The 70 kDa ribosomal S6 kinase 1 (p70S6K1), a downstream target of phosphoinositide 3-kinase (PI3K) and ERK mitogen-activated protein kinase (MAPK), is an important regulator of cell cycle progression, and cell proliferation. Recent studies indicated an important role of p70S6K1 in PTEN-negative and AKT-overexpressing tumors. However, the mechanism of p70S6K1 in tumor angiogenesis remains to be elucidated. In this study, we specifically inhibited p70S6K1 activity in ovarian cancer cells using vector-based small interfering RNA (siRNA) against p70S6K1. We found that knockdown of p70S6K1 significantly decreased VEGF protein expression and VEGF transcriptional activation through the HIF-1 alpha binding site at its enhancer region. The expression of p70S6K1 siRNA specifically inhibited but HIF-1 alpha, but not HIF-1 beta protein expression. We also found that p70S6K1 down-regulation inhibited ovarian tumor growth and angiogenesis, and decreased cell proliferation and levels of VEGF and HIF-1 alpha expression in tumor tissues. Our results suggest that p70S6K1 is required for tumor growth and angiogenesis through HIF-1 alpha and VEGF expression, providing a molecular mechanism of human ovarian cancer mediated by p70S6K1 signaling. (C) 2010 Elsevier Inc. All rights reserved.