A Panel of Protein Kinase Chemosensors Distinguishes Different Types of Fatty Liver Disease

Phosphorylation-Assisted Luciferase Complementation Assay Designed to Monitor Kinase Activity and Kinase-Domain-Mediated Protein-Protein Binding

Protein kinases are key regulators of cell signaling and have been important therapeutic targets for three decades. ATP-competitive drugs directly inhibit the activity of kinases but these enzymes work as part of complex protein networks in which protein-protein interactions (often referred to as kinase docking) may govern a more complex activation pattern. Kinase docking is indispensable for many signaling disease-relevant Ser/Thr kinases and it is mediated by a dedicated surface groove on the kinase domain which is distinct from the substrate-binding pocket. Thus, interfering with kinase docking provides an alternative strategy to control kinases. We describe activity sensors developed for p90 ribosomal S6 kinase (RSK) and mitogen-activated protein kinases (MAPKs: ERK, p38, and JNK) whose substrate phosphorylation is known to depend on kinase-docking-groove-mediated protein-protein binding. The in vitro assays were based on fragment complementation of the NanoBit luciferase, which is facilitated upon substrate motif phosphorylation. The new phosphorylation-assisted luciferase complementation (PhALC) sensors are highly selective and the PhALC assay is a useful tool for the quantitative analysis of kinase activity or kinase docking, and even for high-throughput screening of academic compound collections.

A fluorescent probe for monitoring PTP-PEST enzymatic activity

Phosphatase non-receptor type 12 (PTPN12 or PTP-PEST) is a critical regulator of cell migration, acting as a tumor suppressor in cancer. Decreases in PTP-PEST expression correlate with aggressive phenotypes in hepatocellular carcinoma (HCC). Despite the importance of PTP-PEST in cellular signaling, methods to directly monitor its enzymatic activity are lacking. Herein, we report the design, synthesis, and optimization of a probe to directly monitor PTP-PEST enzymatic activity via a fluorescent readout. This activity sensor, termed pPEST1tide, is capable of detecting as little as 0.2 nM recombinant PTP-PEST. In addition, we demonstrate that this probe can selectively report on PTP-PEST activity using a panel of potential off-target enzymes. In the long-term, this activity probe could be utilized to identify small molecule modulators of PTP-PEST activity as well as provide a prognostic readout for HCC.

New aspects of hepatic endothelial cells in physiology and nonalcoholic fatty liver disease

The liver is the central metabolic hub for carbohydrate, lipid, and protein metabolism. It is composed of four major types of cells, including hepatocytes, endothelial cells (ECs), Kupffer cells, and stellate cells. Hepatic ECs are highly heterogeneous in both mice and humans, representing the second largest population of cells in liver. The majority of them line hepatic sinusoids known as liver sinusoidal ECs (LSECs). The structure and biology of LSECs and their roles in physiology and liver disease were reviewed recently. Here, we do not give a comprehensive review of LSEC structure, function, or pathophysiology. Instead, we focus on the recent progress in LSEC research and other hepatic ECs in physiology and nonalcoholic fatty liver disease and other hepatic fibrosis-related conditions. We discuss several current areas of interest, including capillarization, scavenger function, autophagy, cellular senescence, paracrine effects, and mechanotransduction. In addition, we summarize the strengths and weaknesses of evidence for the potential role of endothelial-to-mesenchymal transition in liver fibrosis.