**5. Involvement of AQPs in the etiology of cancer**

**4.3. Glycerol transport**

64 Tumor Metastasis

the deficient mice [111].

**4.4. Cell proliferation**

**4.5. Cell adhesion**

lular adhesion.

**4.6. Cell migration**

AQP 3 was shown to be expressed in the stratum corneum (SC) at the basal layer of the keratinocytes and plays a role in skin hydration. In AQP-3-deficient mice, SC hydration was significantly reduced due to reduced water content, decreased skin elasticity and wound healing [108]. An important factor which was also attributed to reduced skin hydration in AQP-3-deficient mice is the impaired glycerol transport from the blood to the epidermis through the basal keratinocytes, suggesting the importance of AQP 3 in glycerol transport. Dysregulated expression of AQP 3 has been found in various skin disorders associated with altered epidermal proliferation [109, 110]. In fact, topical or systemic replacement of glycerol prevented skin abnormalities (less hydration and elasticity and impaired barrier function) in

A role for AQP 3 in cell proliferation has been suggested in various cell types. Using corneal epithelial cells, delayed restoration of full-thickness epithelia was seen in AQP-3-deficient mice after scraping. This was confirmed by reduction in proliferating BrdU-positive cells during healing [112]. Reduced keratinocyte cell proliferation was also evident in AQP-3-deficient mice or with siRNA-mediated knockout of AQP 3 in keratinocytes in part through reduction of p38 MAPK activity [113]. Furthermore, the proliferative rate of mouse colonic epithelial cells was significantly reduced in AQP-3-deficient mice, which might explain the enhanced colitis severity in these mice compared to WT mice in the dextran sulfate sodium model of colitis [114].

AQP 0 is thought to be involved in cell-cell adhesion. It has been found to be expressed in lens fiber cells in the eye and plays a role in maintaining their structure [115]. Loss-of-function mutation of AQP 0 in humans and mice resulted in congenital cataracts [34, 92]. In addition, AQP 4 was shown to mediate weak cell-cell interaction through its short helix in the extracel‐ lular loop [116]. Overexpression of AQP 4 in L-cells (which lack endogenous adhesion molecules) resulted in cell cluster formation, which supports the role of this AQP in intercel‐

Various AQPs have been shown to be involved in the cell migrative process. AQP 1 is expressed on the leading edge of migrating cultured endothelial cells in association with increased lamellipodia formation. AQP 1 deficiency in cultured endothelial cells results in significant reduction in their migration. Overexpression of AQP 1 or 4 enhanced cell migration along with prominent membrane ruffling at the leading edge [53]. The role of AQP 1 in cell migration was also confirmed using kidney proximal tubule cells where its deficiency reduced cell migration and its overexpression led to enhanced cell migration through the formation of lamella-like membrane protrusions at the cell leading edge [50]. Furthermore, AQP 4 was localized on the leading edge of migrating cultured astroglia cells, and its expression was increased by inducing There is accumulating evidence for the involvement of several forms of AQPs in various types of cancer which also correlates with tumor stage.

With respect to tumor proliferation, AQP 5 interacts with the Ras-MAPK pathway and cyclin D1/CDK4 complexes in colon cancer [121] and with the EGFR/ERK1/2/p38 MAPK signaling cascade in lung cancer [122], resulting in enhanced proliferation, differentiation and survival. A role for AQP 3 has also been suggested for controlling proliferation of epidermal cancer cells through the facilitation of glycerol transport and increase in ATP generation [123]. In nonsmall-cell lung cancer cells, its effects appear to be associated with enhancement of the expression of p53, increase in the ratio of cleaved to procaspase 3 and reduction in the expression of proliferating cell nuclear antigen and B-cell lymphoma-2 (Bcl-2) [124]. AQP 4 is involved in glioblastoma cell proliferation; siRNA-mediated knockdown of AQP 4 induced cell apoptosis in part through modulation of key proteins involved in this process such as cytochrome c, Bcl-2 and Bad [125].

With regard to tumor migration/invasion and angiogenesis, AQP 3 silencing in non-small lung cancer cells resulted in significant inhibition of cell invasion through reduction of the activity of matrix metalloproteinases (MMPs) 2 and 9 and AKT phosphorylation, as well as reduction in angiogenesis through interaction with the HIF-2α-VEGF pathway [124]. Overexpression of AQP 1 in B16F10 melanoma cells and 4T1 breast cancer cells resulted in enhanced cell invasion and tumor spread when injected through the tail vein in mice [53, 126]. siRNA-mediated knockdown of AQP 1 in melanoma cells also resulted in reduced cell proliferation and invasion [127]. Overexpression of AQP 1 in colon cancer cells increased their invasive potential through actin relocalization and RhoA and Rac activation [128]. In glioma cells, AQP 1 facilitated the shunting of H+ from the intracellular to the extracellular compartment and the release of lactate dehydrogenase (LDH) and cathepsin B, which results in the acidification of the tumor microenvironment leading to enhanced tumor angiogenesis and invasion [129]. AQP 4 also plays a role in glioblastoma cell migration and invasion through rearrangement of the actin cytoskeleton [130]. Furthermore, overexpression of AQP 5 in non-small lung cancer cells enhanced cell metastasis through c-Src activation and induction of the EMT process [122].
