Certain tumor cell responses to the growth factor-inducible early response gene 
product CCN1/Cyr61 overlap with those induced by the hepatocyte growth factor 
(HGF)/c-Met signaling pathway. In this study, we investigate if Cyr61 is a 
downstream effector of HGF/c-Met pathway activation in human glioma cells. A 
semiquantitative immunohistochemical analysis of 112 human glioma and normal 
brain specimens showed that levels of tumor-associated Cyr61 protein correlate 
with tumor grade (P < 0.001) and with c-Met protein expression (r(2) = 0.4791, P 
< 0.0001). Purified HGF rapidly upregulated Cyr61 mRNA (peak at 30 minutes) and 
protein expression (peak at 2 hours) in HGF(-)/c-Met(+) human glioma cell lines 
via a transcription- and translation-dependent mechanism. Conversely, HGF/c-Met 
pathway inhibitors reduced Cyr61 expression in HGF(+)/c-Met(+) human glioma cell 
lines in vitro and in HGF(+)/c-Met(+) glioma xenografts. Targeting Cyr61 
expression with small interfering RNA (siRNA) inhibited HGF-induced cell 
migration (P < 0.01) and cell growth (P < 0.001) in vitro. The effect of Cyr61 
on HGF-induced Akt pathway activation was also examined. Cyr61 siRNA had no 
effect on the early phase of HGF-induced Akt phosphorylation (Ser(473)) 30 
minutes after stimulation with HGF. Cyr61 siRNA inhibited a second phase of Akt 
phosphorylation measured 12 hours after cell stimulation with HGF and also 
inhibited HGF-induced phosphorylation of the Akt target glycogen synthase kinase 
3alpha. We treated preestablished subcutaneous glioma xenografts with Cyr61 
siRNA or control siRNA by direct intratumoral delivery. Cyr61 siRNA inhibited 
Cyr61 expression and glioma xenograft growth by up to 40% in a dose-dependent 
manner (P < 0.05). These results identify a Cyr61-dependent pathway by which 
c-Met activation mediates cell growth, cell migration, and long-lasting 
signaling events in glioma cell lines and possibly astroglial malignancies.