Brain Metastases Are Regulated by Immuno-inflammatory Signaling Pathways Governed by STAT3, MAPK and Tumor Suppressor p53 Status: Possible Therapeutic Targets

Abstract

Background/aim: Brain metastasis (BM) is a complex multi-step process involving various immune checkpoint proteins. Mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinases 1/2 (ERK1/2), and signal transducer and activator of transcription 3 (STAT3) are implicated in tumorigenesis and are critical upstream regulators of Programmed Death Ligand 1 (PD-L1), an immunotherapy target. Tumor suppressor p53, dysregulated in cancers, regulates STAT3 and ERK1/2 signaling. This study examined the roles of STAT3, MAPK and p53 status in BM initiation and maintenance.

Materials and methods: Twenty-six BM, with various primary malignancies, were used (IRB-approved) to determine mutant p53 (p53^mt), pSTAT3^Tyr705, pERK1/2^{Thr202/Tyr204}, and PD-L1 expression using immunohistochemistry. cDNA microarray was used for gene expression analysis. Brain-metastatic breast cancer cells (MDA-MB-231) were treated with STAT3 (NSC74859) or MAPK/ERK1/2 (U0126) inhibitors in regular or astrocytic media. ERK1/2 pathway was assessed using western blotting, and cell proliferation and migration were determined using MTT and scratch-wound assays, respectively.

Results: pSTAT3^Tyr705 and pERK1/2^{Thr202/Tyr204} were expressed at tumor margins, whereas p53^mt and PD-L1 were uniformly expressed, with significant overlap between expression of these proteins. Gene expression analysis identified alterations in 18 p53- and 32 STAT3- or MAPK-associated genes contributing to dysregulated immune responses and cell cycle regulation. U0126 and NSC74859 reduced pERK1/2^{Thr202/Tyr204} expression. Cell proliferation decreased following each treatment (p≤0.01). Migration stagnated following U0126 treatment in astrocytic media (p≤0.01).

Conclusion: Activation of STAT3 and ERK1/2 promotes BM and provides compelling evidence for use of STAT3, ERK1/2 and p53 status as potential immunotherapeutic targets in BM.

Keywords: MAPK; Metastatic brain tumors; PD-L1; STAT3; immunotherapeutic targets; p53.