Water channel (aquaporin 1) expression and distribution in mammary carcinomas and glioblastomas

The aquaporins represent a family of transmembrane water channel proteins that are widely distributed in various tissues throughout the body and play a major role in transcellular and transepithelial water movement. Most tumors have been shown to exhibit high vascular permeability and high interstitial fluid pressure, but the transport pathways for water within tumors remain unknown. In this study, we examined the distribution of the aquaporin 1 (AQP1) water channel protein in several types of transplanted tumor. Two mammary carcinomas, MCaIV and R3230AC, and three glioblastomas, HGL21, U87, and F98, were implanted in rats and mice. Two sites of implantation in rodents were chosen: a cranial window (CW) region and a subcutaneous (SC) region. Tissues were studied using immunoblot analysis and immunofluorescence staining. In the mammary carcinomas, AQP1 was localized in vascular structures; no differences between CW and SC regions were observed. Among the three glioblastomas, HGL21 and U87 exhibited similar AQP1 localization in vascular structures, whereas the center of F98 did not show vascular staining. Cell membranes of normal epithelial cells did not show AQP1 expression, while membranes of most tumor cells exhibited significant AQP1 expression. Interestingly, however, HGL21 and F98 in the CW locations showed no AQP1 expression on tumor cell membranes. These results show that the AQP1 water channel is heterogeneously expressed in tumor cells and their vasculature, and that the level of expression is determined not only by the specific cellular origin of the tumor, but also by the location of the tumor in the host animal.