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This report demonstrates that expression of the P140A O6-methylguanine DNA methyl transferase (MGMT) mutant via retrovirus-mediated gene transfer leads to significant, but modest, resistance of cells to both 6-benzylguanine (6-BG) depletion and treatment with 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU). Expression of the P140A/G156A double mutant appeared to be associated with reduced or unstable protein in hematopoietic cells. [ABSTRACT FROM AUTHOR]

[This corrects the article DOI: 10.18632/oncotarget.13466.].

Epithelial ovarian cancer (EOC) is the leading cause of gynecologic cancer mortality worldwide. Platinum-based therapy is the standard first line treatment and while most patients initially respond, resistance to chemotherapy usually arises. Major signaling pathways frequently upregulated in chemoresistant cells and important in the maintenance of cancer stem cells (CSCs) include Wnt/β-catenin, mTOR, and STAT3. The major objective of our study was to investigate the treatment of ovarian cancer with targeted agents that inhibit these three pathways. Here we demonstrate that niclosamide, a salicylamide derivative, and two synthetically manufactured niclosamide analogs (analog 11 and 32) caused significant inhibition of proliferation of two chemoresistant ovarian cancer cell lines (A2780cp20 and SKOV3Trip2), tumorspheres isolated from the ascites of EOC patients, and cells from a chemoresistant patient-derived xenograft (PDX). This work shows that all three agents significantly decreased the expression of proteins in the Wnt/β-catenin, mTOR and STAT3 pathways and preferentially targeted cells that expressed the ovarian CSC surface protein CD133. It also illustrates the potential of drug repurposing for chemoresistant EOC and can serve as a basis for pathway-oriented in vivo studies.

We previously demonstrated that sustained depletion of methylguanine DNA methyltransferase (MGMT) activity is required for optimal reversal of chloroethylnitrosourea resistance in tumor cells. The purpose of this study was to design O(6)-benzylguanine (BG) treatments that deplete MGMT activity in tumor cells and xenograft tumors in a prolonged manner. When SF767 cells were treated with a bolus dose of BG (25 microM for 1 h), >95% of MGMT activity was depleted but 33% of the activity recovered within 24 h. In contrast, MGMT activity was completely depleted for 24 h when cells were pretreated with a low dose of BG (2.5 microM) for 24 h, followed by the bolus dose and same low-dose treatment for 24 h. This combination regimen of pre- and post-treatments with a bolus dose sensitized cells N,N'-bis(2-chloroethyl)-N-nitrosourea in vitro by approximately 2-fold more than the bolus dose alone. Similar BG treatment with Alzet micro-osmotic pumps produced sustained inhibition of MGMT activity in vivo. In xenograft SF767 tumors, low-dose pre- and post-treatments (8 mg/kg over 24 h) combined with an i.p. bolus dose (80 mg/kg) of BG inhibited >95% of MGMT activity for 24 h after the bolus. The bolus dose alone did not deplete MGMT for 24 h. These results demonstrate that combination low-dose and bolus BG treatment is superior to the bolus dose alone in depleting MGMT activity in a sustained manner in vitro and in vivo. When combined with N,N'-bis(2-chloroethyl)-N-nitrosourea treatment, this BG regimen also should also produce greater antitumor activity than the single bolus dose evaluated clinically.

Human parvovirus B19 gene expression from the viral p6 promoter (B19p6) is restricted to primary human hematopoietic cells undergoing erythroid differentiation. We have demonstrated that expression from this promoter does not occur in established human erythroid cell lines in the context of a recombinant parvovirus genome (Ponnazhagan et al. J Virol 69:8096-8101, 1995). However, abundant expression from this promoter can be readily detected in primary human bone marrow cells (Wang et al. Proc Natl Acad Sci USA 92:12416-12420, 1995; Ponnazhagan et al. J Gen Virol 77:1111-1122, 1996). In the present studies, we investigated the pattern of expression from the B19p6 promoter in primary human bone marrow-derived CD34+ HPC undergoing differentiation into myeloid and erythroid lineages. CD34+ cells were transduced with recombinant adeno-associated virus 2 (AAV) vectors containing the beta-galactosidase (lacZ) gene under the control of the following promoters/enhancers: the cytomegalovirus promoter (vCMVp-lacZ), B19p6 promoter (vB19p6-lacZ), B19p6 promoter with an upstream erythroid cell-specific enhancer element (HS-2) from the locus control region (LCR) from the human beta-globin gene cluster (vHS2-B19p6-lacZ), and the human beta-globin gene promoter with the HS-2 enhancer (vHS2-beta p-lacZ). Transgene expression was evaluated either 48 h after infection or following erythroid differentiation in vitro for 3 weeks. Whereas high-level expression from the CMV promoter 48 h after infection diminished with time, low-level expression from the B19p6 and the beta-globin promoters increased significantly following erythroid differentiation. Furthermore, in HPC assays, there was no significant difference in the level of expression from the CMV promoter in myeloid or erythroid cell-derived colonies. Expression from the B19p6 and the beta-globin promoters, on the other hand, was restricted to erythroid cell colonies. These data further corroborate that the B19p6 promoter is erythroid cell-specific and suggest that the recombinant AAV-B19 hybrid vectors may prove useful in gene therapy of human hemoglobinopathies in general and sickle cell anemia and beta-thalassemia in particular.

O(6)-Benzylguanine (6-BG) inactivates mammalian O(6)-methylguanine DNA methyltransferase (MGMT), an important DNA repair protein that protects cells against chloroethylnitrosourea (CENU) cytotoxicity. 6-BG is being tested as an approach to treat CENU-resistant tumors that overexpress endogenous MGMT. However, in addition to restoring CENU tumor cell sensitivity, 6-BG also increases the cytotoxic effects of CENUs on hematopoietic cells. Several 6-BG-resistant human MGMT mutants have been characterized in Escherichia coli and are predicted to protect mammalian cells against the combination of 6-BG and CENU treatment in vivo. Two mutants, P140A and P140A/G156A, demonstrated 20- and 1200-fold more resistance to 6-BG depletion of MGMT activity compared with wild-type MGMT (WTMGMT). Here, we analyzed retroviral vectors that express either WTMGMT, the P140A or P140A/G156A mutant forms of MGMT. Retroviral-infected L1210 hematopoietic cells demonstrated similar levels of RNA in all transduced clones. However, the amount of MGMT protein and DNA repair activity was reduced in clones expressing the P140A/G156A mutant compared with those expressing WTMGMT or P140A. Expression of P140A was associated with a 4- to 8-fold increase in resistance to 6-BG depletion of MGMT in transduced L1210 clones and a 1, 3-bis(2-chloroethyl)-1-nitrosourea IC(50) of 50 microM (compared with 27.5 microM for WTMGMT) in primary murine hematopoietic cells. These results demonstrate the utility of screening 6-BG-resistant MGMT proteins in hematopoietic cells and provide evidence that the P140A mutant form of MGMT generates 6-BG- and CENU-resistant hematopoietic cells. Retrovirus vectors expressing this mutant may be useful in future human gene therapy trials.