Abstract
Hypoxia is a key parameter that controls tumor angiogenesis and malignant progression by regulating the expression of several oncogenic molecules. The nonreceptor protein-tyrosine kinases Syk and Lck play crucial roles in the signaling mechanism of various cellular processes. The enhanced expression of Syk in normal breast tissue but not in malignant breast carcinoma has prompted us to investigate its potential role in mammary carcinogenesis. Accordingly, we hypothesized that hypoxia/reoxygenation (H/R) may play an important role in regulating Syk activation, and Lck may be involved in this process. In this study, we have demonstrated that H/R differentially regulates Syk phosphorylation and its subsequent interaction and cross-talk with Lck in MCF-7 cells. Moreover, Syk and Lck play differential roles in regulating Sp1 activation and expressions of melanoma cell adhesion molecule (MelCAM), urokinase-type plasminogen activator (uPA), matrix metalloproteinase-9 (MMP-9), and vascular endothelial growth factor (VEGF) in response to H/R. Overexpression of wild type Syk inhibited the H/R-induced uPA, MMP-9, and VEGF expression but up-regulated MelCAM expression. Our data also indicated that MelCAM acts as a tumor suppressor by negatively regulating H/R-induced uPA secretion and MMP-9 activation. The mice xenograft study showed the cross-talk between Syk and Lck regulated H/R-induced breast tumor progression and further correlated with the expressions of MelCAM, uPA, MMP-9, and VEGF. Human clinical specimen analysis supported the in vitro and in vivo findings. To our knowledge, this is first report that the cross-talk between Syk and Lck regulates H/R-induced breast cancer progression and further suggests that Syk may act as potential therapeutic target for the treatment of breast cancer.
Highlights
Previous reports have indicated that areas of hypoxia/reoxygenation (H/R)3 are a typical feature of rapidly growing and metastasizing tumors [11, 12]
Degradation of extracellular matrix plays an important role in tumor metastasis. urokinase-type plasminogen activator (uPA) is a member of the serine protease family that interacts with uPA receptor and facilitates the conversion of inert zymogen plasminogen into widely acting serine protease plasmin [35, 36]
H/R Regulates the Cellular Localization of Syk—To determine whether H/R regulates the cellular localization of Syk, MCF-7 cells (2 ϫ 105 cells/ml) seeded on 35-mm tissue culture plate were exposed to hypoxia for 2 h and reoxygenated for 45 min
Summary
Earlier reports have indicated that MelCAM acts differently in the progression of breast carcinomas. The molecular mechanism by which H/R regulates Syk/Lck-dependent MelCAM expression and uPA secretion and the uPA-dependent pro-MMP-9 activation in breast carcinoma cells is not well understood. The role of Lck and Syk in H/R-induced VEGF expression in breast cancer is not defined clearly. In this study we have demonstrated the differential role of Syk and Lck in the H/R-induced uPA, MMP-9, and VEGF expression. Our findings suggested that H/R down-regulates Syk activation leading to enhanced uPA, MMP-9, and VEGF expression. Clinical data indicated that the higher grades of tumors showed significant HIF-1␣ expression compared with that of lower grades or normal breast tissue and demonstrated an inverse correlation between Syk/MelCAM and uPA/MMP-9/VEGF expression, which further correlates with enhanced tumorigenic potential and neovascularization
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