Aquaporins in Cancer Biology

Abstract

Aquaporins (AQPs) are a family of transmembrane water channel proteins, which were initially characterized as a novel protein family that plays a vital role in transcellular and transepithelial water movement. AQP1, AQP2, AQP4, AQP5, and AQP8 are primarily water selective, whereas AQP3, AQP7, AQP9, and AQP10 (called "aqua-glyceroporins") also transport glycerol and other small solutes. Recently, multiple reports have suggested that AQPs have important roles in cancer cell growth, migration, invasion, and angiogenesis, each of which is important in human carcinogenesis. Here, we review recent data concerning the involvement of AQPs in tumor growth, angiogenesis, and metastasis and explore the expression profiles from various resected cancer samples to further dissect the underlying molecular mechanisms. Moreover, we discuss the potential role of AQPs during the development of genomic instability and performed modeling to describe the integration of binding between AQPs with various SH3 domain binning adaptor molecules. Throughout review and discussion of numerous reports, we have tried to provide key evidence that AQPs play key roles in tumor biology, which may provide a unique opportunity in designing a novel class of anti-tumor agents.

Keywords: AQP expression; AQP in carcinogenesis; genomic instability; glycerol channel; hallmarks of cancer; targeted therapy; water channel.

Figure 1

During colorectal carcinogenesis, expression of AQP1 and AQP5 is induced, which is examined by in situ hybridization. To confirm these findings in vivo and to evaluate the pattern of AQP induction during carcinogenesis, in situ hybridization with AQP1, AQP3, and AQP5 riboprobes generated by cloning the RT–PCR products, which was used to study the expression of these AQPs in colon cancer and in different stages of preneoplastic lesions. From five patients with colon cancer, 16 tissue samples were studied, including one patient with metastasis to the liver. Of these samples, 12 consisted of adenocarcinomas and the surrounding normal colonic tissue, whereas four were metastatic lesions from the liver. Of the five patients, staged surgically resected series were available for three patients. Expression of The AQP1 detected from staining with antisense riboprobe was identified colonic adenoma (A, arrow), primary colon cancer (B, arrow), and metastatic lesions in the liver (C, arrow). Likewise, the AQP5 expression is also detected in early adenoma with moderately dysplastic cells (D, arrow), late adenoma with severe dysplastic cells (E, arrow), and adenocarcinoma (F, arrow). There is almost no expression in the surrounding normal (A, F, star). As positive controls, the germinal centers of the tonsils were stained with antisense riboprobes of AQP5 (G). Sense riboprobes of AQP1 (data not shown) and AQP5(h) is shown as negative control (H). As an internal control, expression of AQP1 in the vascular endothelium is shown (I). Adapted from (26).

Figure 2

Mechanism of tumor promotion by AQP3 is demonstrated by studying AQP3 KO cell models for its proliferation assay from skin tumorigenesis. ATP production by AQP3-mediated glycerol transport may play an important role in the transformation of normal skin epithelial cells and growth of skin cancers. Modified from Reference 91. G3P, glycerol 3-phosphate; DMBA, 1,3-dimethylbutylamine; TPA, 12-O-tetradecanoylphorbol-13-acetate.

Figure 3

AQP5 increases cell proliferation, which depends on Ser156 phosphorylation dependent. (A) Cell proliferation is significantly increased in stable cells with wild-type AQP5 or the N185D mutant as compared with cells with the S156A mutant or the control vector. (B, C) Tumor growth in athymic mice. Significant tumor growth was detected within 8 weeks after subcutaneously injected to athymic mice model with stable cells with wild-type AQP5 or the N185D mutant. No tumor growth is detected in mice injected with stable cells with the S156A cells or MOCK (C). (D) Phosphorylation of AQP5 in cancer cells and resected lung cancer cells. AQP5 is phosphorylated in the two lung cancer cell lines (H1975 and H1838), and in two of three primary non–small cell lung cancer tissue samples. Such phosphorylation is not detected in three primary normal tissues. Adapted from (148).

Figure 4

Development toward tumor growth and cancer metastasis. (A) Although various barriers to tumor progression exist, including DNA repair processes, accumulated mutators can promote the availability of securing nutrition, fulfilling the requirement of angiogenesis, and thus allow the tumor to increase in size. A variety of roles from AQPs during these steps are beginning to be elucidated. (B) AQP can play a crucial role during a series of processes, leading to various stages of accumulated DNA mutations through the activation of RAS, AKT, and MYC activations. Modified from reference (23).

Figure 5

In this study, from SH3 microarray, the purified AQP5 from stable BEAS-2B cells showed binding activity to the SH3 domain of c-Src, Lyn, and Grap2 C-terminal in phosphorylation dependent manner (A). GST pull-down assay confirmed a direct interaction between AQP5 and the Src family molecules (B). c-Src co-immunoprecipitated with AQP5 turned out to be an activated form of c-Src, phosphorylated at Tyr416 (C). Cells expressing AQP5 have more activated c-Src than the cells carrying N185D or S156A mutant (C). 1, Mock; 2, N185D; 3, S156A; 4, AQP5; M, molecular weight marker. AQP5 contains SH3 binding domain, and we pointed out that the downstream pathways are very similar to the one from EGFR or other transmembrane tyrosine kinases. In cancer cells, various adapter proteins (e.g., SHC and GRB-2) can bind and subsequently turns on various downstream pathways. Adapted from reference (124).