Integrins play a role in the resistance of advanced cancers to radiotherapy and chemotherapy. In this study, we show that high expression of the a5 integrin subunit compromises temozolomide-induced tumor suppressor p53 activity in human glioblastoma cells. We found that depletion of the a5 integrin subunit increased p53 activity and temozolomide sensitivity. However, when cells were treated with the p53 activator nutlin-3a, the protective effect of a5 integrin on p53 activation and cell survival was lost. In a functional p53 background, nutlin-3a downregulated the a5 integrin subunit, thereby increasing the cytotoxic effect of temozolomide. Clinically, a5b1 integrin expression was associated with a more aggressive phenotype in brain tumors, and high a5 integrin gene expression was associated with decreased survival of patients with high-grade glioma. Taken together, our findings indicate that negative cross-talk between a5b1 integrin and p53 supports glioma resistance to temozolomide, providing preclinical proof-of-concept that a5b1 integrin represents a therapeutic target for high-grade brain tumors. Direct activation of p53 may remain a therapeutic option in the subset of patients with high-grade gliomas that express both functional p53 and a high level of a5b1 integrin. Cancer Res; 72(14); 3463-70. Ó2012 AACR.
Caveolin-1 plays a checkpoint function in the regulation of processes often altered in cancer. Although increased expression of caveolin-1 seems to be the norm in the glioma family of malignancies, populations of caveolin-1 positive and negative cells coexist among glioblastoma specimens. As no data are available to date on the contribution of such cells to the phenotype of glioblastoma, we manipulated caveolin-1 in the glioblastoma cell line U87MG. We showed that caveolin-1 plays a critical role in the aggressiveness of glioblastoma. We identified integrins as the main set of genes affected by caveolin-1. We reported here that the phenotypic changes observed after caveolin-1 modulation were mediated by alpha(5)beta(1) integrins. As a consequence of the regulation of alpha(5)beta(1) levels by caveolin-1, the sensitivity of cells to the specific alpha(5)beta(1) integrin antagonist, SJ749, was affected. Mediator of caveolin-1 effects, alpha(5)beta(1) integrin, is also a marker for glioma aggressiveness and an efficient target for the treatment of glioma especially the ones exerting the highest aggressive phenotype.
The potential role of A 5 B 1 integrins in cancer has recently attracted much interest. However, few A 5 B 1 -selective antagonists have been developed compared with other integrins. The most specific nonpeptidic A 5 B 1 antagonist described thus far, SJ749, inhibits angiogenesis by affecting adhesion and migration of endothelial cells. We investigated the effects of SJ749 in two human astrocytoma cell lines, A172 and U87, which express different levels of A 5 B 1 . SJ749 dose-dependently inhibited adhesion of both cell types on fibronectin. Application of SJ749 to spread cells led to formation of nonadherent spheroids for A172 cells but had no effect on U87 cell morphology. SJ749 also reduced proliferation of A172 cells due to a long lasting G 0 -G 1 arrest, whereas U87 cells were only slightly affected. However, under nonadherent culture conditions (soft agar), SJ749 significantly reduced the number of colonies formed only by U87 cells. As U87 cells express more A 5 B 1 than A172 cells, we specifically examined the effect of SJ749 on A172 cells overexpressing A 5 . Treatment of A 5 -A172 cells with SJ749 decreased colony formation similarly to that observed in U87 cells. Therefore, in nonadherent conditions, the effect of SJ749 on tumor cell growth characteristics depends on the level of A 5 B 1 expression. Our study highlights the importance of A 5 B 1 as an anticancer target and shows for the first time that a small nonpeptidic A 5 B 1 -specific antagonist affects proliferation of tumor cells. (Cancer Res 2006; 66(12): 6002-7)
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