Increased generation of reactive oxygen species (ROS) and an altered redox status have long been observed in cancer cells, and recent studies suggest that this biochemical property of cancer cells can be exploited for therapeutic benefits. Cancer cells in advanced stage tumours frequently exhibit multiple genetic alterations and high oxidative stress, suggesting that it might be possible to preferentially eliminate these cells by pharmacological ROS insults. However, the upregulation of antioxidant capacity in adaptation to intrinsic oxidative stress in cancer cells can confer drug resistance. Abrogation of such drug-resistant mechanisms by redox modulation could have significant therapeutic implications. We argue that modulating the unique redox regulatory mechanisms of cancer cells might be an effective strategy to eliminate these cells.

Targeting cancer cells by ROS-mediated mechanisms: a radical therapeutic approach?

Mechanism of Activation of the Raf-Erk Signaling Pathway by Oncogenic Mutations of B-Raf

How does the coagulation protease thrombin regulate cellular behaviour? The protease-activated receptors (PARs) provide one answer. In concert with the coagulation cascade, these receptors provide an elegant mechanism linking mechanical information in the form of tissue injury or vascular leakage to cellular responses. Roles for PARs are beginning to emerge in haemostasis and thrombosis, inflammation, and perhaps even blood vessel development.

Thrombin signalling and protease-activated receptors

Immune responses participate in every phase of atherosclerosis. There is increasing evidence that both adaptive and innate immunity tightly regulate atherogenesis. Although improved treatment of hyperlipidaemia reduces the risk for cardiac and cerebral complications of atherosclerosis, these remain among the most prevalent of diseases and will probably become the most common cause of death globally within 15 years. This Review focuses on the role of immune mechanisms in the formation and activation of atherosclerotic plaques, and also includes a discussion of the use of inflammatory markers for predicting cardiovascular events. We also outline possible future targets for prevention, diagnosis and treatment of atherosclerosis.

The immune response in atherosclerosis: a double-edged sword

This review of the role of platelets in atherothrombosis illuminates the surprisingly numerous activities of these tiny, anucleate cells and stresses their participation in the inflammatory component of atherothrombosis. The information is presented in the context of the prevention of atherothrombosis by antiplatelet agents now in clinical use.

/pdf/platelet-activation-and-atherothrombosis-4afsrhs7f0.pdf

Platelet Activation and Atherothrombosis

CD40 ligand (CD40L, CD154), a transmembrane protein structurally related to the cytokine TNF-alpha, was originally identified on stimulated CD4+ T cells, and later on stimulated mast cells and basophils. Interaction of CD40L on T cells with CD40 on B cells is of paramount importance for the development and function of the humoral immune system. CD40 is not only constitutively present on B cells, but it is also found on monocytes, macrophages and endothelial cells, suggesting that CD40L has a broader function in vivo. We now report that platelets express CD40L within seconds of activation in vitro and in the process of thrombus formation in vivo. Like TNF-alpha and interleukin-1, CD40L on platelets induces endothelial cells to secrete chemokines and to express adhesion molecules, thereby generating signals for the recruitment and extravasation of leukocytes at the site of injury. Our results indicate that platelets are not only involved in haemostasis but that they also directly initiate an inflammatory response of the vessel wall.

CD40 ligand on activated platelets triggers an inflammatory reaction of endothelial cells

Growth factor-induced signalling leads to activation of members of the Ras family and subsequent stimulation of different Raf isoforms. Within the mechanism of Raf activation, two isoforms of Raf, cRaf and BRaf, may cooperate. We investigated the relationship between cRaf and BRaf and found that active Ras induced heterodimerization of cRaf and BRaf, an effect that was dependent on the serine residue at position 621 of cRAF: Moreover, we also found that cRaf COOH-terminus constitutively associated with BRaf, whereas the NH(2) terminus did not, even in the presence of active RAS: These data suggest that Ras induces the cRaf-BRaf complex formation through the exposure of 14-3-3 binding sites in the COOH-terminus of cRAF: Thus, Ras-induced cRaf-Braf heterodimerization may explain the observed cooperativity of cRaf and BRaf in cells responding to growth factor signals.

https://cancerres.aacrjournals.org/content/canres/61/9/3595.full.pdf

Active Ras induces heterodimerization of cRaf and BRaf.

CD40 is a type I member of the tumour necrosis factor (TNF) receptor superfamily of proteins, and is present on a wide variety of cells including vascular endothelial cells. Ligation of this receptor on endothelial cells is known to increase expression of inflammatory adhesion molecules. We have recently demonstrated that platelets express the ligand of CD40 (CD154) within seconds of exposure to agonist, and interact with endothelial cells to participate directly in the induction of an inflammatory response. Here we show that activated platelets induce tissue factor (TF) expression on endothelial cells in a CD40/CD154-dependent manner, and that the magnitude of this response can equal that induced by TNFα. Moreover, CD40 ligation on endothelial cells downregulates the expression of thrombomodulin. We also show that CD40-mediated TF expression is less sensitive to inhibition with the oxidative radical scavenger pyrrolidine dithiocarbamate than is that mediated by TNFα, indicating that CD40 has a distinct signalling pathway. Tissue factor is a cell membrane protein which functions as the main trigger of the extrinsic pathway of blood coagulation, and its expression on endothelial cells is implicated in wound healing and angiogenesis. Since platelets are among the first cells involved in haemostasis following tissue injury, our data showing that ligation of CD40 by CD154 induces a procoagulant phenotype on vascular endothelial cells suggests that platelets may play an important role in the induction of wound healing.

Activated Platelets Induce Tissue Factor Expression on Human Umbilical Vein Endothelial Cells by Ligation of CD40

Differential expression and regulation of protease-activated receptors in human peripheral monocytes and monocyte-derived antigen-presenting cells

Acetyl-boswellic acids (acetyl-BA) are pentacyclic triterpenes derived from the gum resin of frankincense. We have previously shown that these compounds are effective cytotoxic agents, acting through a mechanism that appears to involve the inhibition of topoisomerase activity. We have now investigated the mechanism of action of acetyl-BA and show that these compounds are more potent inhibitors of human topoisomerases I and IIalpha than camptothecin, and amsacrine or etoposide, respectively. Our data demonstrate that acetyl-BA and, to a lesser extent, some other pentacyclic triterpenes, such as betulinic acid, ursolic acid, and oleanolic acid, inhibit topoisomerases I and IIalpha through a mechanism that does not involve stabilization of the cleavable complex or the intercalation of DNA. Surface plasmon resonance analysis revealed that topoisomerases I and IIalpha bind directly to an immobilized derivative of acetyl-BA. This acetyl-BA derivative interacts with human topoisomerases through high-affinity binding sites yielding K(D) values of 70.6 nM for topoisomerase I and 7.6 nM for topoisomerase IIalpha. Based on our data, we propose that acetyl-BA inhibit topoisomerases I and IIalpha through competition with DNA for binding to the enzyme. Thus, acetyl-BA are a unique class of dual catalytic inhibitors of human topoisomerases I and IIalpha.

Joseph R. Slupsky

Papers

CD40 ligand on activated platelets triggers an inflammatory reaction of endothelial cells

Active Ras induces heterodimerization of cRaf and BRaf.

Activated Platelets Induce Tissue Factor Expression on Human Umbilical Vein Endothelial Cells by Ligation of CD40

Differential expression and regulation of protease-activated receptors in human peripheral monocytes and monocyte-derived antigen-presenting cells

Acetyl-boswellic acids are novel catalytic inhibitors of human topoisomerases I and IIalpha.