**6. Role of AQPs in the pathogenesis of breast cancer**

While AQPs have been shown to be involved in the delivery of water to the mammary glands which is critical for milk production and secretion during lactation [131], their expression in breast tumors is modified and correlates with tumor grade.

#### **6.1. AQP 1**

Immunostaining indicates a predominantly membranous localization with some presence in the cytoplasm in large tumor cells (more pronounced at the tumor invasion front), but no expression was seen in smaller tumor cells. All of the AQP 1 positive invasive carcinomas are found to be of ductal type, ER−ve and HER2/neu −ve (triple −ve form), and its expression was significantly associated with poor clinical prognosis [132, 133]. A recent report suggested that the cytoplasmic expression of AQP 1 promotes breast cancer progression and was associated with a shorter survival rate especially in luminal subtype patients [134]. Its cytoplasmic expression was positively correlated with advanced pathological features of invasive ductal carcinoma and lymph node metastasis [134]. Another study reported that AQP 1 was highly expressed in blood vessels (mainly in CD31+ve endothelial cells) of human breast and endometrial carcinoma tissues, suggesting a role in tumor angiogenesis [135]. Using human umbilical vein endothelial cells (HUVECs), Zou et al. [135] showed that estrogen treatment significantly up-regulated AQP 1 expression in a time- and dose-dependent fashion, which was mediated through a functional estrogen response element motif in the promoter region of the AQP1 gene. Estrogen treatment significantly increased HUVEC proliferation, migration, invasion and tubule formation; all of these effects were inhibited by pretreatment of cells with AQP1-specific siRNA. These data suggest an important role of AQP 1 in cell invasion in part through regulating actin stress fiber formation through colocalization with the ezrin/radixin/ moesin protein complex [135]. Qin et al. [134] showed that overexpression of AQP 1 in MCF-7 and MDA-MB-231 cells significantly enhanced (by approximately 2 fold) cell proliferation and invasion. Epidermal growth factor (EGF) stimulation induced AQP 1 redistribution from the cytoplasm to the cell membrane, further supporting a role in promoting cell invasion. In the mouse mammary tumor virus-driven polyoma middle T oncogene (MMTV-PyVT) model (which spontaneously develops a well-differentiated luminal-type breast carcinoma with lung metastasis), AQP 1 deficiency significantly reduced the breast tumor mass (by 46%) and volume (by 50%), vessel density and the number of lung metastases compared to the control group [136]. This effect was in part due to decreased expression of vascular endothelial growth factor receptor-2 (VEGFR2) and increased levels of hypoxia inducible factor-1α (HIF-1α) in the AQP 1 knockout mice [136].
