Distinct molecular mechanisms of acquired resistance to temozolomide in glioblastoma cells

Abstract

Despite recent therapeutic advances, the prognosis of patients afflicted by glioblastoma remains poor, with a progression-free survival in the range of months, even with multimodal therapy including surgery, radio-and chemotherapy. Temozolomide (TMZ), an oral alkylating agent, has demonstrated activity against recurrent and newly diagnosed glioblastoma (Yung et al. 2000;Stupp et al. 2005), and is being used as the standard of care for newly diagnosed glioblastoma since 2005.The DNA repair protein O 6 -methylguanine methyltransferase (MGMT) removes O 6 -alkyl-guanine adducts from DNA through irreversible binding and degradation, thereby minimizing the DNA-damaging effects of alkylating agent chemotherapy (Wang et al. 1996;Phillips et al. 1997 Abstract Temozolomide (TMZ) is an alkylating chemotherapeutic agent that prolongs the survival of patients with glioblastoma. Clinical benefit is more prominent in patients with methylation of the O 6 -methyl-guanine DNA methyltransferase (MGMT) promoter. However, all patients eventually suffer from tumor progression because their tumors become resistant to TMZ. Here, we modeled acquired TMZ resistance in glioma cells in vitro to identify underlying molecular mechanisms. To this end, the glioma cell lines LNT-229, LN-308, and LN-18 were exposed repetitively to increasing concentrations of TMZ to induce a stable resistant phenotype (R) defined by clonogenic survival assays. The molecular mechanisms mediating acquired resistance were assessed by immunoblot, PCR, and flow cytometry. Rescue experiments were performed with siRNA-mediated candidate gene silencing. We found in LN-18 cells constitutively expressing MGMT a strong up-regulation of MGMT levels in TMZ-resistant cells. TMZ resistance in the MGMT-negative cell lines LNT-229 and LN-308 was not associated with de novo expression of MGMT. Instead, we found a down-regulation of several DNA mismatchrepair proteins in resistant LNT-229 cells. A TMZ-resistant phenotype was also achieved by silencing selected DNA mismatch repair proteins in parental LNT-229 cells. No obvious mechanism of resistance was identified in the third cell line, LN-308, except for reduced methylation of LINE-1 repetitive elements. In conclusion, we demonstrate that different molecular mechanisms may contribute to the development of acquired TMZ resistance in glioma cells, indicating the need to develop distinct strategies to overcome resistance.

FBW7 is associated with prognosis, inhibits malignancies and enhances temozolomide sensitivity in glioblastoma cells

“…Since the establishment of standard TMZ chemotherapy in glioma treatment, acquired chemotherapeutic resistance has emerged as one of the most important obstacles leading to tumor reoccurrence and refractoriness, which can be regulated by a series of mechanisms such as aberrantly enhanced DNA repair, cell stemness conversion, alteration of metabolism, elevated multidrug resistant phenotype, and imbalance of apoptosis or autophagy 17, 18, 29. Therefore, enhancing the sensitivity to TMZ was a pivotal measure to improve the treatment outcome of glioma.…”

Involvement of HIF-1α activation in the doxorubicin resistance of human osteosarcoma cells

“…Furthermore, high levels of p21 have also been associated with drug resistance in acute myelogenous leukaemia, head and neck carcinomas, and colon carcinoma (45)(46)(47). Moreover, in many late-stage glioblastomas, known to be extraordinarily resistant to chemotherapy and radiation (48,49), resistance to apoptosis has been linked to elevated p21 levels (50,51). Altogether, these data support a positive role for p21 in tumour cell survival; this protein may be induced by chemotherapeutic agents promoting its expression, as observed in our doxorubicin-resistant MG-63 cells.…”

Immunological effects of chemotherapy and radiotherapy against brain tumors

“…CRT, HMGB1 and several other intracellular Brain tumors with high mutational load are particularly responsive to immune checkpoint inhibition (34). TMZ can induce a "hypermutation phenotype" with alterations of DNA mismatch repair (MMR) genes (35,36). Hence, it needs to be assessed whether brain tumor patients that were treated with TMZ will respond better to immune checkpoint inhibition.…”