Interaction between p21-activated protein kinase and Rac during differentiation of HL-60 human promyelocytic leukemia cell induced by all-trans-retinoic acid

Normal and Neoplastic Growth Suppression by the Extended Myc Network

Among the first discovered and most prominent cellular oncogenes is MYC, which encodes a bHLH-ZIP transcription factor (Myc) that both activates and suppresses numerous genes involved in proliferation, energy production, metabolism and translation. Myc belongs to a small group of bHLH-ZIP transcriptional regulators (the Myc Network) that includes its obligate heterodimerization partner Max and six "Mxd proteins" (Mxd1-4, Mnt and Mga), each of which heterodimerizes with Max and largely opposes Myc's functions. More recently, a second group of bHLH-ZIP proteins (the Mlx Network) has emerged that bears many parallels with the Myc Network. It is comprised of the Myc-like factors ChREBP and MondoA, which, in association with the Max-like member Mlx, regulate smaller and more functionally restricted repertoires of target genes, some of which are shared with Myc. Opposing ChREBP and MondoA are heterodimers comprised of Mlx and Mxd1, Mxd4 and Mnt, which also structurally and operationally link the two Networks. We discuss here the functions of these "Extended Myc Network" members, with particular emphasis on their roles in suppressing normal and neoplastic growth. These roles are complex due to the temporal- and tissue-restricted expression of Extended Myc Network proteins in normal cells, their regulation of both common and unique target genes and, in some cases, their functional redundancy.

Strategies to generate functionally normal neutrophils to reduce infection and infection-related mortality in cancer chemotherapy

Neutrophils form an essential part of innate immunity against infection. Cancer chemotherapy-induced neutropenia (CCIN) is a condition in which the number of neutrophils in a patient's bloodstream is decreased, leading to increased susceptibility to infection. Granulocyte colony-stimulating factor (GCSF) has been the only approved treatment for CCIN over two decades. To date, CCIN-related infection and mortality remain a significant concern, as neutrophils generated in response to administered GCSF are functionally immature and cannot effectively fight infection. This review summarizes the molecular regulatory mechanisms of neutrophil granulocytic differentiation and innate immunity development, dissects the biology of GCSF in myeloid expansion, highlights the shortcomings of GCSF in CCIN treatment, updates the recent advance of a selective retinoid agonist that promotes neutrophil granulocytic differentiation, and evaluates the benefits of developing GCSF biosimilars to increase access to GCSF biologics versus seeking a new mode to fundamentally advance GCSF therapy for treatment of CCIN.

Oxyradical stress increases the biosynthesis of 2-arachidonoylglycerol: involvement of NADPH oxidase

NADPH oxidase (Nox)-derived oxyradicals contribute to atherosclerosis by oxidizing low-density lipoproteins (LDL), leading to their phagocytosis by vascular macrophages. Endocannabinoids, such as 2-arachidonoylglycerol (2-AG), might be an important link between oxidative stress and atherosclerosis. We hypothesized that 2-AG biosynthesis in macrophages is enhanced following ligation of oxidized LDL by scavenger receptors via a signal transduction pathway involving Nox-derived ROS that activates diacylglycerol lipase-β (DAGL-β), the 2-AG biosynthetic enzyme. To test this idea, we challenged macrophage cell lines and murine primary macrophages with a xanthine oxidase system or with nonphysiological and physiological Nox stimulants [phorbol 12-myristate 13-acetate (PMA) and arachidonic acid (AA)]. Each stressor increased cellular superoxide levels and enhanced 2-AG biosynthetic activity in a Nox-dependent manner. Levels of cytosolic phospholipase A₂-dependent AA metabolites (eicosanoids) in primary macrophages were also dependent on Nox-mediated ROS. In addition, 2-AG levels in DAGL-β-overexpressing COS7 cells were attenuated by inhibitors of Nox and DAGL-β. Furthermore, ROS induced by menadione (a redox cycling agent) or PMA could be partially attenuated by the cannabinoid 1/2 receptor agonist (WIN 55,212-2). Finally, cells that overexpress Nox2 components (Phox-COS7) synthesized larger amounts of 2-AG compared with the parental COS7 cells. Together, the results suggest a positive correlation between heightened oxygen radical flux and 2-AG biosynthesis in macrophage cell lines and primary macrophages. Because of the antioxidant and anti-inflammatory effects associated with 2-AG, the increased levels of this bioactive lipid might be an adaptive response to oxidative stress. Thus oxyradical stress may be counteracted by the enhanced endocannabinoid tone.

From oncogene to tumor suppressor: the dual role of Myc in leukemia

The transcription factor c-Myc strongly stimulates cell proliferation but also regulates apoptosis, senescence, cell competition and cell differentiation, and its elevated activity is a hallmark for human tumorigenesis. c-Myc induces transcription by forming heterodimers with Max and then directly binding DNA at E-box sequences. Conversely, transcription repression depends primarily on the inhibitory interaction of c-Myc/Max with Miz-1 at DNA initiator elements. We recently described a distinct mechanism of c-Myc gene regulation, in which c-Myc interacts with the retinoic acid receptor α (RARα) and is recruited to RAR DNA binding sequences (RAREs). In leukemia cells, this c-Myc/RARα complex functions either as an activator or a repressor of RARα-dependent targets through a phosphorylation switch. Unphosphorylated c-Myc interacts with RARα to repress the expression of RAR targets required for differentiation, thereby aggravating leukemia malignancy. However, if c-Myc is phosphorylated by the kinase Pak2, the c-Myc/RARα complex activates transcription of those same genes to stimulate differentiation, thus reducing tumor burden. Here, we discuss the role of c-Myc in balancing proliferation and differentiation and how modulating this previously unidentified c-Myc activity might provide alternative therapies against leukemia and possibly other types of tumors.

Retinoic acid as a modulator of the activity of protein kinase Calpha

All-trans-retinoic acid (atRA) is a derivative of vitamin A and possesses antitumor activity. We demonstrate that atRA is able to modulate the activity of protein kinase C alpha (PKCalpha), which is related to tumor development. In vitro, it was found that atRA activated PKCalpha in the presence of Ca(2+) and in the absence of phosphatidylserine, although such activity is considerably inhibited in mutations affecting residues D246 and D248 and also residue N189, all of which are known to be essential for the interaction with Ca(2+) and phosphatidylserine in the C2 domain. It was concluded that atRA substitutes phosphatidylserine although with lower specific activities. However, atRA had a biphasic effect on PKCalpha activity in the presence of activating phospholipids, such as phosphatidylserine and phosphatidylinositol 4,5-bisphosphate, yielding activation at low concentrations but inactivation at higher ones. This second inhibitory characteristic was not shown with K209 and K211 mutations (residues located in the Lys-rich cluster in the C2 domain) in PKCalpha. This interesting effect revealed the importance of phospholipid binding at this site to ensure maximum activity for the wild-type PKCalpha. The C1 domain was not related with the atRA effect on PKCalpha. It was concluded that whereas atRA may activate PKCalpha through the Ca(2+)-phosphatidylserine-binding site of the C2 domain, it may also inhibit the activity of this enzyme when displacing the phospholipid from the Lys-rich cluster also located in the C2 domain.

N-Ras–Induced Growth Suppression of Myeloid Cells Is Mediated by IRF-1

Activating mutations in ras oncogenes occur at high frequency in human malignancies and expression of activated ras in immortalized cells lines is generally transforming. However, somewhat paradoxically, ectopic expression of ras in some myeloid cell lines has been shown to induce growth suppression associated with up-regulation of the cyclin-dependent kinase inhibitor p21CIP1/WAF1 in a p16INK4a, p15INK4b, and p53 independent fashion. We have used cDNA array technology to compare the expression profile induced by activated N-ras (N-rasG13R) in growth-suppressed myeloid cells with that induced in myeloid cells, which are transformed by N-rasG13R. The expression profile induced in growth suppressed cells was consistent with differentiation and included the up-regulation of the transcription factor IFN regulatory factor-1 (IRF-1), a known transcriptional activator of p21CIP/WAF1 expression and a target of oncogenic mutations associated with myeloid leukemia. Antisense suppression of IRF-1 prevented N-rasG13R–associated growth arrest and up-regulation of p21CIP1/WAF1. These results define a novel tumor suppressive response to oncogenic signaling and provide a mechanistic link between growth suppression and differentiation in myeloid cells.

Unravelling the signal-transduction network in B lymphocytes

The Alliance for Cellular Signaling has chosen the mouse B lymphocyte as a model system to understand basic principles that govern cellular signalling. Progress to that end has focused initially on establishing a reproducible experimental cell system and characterizing essential signalling responses. Although unravelling this complex network will take years, findings revealed in the interim will prove immensely useful to the scientific community at large.