HCV-induced PKR activation is stimulated by the mitogen- and stress-activated protein kinase MSK2

Identification of p90 Ribosomal S6 Kinase 2 as a Novel Host Protein in HBx Augmenting HBV Replication by iTRAQ-Based Quantitative Comparative Proteomics

PURPOSE

The aim of this study was to screen for novel host proteins that play a role in HBx augmenting Hepatitis B virus (HBV) replication.

EXPERIMENTAL DESIGN

Three HepG2 cell lines stably harboring different functional domains of HBx (HBx, HBx-Cm6, and HBx-Cm16) were cultured. ITRAQ technology integrated with LC-MS/MS analysis was applied to identify the proteome differences among these three cell lines.

RESULTS

In brief, a total of 70 different proteins were identified among HepG2-HBx, HepG2-HBx-Cm6, and HepG2-HBx-Cm16 by double repetition. Several differentially expressed proteins, including p90 ribosomal S6 kinase 2 (RSK2), were further validated. RSK2 was expressed at higher levels in HepG2-HBx and HepG2-HBx-Cm6 compared with HepG2-HBx-Cm16. Furthermore, levels of HBV replication intermediates were decreased after silencing RSK2 in HepG2.2.15. An HBx-minus HBV mutant genome led to decreased levels of HBV replication intermediates and these decreases were restored to levels similar to wild-type HBV by transient ectopic expression of HBx. After silencing RSK2 expression, the levels of HBV replication intermediates synthesized from the HBx-minus HBV mutant genome were not restored to levels that were observed with wild-type HBV by transient HBx expression.

CONCLUSION AND CLINICAL RELEVANCE

Based on iTRAQ quantitative comparative proteomics, RSK2 was identified as a novel host protein that plays a role in HBx augmenting HBV replication.

Structural Basis for the Non-catalytic Functions of Protein Kinases

Protein kinases are known primarily for their ability to phosphorylate protein substrates, which constitutes an essential biological process. Recently, compelling evidence has accumulated that the functions of many protein kinases extend beyond phosphorylation and include an impressive spectrum of non-catalytic roles, such as scaffolding, allosteric regulation, or even protein-DNA interactions. How the conserved kinase fold shared by all metazoan protein kinases can accomplish these diverse tasks in a specific and regulated manner is poorly understood. In this review, we analyze the molecular mechanisms supporting phosphorylation-independent signaling by kinases and attempt to identify common and unique structural characteristics that enable kinases to perform non-catalytic functions. We also discuss how post-translational modifications, protein-protein interactions, and small molecules modulate these non-canonical kinase functions. Finally, we highlight current efforts in the targeted design of small-molecule modulators of non-catalytic kinase functions, a new pharmacological challenge for which structural considerations are more important than ever.

Toll-like receptor signalling in liver disease: ER stress the missing link?

Toll-like receptors induce a complex inflammatory response that can function to alert the body to infection, neutralize pathogens and repair damaged tissues. Toll-like receptors are expressed on kupffer, endothelial, dendritic, biliary epithelial, hepatic stellate cells, and hepatocytes in the liver. The endoplasmic reticulum (ER) is a central organelle of eukaryotic cells that exists as a place of lipid synthesis, protein folding and protein maturation. The ER is a major signal transduction organelle that senses and responds to changes in homeostasis. Conditions interfering with the function of the ER are collectively known as ER stress and can be induced by accumulation of unfolded protein aggregates or by excessive protein traffic as can occur during viral infection. The ability of ER stress to induce an inflammatory response is considered to play a role in disease pathogenesis. Importantly, ER stress is viewed as a contributor to the pathogenesis of liver diseases with evidence linking components of ER homeostasis as requirements for optimal Toll-like receptor function. In this context this review discusses the association of Toll-like receptors with ER stress. This is an emerging paradigm in the understanding of Toll-like receptor signalling which may have an underlying role in the pathogenesis of liver disease.

The secret life of kinases: functions beyond catalysis

Protein phosphorylation participates in the regulation of all fundamental biological processes, and protein kinases have been intensively studied. However, while the focus was on catalytic activities, accumulating evidence suggests that non-catalytic properties of protein kinases are essential, and in some cases even sufficient for their functions. These non-catalytic functions include the scaffolding of protein complexes, the competition for protein interactions, allosteric effects on other enzymes, subcellular targeting, and DNA binding. This rich repertoire often is used to coordinate phosphorylation events and enhance the specificity of substrate phosphorylation, but also can adopt functions that do not rely on kinase activity. Here, we discuss such kinase independent functions of protein and lipid kinases focussing on kinases that play a role in the regulation of cell proliferation, differentiation, apoptosis, and motility.