Integrin αVβ3 silencing sensitizes malignant glioma cells to temozolomide by suppression of homologous recombination repair

Integrins have been suggested as possible targets in anticancer therapy. Here we show that knockdown of integrins alpha V beta 3, alpha V beta 5, alpha 3 beta 1 and alpha 4 beta 1 and pharmacological inhibition using a cyclo-RGD integrin alpha V beta 3/alpha V beta 5 antagonist sensitized multiple high-grade glioma cell lines to temozolomide (TMZ)-induced cytotoxicity. The greatest effect was observed in LN229 cells upon integrin beta 3 silencing, which led to inhibition of the FAK/Src/Akt/NF.B signaling pathway and increased formation of gamma H2AX foci. The integrin beta 3 knockdown led to the proteasomal degradation of Rad51, reduction of Rad51 foci and reduced repair of TMZ-induced DNA double-strand breaks by impairing homologous recombination efficiency. The down-regulation of beta 3 in Rad51 knockdown (LN229-Rad51kd) cells neither further sensitized them to TMZ nor increased the number of gamma H2AX foci, confirming causality between beta 3 silencing and Rad51 reduction. RIP1 was found cleaved and I kappa Ba significantly less degraded in beta 3-silenced/TMZ-exposed cells, indicating inactivation of NF kappa B signaling. The anti-apoptotic proteins Bcl-xL, survivin and XIAP were proteasomally degraded and caspase-3/-2 cleaved. Increased H2AX phosphorylation, caspase-3 cleavage, reduced Rad51 and RIP1 expression, as well as sustained I kappa Ba expression were also observed in mouse glioma xenografts treated with the cyclo-RGD inhibitor and TMZ, confirming the molecular mechanism in vivo. Our data indicates that beta 3 silencing in glioma cells represents a promising strategy to sensitize high-grade gliomas to TMZ therapy.