Primary human GBM neurospheres expressed high levels of the
Primary human GBM neurospheres expressed high levels of the metabolic enzyme fatty acyl-CoA synthetase VL3 (ACSVL3) that were associated with CSCs self-renewal and tumorigenic capacity. ACSVL3 knockdown significantly decreased ALDH levels, indicating its contribution to cell metabolism and maintenance of stem cell phenotype.
Role of ALDH in Therapy Resistance and Pharmacological Targeting In addition, GBM cell lines with high ALDH1A1 expression but without MGMT promoter methylation (ALDH1A1+/MGMT−) exhibited significant resistance to temozolomide (TMZ) compared with ALDH1A1/MGMT+, ALDH1A1−/MGMT+, and ALDH1A1+/MGMT− cell lines. However, when these cells were treated with both TMZ and DEAB, their clonogenic capacity was notably reduced and neurosphere formation was attenuated. ALDH1A1 knockdown by shRNA and concomitant administration of TMZ significantly decreased cell proliferation and induced G2/M arrest . GBM cells are also highly resistant to the anticancer deoxyribonucleoside analog gemcitabine [2,2′-difluorodeoxycytidine (dFdC)]. Interestingly, the cytotoxicity of dFdC was markedly enhanced in human GBM cell lines and ALDH+ CSCs when administered in combination with the specific ALDH inhibitor tetraethylthiuram disulfide (disulfiram/copper, DS/Cu), mainly by increasing their apoptosis. Moreover, DS/Cu/dFdC treatment abolished CSCs capacity to form neurospheres and their metastatic potential . Apart from the irreversible inhibition of ALDH1A1, DS/Cu blocks NF-κB translocation and PI3K/AKT signaling, thus affecting cell proliferation while also inactivating metastasis-associated genes. Several studies indicate DS/Cu efficacy for Hydroxyurea tumors and two clinical trials (NCT01907165 and NCT01777919) are currently evaluating its effect in GBM patients 8, 10. Dual inhibition of ALDH and oxidative phosphorylation was performed recently in GBM tumor spheres using the polyphenol gossypol and the biguanide phenformin with encouraging results over viability, stemness, and invasiveness . When ATP depletion was overcome by malate supplementation, the effects were reversed. ALDH1L1 knockdown mimicked their effects, indicating the critical involvement of ALDH in mediating ATP production in GBM. These drugs were further tested in a mouse orthotopic xenograft model with high efficacy, suggesting bioenergetic deprivation as a novel promising therapeutic strategy. Two novel ALDH inhibitors, GA11 and GA23, derivatives of the natural product daidzin, able to penetrate the blood–brain barrier, were shown to specifically reduce growth of MES83 glioma spheres with high ALDH expression (ALDHhigh) compared with PN. Regular administration of GA11 to MES83 and MES267 glioma sphere lines-based mouse brain tumors impaired tumor development and prolonged survival . In addition, a Korean herbal recipe MSC500 inhibited high ALDH1 activity in CSCs of the GBM8401 cell line and suppressed stemness genes along with CSC markers . Finally, a garlic-originated compound Z-ajoene with tumor antiproliferative effects suppressed ALDH+ CSCs in a U87GBM cell line.
Concluding Remarks and Future Perspectives
Introduction A microbial formation of 3-ketoglycosides in Agrobacterium tumefaciens was recognized in the early 1960s and the enzyme responsible for this reaction was first identified by Bernaerts and De Ley in 1963 . However, the enzyme was purified and given the trivial name “d-glucoside 3-dehydrogenase” by Hayano and Fukui in 1967 . d-glucoside 3-dehydrogenase (EC.126.96.36.199; G3DH) is a flavine adenine dinucleotide related (FAD-related) enzyme that converts various types of glucoside by oxidizing the hydroxyl group at the C-3 position to their corresponding 3-ketoglucosides [, , ]. G3DH transfers electrons via cytochrome to terminate the oxidation reaction; however, this enzyme can use other artificial electron acceptors, for example, 2,6-dichlorophenolindophenol (DCPIP), phenazine methosulfate (PMS), and ferric cyanide . G3DH has physiological importance as a common factor for glucosides and amino acids transportation in the energy-supplying system, as a membrane structure to maintain the transport reactions, and as a member of respiration system that prepares 3-ketoglucosides, the first intermediates in carbohydrate metabolic pathway [7,8].