Downregulation of the Rho GTPase pathway abrogates resistance to ionizing radiation in wild-type p53 glioblastoma by suppressing DNA repair mechanisms.

Glioblastoma (GBM), the most common aggressive brain tumor, is characterized by rapid cellular infiltration and is routinely treated with ionizing radiation (IR), but therapeutic resistance inevitably recurs. The actin cytoskeleton of glioblastoma cells provides their high invasiveness, but it remains unclear whether Rho GTPases modulate DNA damage repair and therapeutic sensitivity. Here, we irradiated glioblastoma cells with different p53 status and explored the effects of Rho pathway inhibition to elucidate how actin cytoskeleton disruption affects the DNA damage response and repair pathways. p53-wild-type and p53-mutant cells were subjected to Rho GTPase pathway modulation by treatment with C3 toxin; knockdown of mDia-1, PFN1 and MYPT1; or treatment with F-actin polymerization inhibitors. Rho inhibition increased the sensitivity of glioma cells to IR by increasing the number of DNA double-strand breaks and delaying DNA repair by nonhomologous end-joining in p53-wild-type cells. p53 knockdown reversed this phenotype by reducing p21 expression and Rho signaling activity, whereas reactivation of p53 in p53-mutant cells by treatment with PRIMA-1 reversed these effects. The interdependence between p53 and Rho is based on nuclear p53 translocation facilitated by G-actin and enhanced by IR. Isolated IR-resistant p53-wild-type cells showed an altered morphology and increased stress fiber formation: inhibition of Rho or actin polymerization decreased cell viability in a p53-dependent manner and reversed the resistance phenotype. p53 silencing reversed the Rho inhibition-induced sensitization of IR-resistant cells. Rho inhibition also impaired the repair of IR-damaged DNA in 3D spheroid models. Rho GTPase activity and actin cytoskeleton dynamics are sensitive targets for the reversal of acquired resistance in GBM tumors with wild-type p53.

Polyoxovanadates as new P-glycoprotein inhibitors: insights into the mechanism of inhibition.

A promising strategy to overcome multidrug resistance is the use of inhibitors of ABC drug transporters. For this reason, we evaluated the polyoxovanadates (POVs) [V10 O28 ]6- (V10 ), [H6 V14 O38 (PO4 )]5- (V14 ), [V15 O36 Cl]6- (V15 ) and [V18 O42 I]7- (V18 ) as inhibitors of three major multidrug resistance-linked ABC transporters: P-glycoprotein (P-gp), ABCG2 and MRP1. All of the POVs selectively inhibited P-gp. V10 and V18 were the two most promising compounds, with IC50 values of transport inhibition of 25.4 and 22.7 µm, respectively. Both compounds inhibited P-gp ATPase activity, with the same IC50 value of 1.26 µm. V10 and V18 triggered different conformational changes in the P-gp protein with time-dependent inhibition, which was confirmed using the synthesized salt of V10 with rhodamine B, RhoB-V10 . The hydrophilic nature of POVs supports the hypothesis that these compounds target an unusual ligand-binding site, opening new possibilities in the development of potent modulators of ABC transporters.