Down-regulation of Aquaporin-1 mediates a microglial phenotype switch affecting glioma growth

Highlights

• Microglial cells participate in glioma progression through multiple interaction pathways.

• Aquaporin 1 knock-out on microglial cells affect microglial function including migration and phenotype change.

• Aquaporin 1 deletion on microglia promote glioma growth in the tumor micro-environment.

• MEK/ERK signaling cascades regulate aquaporin 1 expression in microglia.

• Inhibition of MEK/ERK signaling pathway by trametinib suppress microglia induced glioma growth.

Abstract

Aquaporin 1 (AQP1), a transmembrane protein that forms water channels, has previously been shown to facilitate growth and progression of many types of tumors by modulating tumor cell migration, proliferation and angiogenesis. Here, we determined the impact of AQP1 expression in the tumor environment on the progression of brain tumors. Primary microglia from wild type(WT) and AQP1 knockout(KO) mice were used to test AQP1 effect on microglia function by using Western blot, quantative PCR, in an experimental in vivo mouse glioma model and organotypic brain slice culture. Deletion of AQP1 in the host tissue significantly reduced the survival of the mice implanted with GL261 glioma cells. The density of glioma-associated microglia/macrophages was almost doubled in AQP1KO mice. We found that factors secreted from GL261 cells decrease microglial AQP1 expression via the MEK/ERK pathway, and that inhibition of this pathway with Trametinib reduced tumor growth and prolonged the survival of tumor bearing mice, an effect which required the presence of microglia. Deletion of AQP1 in cultured microglia resulted in an increase in migratory activity and a decrease in TLR4-dependent innate immune responses. Our study demonstrates a functional relevance of AQP1 expression in microglia and hints to AQP1 as a potential novel target for glioma therapy.

Introduction

Glioblastoma (GBM) is a WHO grade IV brain tumor characterized by a high tumor heterogeneity, high recurrence and poor prognosis. It is the most common malignant tumor in the central nervous system (CNS) [1]. Microglia are the resident immune cells of the brain, sense any pathologic change and react quickly to maintain the stability of the CNS [2,3]. The glioma tissue is infiltrated by microglia and peripheral macrophages (glioma associated microglia and macrophages, GAMs) which show a tumor-supporting phenotype contributing to glioma growth, invasion, and tolerance to chemo- and radiotherapy [2,4]. We have previously shown that GAMs acquire a gene expression profile which is distinct from the classical M1/M2 phenotype [5,6]. We and others have shown that glioma exploit GAMs via different mechanisms to promote glioma growth [[7], [8], [9]].

Aquaporin 1 (AQP1), a member of the aquaporin family, is a transmembrane water channel that plays a leading role in transportation of water molecules in and out of cells [10]. It also facilitates cerebrospinal fluid (CSF) formation and cell migration under physiological and pathological conditions [[11], [12], [13]]. Recent evidence indicates that the expression of AQP1 is closely related to the clinical characteristics of many types of tumors, and its expression may even be a prognostic predictor of certain tumors such as colorectal cancer [[14], [15], [16]]. In the case of gliomas, there is already evidence that AQP1 is highly expressed in gliomas, and its expression is positively correlated with tumor grade and prognosis [15,17]. However, little is known about AQP1 in the tumor microenvironment. For macrophages it has been shown that AQP1 oppositely regulates migration depending on the polarization by activating factors such as LPS [18,19].

In the present study, we determined the role of AQP1 in regulating microglial properties, and to explore its role in the development of glioma. We show that deletion of AQP1 mediates GAMs into a tumor supportive phenotype via ERK signaling and an established MEK inhibitor can significantly reduce tumor growth.