomous inositol lipid metabolism and PI-3K signaling within the nucleus. Much less, however, is known about the role and regulation of this nuclear PI-3K pathway. Components of the PI-3K signaling pathway, including PI 3-kinase and its downstream kinase Akt, have been identified at the nuclear level. Consistent with the presence of a complete PI-3K signaling pathway in the nucleus, we have recently found that phosphoinositide-dependent kinase 1 (PDK1), a kinase functioning downstream of PI-3K and upstream of Akt, is a nucleo-cytoplasmic shuttling protein. In the present review, we update our current knowledge on the regulatory mechanisms and the functional roles of PDK1 nuclear translocation. We also summarize some of the kinase-independent activities of PDK1 in cell signaling.     

Fibronectin–integrin mediated signaling in human cervical cancer cells (SiHa)

Interaction between cell surface integrin receptors and extracellular matrix (ECM) components plays an important role in cell survival, proliferation, and migration, including tumor development and invasion of tumor cells. Matrix metalloproteinases (MMPs) are a family of metalloproteinases capable of digesting ECM components and are important molecules for cell migration. Binding of ECM to integrins initiates cascades of cell signaling events modulating expression and activity of different MMPs. The aim of this study is to investigate fibronectin–integrin-mediated signaling and modulation of MMPs. Our findings indicated that culture of human cervical cancer cell (SiHa) on fibronectin-coated surface perhaps sends signals via fibronectin–integrin-mediated signaling pathways recruiting focal adhesion kinase (FAK) extracellular signal regulated kinase (ERK), phosphatidyl inositol 3 kinase (PI-3K), integrin-linked kinase (ILK), nuclear factor-kappa B (NF-κB), and modulates expression and activation of mainly pro-MMP-9, and moderately pro-MMP-2 in serum-free culture medium.     

Cell cycle regulators in cancer cell metabolism

Cancer proliferation and progression involves altered metabolic pathways as a result of continuous demand for energy and nutrients. In the last years, cell cycle regulators have been involved in the control of metabolic processes, such as glucose and insulin pathways and lipid synthesis, in addition to their canonical function controlling cell cycle progression. Here we describe recent data demonstrating the role of cell cycle regulators in the metabolic control especially in studies performed in cancer models. Moreover, we discuss the importance of these findings in the context of current cancer therapies to provide an overview of the relevance of targeting metabolism using inhibitors of the cell cycle regulation.     

Nuclear prostaglandin receptors: role in pregnancy and parturition?

The key regulatory role of prostanoids [prostaglandins (PGs) and thromboxanes (TXs)] in the maintenance of pregnancy and initiation of parturition has been established. However, our understanding of how these events are fine-tuned by the recruitment of specific signaling pathways remains unclear. Whereas, initial thoughts were that PGs were lipophilic and would easily cross cell membranes without specific receptors or transport processes, it has since been realized that PG signaling occurs via specific cell surface G-protein coupled receptors (GPCRs) coupled to classical adenylate cyclase or inositol phosphate signaling pathways. Furthermore, specific PG transporters have been identified and cloned adding a further level of complexity to the regulation of paracrine action of these potent bioactive molecules. It is now apparent that PGs also activate nuclear receptors, opening the possibility of novel intracrine signaling mechanisms. The existence of intracrine signaling pathways is further supported by accumulating evidence linking the perinuclear localization of PG synthesizing enzymes with intracellular PG synthesis. This review will focus on the evidence for a role of nuclear actions of PGs in the regulation of pregnancy and parturition.     

Rapid regulatory actions of sex steroids on cell movement through the actin cytoskeleton

Cell movement is required in relevant physiological processes such as embryonic development, tissue and organ differentiation, inflammation, immune response and wound healing, along with pathological phenomena, such as cancer metastatic spread. Cell motility is tightly controlled by a complex and often redundant array of intracellular signaling pathways largely devoted to the dynamic regulation of the actin cytoskeletal network and of its relationship with the cell membrane and the extracellular matrix. Sex steroids, particularly estrogen and progesterone, are effective regulators of cell migration and tissue organization, and recent evidence indicates that this is in part obtained through the regulation of the cytoskeleton. Intriguingly, many of these regulatory actions related to cell movement are achieved through rapid, non-classical signaling of sex steroid receptors to kinase cascades, independently from nuclear alteration of gene expression or protein synthesis. The identification of the mechanistic basis for these rapid actions on cell cytoskeleton and cell movement has special relevance for the characterization of the effects of sex steroids in physiological conditions, such their role in the control of inflammation, brain or vascular cell remodelling, angiogenesis or wound healing, as well as in the context of pathological conditions such as steroid-sensitive cancer cell invasion and metastasis. This review highlights the physiological and clinical conditions where the regulatory effects on the cytoskeleton and cell movement of sex steroids might have a special importance, as well as the recent advances in the characterization of the mechanisms, providing insights and working hypotheses on possible clinical applications for the modulation of these pathways.     

The many faces of ubiquitinated histone H2A: insights from the DUBs

Increasing knowledge on the cell cycle deregulations in cancers has promoted the introduction of phytochemicals, which can either modulate signaling pathways leading to cell cycle regulation or directly alter cell cycle regulatory molecules, in cancer therapy. Most human malignancies are driven by chromosomal translocations or other genetic alterations that directly affect the function of critical cell cycle proteins such as cyclins as well as tumor suppressors, e.g., p53. In this respect, cell cycle regulation and its modulation by curcumin are gaining widespread attention in recent years. Extensive research has addressed the chemotherapeutic potential of curcumin (diferuloylmethane), a relatively non-toxic plant derived polyphenol. The mechanisms implicated are diverse and appear to involve a combination of cell signaling pathways at multiple levels. In the present review we discuss how alterations in the cell cycle control contribute to the malignant transformation and provide an overview of how curcumin targets cell cycle regulatory molecules to assert anti-proliferative and/or apoptotic effects in cancer cells. The purpose of the current article is to present an appraisal of the current level of knowledge regarding the potential of curcumin as an agent for the chemoprevention of cancer via an understanding of its mechanism of action at the level of cell cycle regulation. Taken together, this review seeks to summarize the unique properties of curcumin that may be exploited for successful clinical cancer prevention.     

Physiology and pathology of nuclear phospholipase C β1

The existence and function of inositide signaling in the nucleus is well documented and we know that the existence of the inositide cycle inside the nucleus has a biological role. An autonomous lipid-dependent signaling system, independently regulated from its plasma membrane counterpart, acts in the nucleus and modulates cell cycle progression and differentiation.We and others focused on PLCβ1, which is the most extensively investigated PLC isoform in the nuclear compartment. PLCβ1 is a key player in the regulation of nuclear inositol lipid signaling, and, as discussed above, its function could also be involved in nuclear structure because it hydrolyses PtdIns(4,5)P2, a well accepted regulator of chromatin remodelling. The evidence, in a number of patients with myelodysplastic syndromes, that the mono-allelic deletion of PLCβ1 is associated with an increased risk of developing acute myeloid leukemia paves the way for an entirely new field of investigation. Indeed the genetic defect evidenced, in addition to being a useful prognostic tool, also suggests that altered expression of this enzyme could have a role in the pathogenesis of this disease, by causing an imbalance between proliferation and apoptosis. The epigenetics of PLCβ1 expression in MDS has been reviewed as well.     

Antiproliferative Effects and Mechanisms of Liver X Receptor Ligands in Pancreatic Ductal Adenocarcinoma Cells

Pancreatic ductal adenocarcinoma (PDAC) is difficult to detect early and is often resistant to standard chemotherapeutic options, contributing to extremely poor disease outcomes. Members of the nuclear receptor superfamily carry out essential biological functions such as hormone signaling and are successfully targeted in the treatment of endocrine-related malignancies. Liver X receptors (LXRs) are nuclear receptors that regulate cholesterol homeostasis, lipid metabolism, and inflammation, and LXR agonists have been developed to regulate LXR function in these processes. Intriguingly, these compounds also exhibit antiproliferative activity in diverse types of cancer cells. In this study, LXR agonist treatments disrupted proliferation, cell-cycle progression, and colony-formation of PDAC cells. At the molecular level, treatments downregulated expression of proteins involved in cell cycle progression and growth factor signaling. Microarray experiments further revealed changes in expression profiles of multiple gene networks involved in biological processes and pathways essential for cell growth and proliferation following LXR activation. These results establish the antiproliferative effects of LXR agonists and potential mechanisms of action in PDAC cells and provide evidence for their potential application in the prevention and treatment of PDAC.