P03.19 AQP4 in brain metastasis: its role and cross talk with the brain microenvironment.

Abstract

AbstractAquaporins (AQPs) are a family of transmembrane channels, involved in transcellular and transepithelial fluids transport; AQP4 is enhanced in the brain, where is the main water channel, localized in the astrocytic endfoot processes. AQP4 is anchored to the blood-brain barrier (BBB) by the dystrophin-associated complex (DAPC) consisting of α-syntrophin and dystrophin. The AQP4 channel is co-expressed on the membrane with the inwardly rectifying K+ channel (Kir4.1) and the Na+/K+/Cl- cotransporter channel (NKCC1) to maintain extracellular homeostasis. In cancer AQPs promote cellular invasion and migration. Previously, in brain metastasis we observed AQP4 expression mostly in glial cells surrounding tumor, but also in metastatic cells (mainly on NSCLC). Therefore, in this study, our principal aim is to investigate the role of AQP4 pathway in tumor microenvironment, their cross talk with the extracellular matrix and its meaning in the metastatic process. So, we have performed a retrospective analysis about the expression of AQP4 and the molecules and channels associated to AQP4, on 87 brain metastases from different tumors: lung (n=36), breast (n=21), colon (n=7), kidney (n=11), melanoma (n=7), ovary (n=3), thyroid (n=1) and prostate (n=1). Dystrophin, α-syntrophin, Kir4.1 and NKCC1 were first evaluated by immunohistochemistry and then by molecular analysis. On 21 patients we analyzed both the primitive and metastatic tumor. We observed a great AQP4 expression at the metastasis-brain interface; high expression of α-syntrophin and NKCC1 was also observed, while Kir4.1 was lower. Western blot confirmed this expression of AQP4 both in tumor and in the surrounding tissue. qPCR showed a downregulation of AQP4 mRNA except for lung metastases. α-syntrophin showed a very high expression in metastatic cells compared to normal brain, but surprisingly mRNA levels were downregulated; dystrophin showed very low levels of expression both of protein and mRNA in tumor. NKCC1 and Kir4.1 channels were overexpressed compared to the brain. These results showed a deregulation of the whole pathway associated to AQP4 in the presence of the metastases. Besides, confocal images showed a polarization of AQP4 expression mostly in lung and breast metastases. The brain metastasis maintain the same profile of original tumor. Our preliminary studies in primitive neoplasms and their counterpart brain metastases suggest a cross talk from AQP4 pathway with adhesion molecules expression and extracellular matrix in response to the presence of tumor mass. In conclusion, our results confirmed a cross talk between AQP4 and the microenvironment during the metastatic process, having a role in to maintain homeostasis, trying to reduce the edema and to limit the spreading of tumor cells in the parenchyma. This study will provide an important step forward in the knowledge of microenvironment’s involvement in brain metastasis.