AKT/PKB signaling: navigating downstream.

The regulation of cell growth and survival can be subverted by a variety of genetic defects that alter transcriptional programs normally responsible for controlling cell number. High throughput analysis of these gene expression patterns should ultimately lead to the identification of minimal expression profiles that will serve as common denominators in assigning a cancer to a given category. In the course of defining the common denominators, though, we should not be too surprised to find that cancers within a single category may nevertheless exhibit seemingly disparate genetic defects. The wnt pathway has already provided an outstanding example of this. We now know of three regulatory genes in this pathway that are mutated in primary human cancers and several others that promote experimental cancers in rodents (Fig. 1). In all of these cases the common denominator is the activation of gene transcription by -catenin. The resulting gene expression profile should provide us with a signature common to those cancers carrying defects in the wnt pathway. In this review, the wnt pathway will be covered from the perspective of cancer, with emphasis placed on molecular defects known to promote neoplastic transformation in humans and in animal models.

/pdf/wnt-signaling-and-cancer-2fdsemer2h.pdf

Wnt signaling and cancer

Beneath the complexity and idiopathy of every cancer lies a limited number of 'mission critical' events that have propelled the tumour cell and its progeny into uncontrolled expansion and invasion One of these is deregulated cell proliferation, which, together with the obligate compensatory suppression of apoptosis needed to support it, provides a minimal 'platform' necessary to support further neoplastic progression Adroit targeting of these critical events should have potent and specific therapeutic consequences

Proliferation, cell cycle and apoptosis in cancer

Phosphatidylinositol 3-kinases (PI3Ks) evolved from a single enzyme that regulates vesicle trafficking in unicellular eukaryotes into a family of enzymes that regulate cellular metabolism and growth in multicellular organisms. In this review, we examine how the PI3K pathway has evolved to control these fundamental processes, and how this pathway is in turn regulated by intricate feedback and crosstalk mechanisms. In light of the recent advances in our understanding of the function of PI3Ks in the pathogenesis of diabetes and cancer, we discuss the exciting therapeutic opportunities for targeting this pathway to treat these diseases.

The evolution of phosphatidylinositol 3-kinases as regulators of growth and metabolism

Physiologically important cell-signalling networks are complex, and contain several points of regulation, signal divergence and crosstalk with other signalling cascades. Here, we use the concept of 'critical nodes' to define the important junctions in these pathways and illustrate their unique role using insulin signalling as a model system.

Critical nodes in signalling pathways: Insights into insulin action

Glycogen synthase kinase 3 (GSK3) was initially described as a key enzyme involved in glycogen metabolism, but is now known to regulate a diverse array of cell functions. The study of the substrate specificity and regulation of GSK3 activity has been important in the quest for therapeutic intervention.

The renaissance of GSK3.

Identified originally as a regulator of glycogen metabolism, glycogen synthase kinase-3 (GSK3) is now a well-established component of the Wnt signalling pathway, which is essential for setting up the entire body pattern during embryonic development. It may also play important roles in protein synthesis, cell proliferation, cell differentiation, microtubule dynamics and cell motility by phosphorylating initiation factors, components of the cell-division cycle, transcription factors and proteins involved in microtubule function and cell adhesion. Generation of the mouse knockout of GSK3beta, as well as studies in neurons, also suggest an important role in apoptosis. The substrate specificity of GSK3 is unusual in that efficient phosphorylation of many of its substrates requires the presence of another phosphorylated residue optimally located four amino acids C-terminal to the site of GSK3 phosphorylation. Recent experiments, including the elucidation of its three-dimensional structure, have enhanced our understanding of the molecular basis for the unique substrate specificity of GSK3. Insulin and growth factors inhibit GSK3 by triggering its phosphorylation, turning the N-terminus into a pseudosubstrate inhibitor that competes for binding with the 'priming phosphate' of substrates. In contrast, Wnt proteins inhibit GSK3 in a completely different way, by disrupting a multiprotein complex comprising GSK3 and its substrates in the Wnt signalling pathway, which do not appear to require a 'priming phosphate'. These latest findings have generated an enormous amount of interest in the development of drugs that inhibit GSK3 and which may have therapeutic potential for the treatment of diabetes, stroke and Alzheimer's disease.

GSK3 takes centre stage more than 20 years after its discovery.

A Common Phosphate Binding Site Explains the Unique Substrate Specificity of GSK3 and Its Inactivation by Phosphorylation

Phosphorylation of the endogenous GSK3alpha (glycogen synthase kinase-3alpha) at Tyr279 and GSK3beta at Tyr216 was suppressed in HEK-293 or SH-SY5Y cells by incubation with pharmacological inhibitors of GSK3, but not by an Src-family inhibitor, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4- d ]pyrimidine (PP2), or a general protein tyrosine kinase inhibitor (genistein). GSK3beta transfected into HEK-293 cells or Escherichia coli became phosphorylated at Tyr216, but catalytically inactive mutants did not. GSK3beta expressed in insect Sf 21 cells or E. coli was extensively phosphorylated at Tyr216, but the few molecules lacking phosphate at this position could autophosphorylate at Tyr216 in vitro after incubation with MgATP. The rate of autophosphorylation was unaffected by dilution and was suppressed by the GSK3 inhibitor kenpaullone. Wild-type GSK3beta was unable to catalyse the tyrosine phosphorylation of catalytically inactive GSK3beta lacking phosphate at Tyr216. Our results indicate that the tyrosine phosphorylation of GSK3 is an intramolecular autophosphorylation event in the cells that we have studied and that this modification enhances the stability of the enzyme.

/pdf/further-evidence-that-the-tyrosine-phosphorylation-of-462fi41wa8.pdf

Further evidence that the tyrosine phosphorylation of glycogen synthase kinase-3 (GSK3) in mammalian cells is an autophosphorylation event.

Seliciclib (CYC202, R-roscovitine) is a cyclin-dependent kinase (CDK) inhibitor that competes for the ATP binding site on the kinase. It has greatest activity against CDK2/cyclin E, CDK7/cyclin H, and CDK9/cyclin T. Seliciclib induces apoptosis from all phases of the cell cycle in tumor cell lines, reduces tumor growth in xenografts in nude mice and is currently in phase II clinical trials. This study investigated the mechanism of cell death in multiple myeloma cells treated with seliciclib. In myeloma cells treated in vitro, seliciclib induced rapid dephosphorylation of the carboxyl-terminal domain of the large subunit of RNA polymerase II. Phosphorylation at these sites is crucial for RNA polymerase II-dependent transcription. Inhibition of transcription would be predicted to exert its greatest effect on gene products where both mRNA and protein have short half-lives, resulting in rapid decline of the protein levels. One such gene product is the antiapoptotic factor Mcl-1, crucial for the survival of a range of cell types including multiple myeloma. As hypothesized, following the inhibition of RNA polymerase II phosphorylation, seliciclib caused rapid Mcl-1 down-regulation, which preceded the induction of apoptosis. The importance of Mcl-1 was confirmed by short interfering RNA, demonstrating that reducing Mcl-1 levels alone was sufficient to induce apoptosis. These results suggest that seliciclib causes myeloma cell death by disrupting the balance between cell survival and apoptosis through the inhibition of transcription and down-regulation of Mcl-1. This study provides the scientific rationale for the clinical development of seliciclib for the treatment of multiple myeloma.

https://cancerres.aacrjournals.org/content/canres/65/12/5399.full.pdf

Seliciclib (CYC202, R-Roscovitine) Induces Cell Death in Multiple Myeloma Cells by Inhibition of RNA Polymerase II–Dependent Transcription and Down-regulation of Mcl-1

Sheelagh Frame

Papers

The renaissance of GSK3.

GSK3 takes centre stage more than 20 years after its discovery.

A Common Phosphate Binding Site Explains the Unique Substrate Specificity of GSK3 and Its Inactivation by Phosphorylation

Further evidence that the tyrosine phosphorylation of glycogen synthase kinase-3 (GSK3) in mammalian cells is an autophosphorylation event.

Seliciclib (CYC202, R-Roscovitine) Induces Cell Death in Multiple Myeloma Cells by Inhibition of RNA Polymerase II–Dependent Transcription and Down-regulation of Mcl-1