Identification and Characterisation of cis-Regulatory Elements Upstream of the Human Receptor Tyrosine Kinase Gene MERTK

A+T rich interaction domain protein 3a (Arid3a) impairs Mertk-mediated efferocytosis in cholestasis

BACKGROUND & AIMS

Macrophages are key elements in the pathogenesis of cholestatic liver diseases. Arid3a plays a prominent role in the biologic properties of hematopoietic stem cells, B lymphocytes and tumor cells, but its ability to modulate macrophage function during cholestasis remains unknown.

METHODS

Gene and protein expression and cellular localization were assessed by q-PCR, immunohistochemistry, immunofluorescence staining and flow cytometry. We generated myeloid-specific Arid3a knockout mice and established three cholestatic murine models. The transcriptome was analyzed by RNA-seq. A specific inhibitor of the Mertk receptor was used in vitro and in vivo. Promoter activity was determined by chromatin immunoprecipitation-seq against Arid3a and a luciferase reporter assay.

RESULTS

In cholestatic murine models, myeloid-specific deletion of Arid3a alleviated cholestatic liver injury (accompanied by decreased accumulation of macrophages). Arid3a-deficient macrophages manifested a more reparative phenotype, which was eliminated by in vitro treatment with UNC2025, a specific inhibitor of the efferocytosis receptor Mertk. Efferocytosis of apoptotic cholangiocytes was enhanced in Arid3a-deficient macrophages via upregulation of Mertk. Arid3a negatively regulated Mertk transcription by directly binding to its promoter. Targeting Mertk in vivo effectively reversed the protective phenotype of Arid3a deficiency in macrophages. Arid3a was upregulated in hepatic macrophages and circulating monocytes in primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC). Mertk was correspondingly upregulated and negatively correlated with Arid3a expression in PBC and PSC. Mertk+ cells were located in close proximity to cholangiocytes, while Arid3a+ cells were scattered among immune cells with greater spatial distances to hyperplastic cholangiocytes in PBC and PSC.

CONCLUSIONS

Arid3a promotes cholestatic liver injury by impairing Mertk-mediated efferocytosis of apoptotic cholangiocytes by macrophages during cholestasis. The Arid3a-Mertk axis is a promising novel therapeutic target for cholestatic liver diseases.

IMPACT AND IMPLICATIONS

Macrophages play an important role in the pathogenesis of cholestatic liver diseases. This study reveals that macrophages with Arid3a upregulation manifest a pro-inflammatory phenotype and promote cholestatic liver injury by impairing Mertk-mediated efferocytosis of apoptotic cholangiocytes during cholestasis. Although we now offer a new paradigm to explain how efferocytosis is regulated in a myeloid cell autonomous manner, the regulatory effects of Arid3a on chronic liver diseases remain to be further elucidated.

Control of tumor-associated macrophage responses by nutrient acquisition and metabolism

Metazoan tissue specification is associated with integration of macrophage lineage cells in sub-tissular niches to promote tissue development and homeostasis. Oncogenic transformation, most prevalently of epithelial cell lineages, results in maladaptation of resident tissue macrophage differentiation pathways to generate parenchymal and interstitial tumor-associated macrophages that largely foster cancer progression. In addition to growth factors, nutrients that can be consumed, stored, recycled, or converted to signaling molecules have emerged as crucial regulators of macrophage responses in tumor. Here, we review how nutrient acquisition through plasma membrane transporters and engulfment pathways control tumor-associated macrophage differentiation and function. We also discuss how nutrient metabolism regulates tumor-associated macrophages and how these processes may be targeted for cancer therapy.

Genome-Wide Association Study Identifies Risk Loci for Cluster Headache

OBJECTIVE

This study was undertaken to identify susceptibility loci for cluster headache and obtain insights into relevant disease pathways.

METHODS

We carried out a genome-wide association study, where 852 UK and 591 Swedish cluster headache cases were compared with 5,614 and 1,134 controls, respectively. Following quality control and imputation, single variant association testing was conducted using a logistic mixed model for each cohort. The 2 cohorts were subsequently combined in a merged analysis. Downstream analyses, such as gene-set enrichment, functional variant annotation, prediction and pathway analyses, were performed.

RESULTS

Initial independent analysis identified 2 replicable cluster headache susceptibility loci on chromosome 2. A merged analysis identified an additional locus on chromosome 1 and confirmed a locus significant in the UK analysis on chromosome 6, which overlaps with a previously known migraine locus. The lead single nucleotide polymorphisms were rs113658130 (p = 1.92 × 10^-17 , odds ratio [OR] = 1.51, 95% confidence interval [CI] = 1.37-1.66) and rs4519530 (p = 6.98 × 10^-17 , OR = 1.47, 95% CI = 1.34-1.61) on chromosome 2, rs12121134 on chromosome 1 (p = 1.66 × 10^-8 , OR = 1.36, 95% CI = 1.22-1.52), and rs11153082 (p = 1.85 × 10^-8 , OR = 1.30, 95% CI = 1.19-1.42) on chromosome 6. Downstream analyses implicated immunological processes in the pathogenesis of cluster headache.

INTERPRETATION

We identified and replicated several genome-wide significant associations supporting a genetic predisposition in cluster headache in a genome-wide association study involving 1,443 cases. Replication in larger independent cohorts combined with comprehensive phenotyping, in relation to, for example, treatment response and cluster headache subtypes, could provide unprecedented insights into genotype-phenotype correlations and the pathophysiological pathways underlying cluster headache. ANN NEUROL 2021;90:193-202.