Influence of signaling kinases on functional dynamics of nuclear receptor CAR

Mitogen-Activated Protein Kinase and Exploratory Nuclear Receptor Crosstalk in Cancer Immunotherapy

The three major mitogen-activated protein kinase (MAPK) pathways (ERK1/2, p38, and JNK/SAPK) are upstream regulators of the nuclear receptor superfamily (NRSF). These ligand-activated transcription factors are divided into subclasses comprising receptors for endocrine hormones, metabolic compounds (e.g., vitamins, diet), xenobiotics, and mediators released from host immune reactions such as tissue injury and inflammation. These internal and external cues place the NRSF at the frontline as sensors and translators of information from the environment towards the genome. For most of the former "orphan" receptors, physiological and synthetic ligands have been identified, opening intriguing opportunities for combination therapies with existing cancer medications. Hitherto, only preclinical data are available, warranting further validation in clinical trials in patients. The current review summarized the existing literature covering the expression and function of NRSF subclasses in human solid tumors and hematopoietic malignancies and their modulatory effects on innate (e.g., macrophages, dendritic cells) and adaptive (i.e., T cell subsets) immune cells, encouraging mechanistic and pharmacological studies in combination with current clinically approved therapeutics against immune checkpoint molecules (e.g., PD1).

Protein phosphatase 2A regulates cytotoxicity and drug resistance by dephosphorylating xenobiotic metabolism enzymes AHR and MDR1

Protein phosphatase 2A (PP2A) is a serine/threonine dephosphorylating enzyme complex that plays numerous roles in biological processes, including cell growth and metabolism. However, its specific actions in many of these critical pathways are unclear. To explore mechanisms underlying metabolic enzyme regulation in the liver, we investigated the key pathways involved in regulation of xenobiotic-metabolizing enzymes in a mouse model with hepatocyte-specific deletion of Ppp2r1a, encoding the Aα subunit of PP2A. We performed transcriptome and phosphoproteome analysis in mouse livers at the age of 3 months and identified 2695 differentially expressed genes and 549 upregulated phosphoproteins in homozygous knockout mouse livers compared with WT littermates. In particular, the expression of metabolic enzymes Cyp2e1, Cyp1a1, Cyp1a2, Mdr1a, and Abcg2 was dramatically altered in homozygous knockout mouse livers. We also demonstrated that activation of PP2A reversed the decline of metabolic enzyme expression in primary mouse hepatocytes. We found that specific PP2A holoenzymes were involved in metabolic enzyme induction through dephosphorylation of transcription factors, nuclear receptors, or the target enzymes themselves, leading to dysregulation of xenobiotic metabolism or drug-induced hepatotoxicity. Notably, we confirmed that a regulatory axis, PP2A B56α–aryl hydrocarbon receptor–Cyp1a1, was involved in benzo(a)pyrene-induced cytotoxicity through dephosphorylation of the metabolic nuclear receptor, aryl hydrocarbon receptor, at serine 36. In addition, we showed that PP2A B56δ complexes directly dephosphorylated the multidrug efflux pump MDR1 (encoded by multi-drug resistance gene 1), contributing to drug resistance against the chemotherapeutic 5-fluorouracil. Taken together, these novel findings demonstrate the involvement of PP2A in the regulation of liver metabolism.