Adenovirus-mediated delivery of bFGF small interfering RNA reduces STAT3 phosphorylation and induces the depolarization of mitochondria and apoptosis in glioma cells U251

Abstract

Glioblastoma multiforme (GBM) carries a dismal prognosis primarily due to its aggressive proliferation in the brain regulated by complex molecular mechanisms. One promising molecular target in GBM is over-expressed basic fibroblast growth factor (bFGF), which has been correlated with growth, progression, and vascularity of human malignant gliomas. Previously, we reported significant antitumor effects of an adenovirus-vector carrying bFGF small interfering RNA (Ad-bFGF-siRNA) in glioma in vivo and in vitro. However, its mechanisms are unknown. Signal transducer and activator of transcription 3 (STAT3) is constitutively active in GBM and correlates positively with the glioma grades. In addition, as a specific transcription factor, STAT3 serves as the convergent point of various signaling pathways activated by multiple growth factors and/or cytokines. Therefore, we hypothesized that the proliferation inhibition and apoptosis induction by Ad-bFGF-siRNA may result from the interruption of STAT3 phosphorylation. In the current study, we found that in glioma cells U251, Ad-bFGF-siRNA impedes the activation of ERK1/2 and JAK2, but not Src, decreases IL-6 secretion, reduces STAT3 phosphorylation, decreases the levels of downstream molecules CyclinD1 and Bcl-xl, and ultimately results in the collapse of mitochondrial membrane potentials as well as the induction of mitochondrial-related apoptosis. Our results offer a potential mechanism for using Ad-bFGF-siRNA as a gene therapy for glioma. To our knowledge, it is the first time that the bFGF knockdown using adenovirus-mediated delivery of bFGF siRNA and its potential underlying mechanisms are reported. Therefore, this finding may open new avenues for developing novel treatments against GBM.

Figure 1

Ad-bFGF-siRNA reduces STAT3 phosphorylation in U251 cells. (A) Western blot analysis revealed that the levels of pSTAT3 (Tyr705) and pJAK2 are higher in U251 cells than in normal human astrocytes (NHA). (B) Ad-bFGF-siRNA (MOI = 100) reduces STAT3 phosphorylation (both Tyr705 and Ser727) in a time-dependent manner in U251 cells. Total STAT3 expression remains stable.

Figure 2

Ad-bFGF-siRNA reduces the activation of upstream molecules and the expression of downstream molecules of STAT3 in U251 cells. (A) Ad-bFGF-siRNA (MOI = 100) reduces the phosphorylation/activation of ERK1/2 and JAK2 in a time-dependent manner in U251 cells. Total ERK1/2 and JAK2 expression remains stable. Total and phosphorylated Src decreases at 48 h in a similar manner. (B) Ad-bFGF-siRNA (MOI = 100) reduces the expression of CyclinD1 and Bcl-xl at 72 h time point.

Figure 3

Ad-bFGF-siRNA reduces IL-6 secretion in U251 cells. (A) ELISA analysis showed that IL-6 secretion in the Ad-bFGF-siRNA group (MOI = 100) was lower than that in the control and Ad-GFP groups during both 24-48 h and 48-72 h periods. **: p < 0.0001. Data are presented as mean ± SD, n = 3. (B) U251 cells infected with Ad-bFGF-siRNA for 48 h were treated with serum-free DMEM in the presence or absence of recombinant IL-6 (100 ng/ml) for 24 h. Cells treated with DMSO for 72 h served as controls. The phosphorylation of STAT3 at both Tyr705 and Ser727 is elevated after stimulated with IL-6 for 24 h.

Figure 4

Ad-bFGF-siRNA reduces the mitochondrial transmembrane potential (ΔΨm) and induces apoptosis in U251 cells. (A) Cytofluorimetric analysis using JC-1 staining demonstrated that Ad-bFGF-siRNA treatment (MOI = 100) induces depolarization of mitochondria. Percentages of cells with high ΔΨm (%) are shown in each column. Data are represented as mean ± SD of three replicates (**: P < 0.0005). Changes in ΔΨm were also detected by fluorescence microscopy. Magnification: 200×. Scale bar: 50 μm. Normal cells that have high ΔΨm show punctuate yellow fluorescence. Apoptotic cells show diffuse green fluorescence because of the decrease in mitochondrial membrane potential. (B) Western blot analysis revealed that Ad-bFGF-siRNA (MOI = 100 for 72 h) increases the expressions of Cytochrome C, Caspase3, and Bax.