Table 2.

Characteristics of acquired TMZ resistant GBM cell lines.

Host cellsa	TMZ resistant cells		IC50 (μM) to TMZ	Molecular events of adapted TMZ resistant cells	TMZ resistant cells are sensitive to	References
	Selection method	Maintenance method
SF188	Stepwise exposure of the cells to TMZ (50–300 μM) for 6 months	Maintain the TMZ resistant cells in TMZ free cell culture medium, but the cells were frequently incubated with 300 μM TMZ	SF188: 426 μM SF188_R: 1854 μM	Increased activity of AGT Reduced expression of pro-apoptotic proteins (Bad, Bax, Bcl-Xs) No change for mismatch repair enzymes Not affected by p53 status, because parent and TMZ resistant cells contain mutant p53	TMZ resistant cells are sensitive to the AGT inhibitor O6-benzylguanine (BG)	Ma J et al.39
Primary tumor	Incremental concentrations of TMZ (2.5, 5, 7.5, 10 μM) for 1 h for 5 consecutive days. This step was repeated several times until the resulting cell population was resistant	The TMZ resistant cells re-treated with 10 μM TMZ every 8-10 passages		Overexpressed MGMT, Decreased expression of TNFAIP3 (NF-kB pathway modulator, encode the zinc finger protein A20), NFKBIA (NF-kB inhibiting IkB family member), C8orf4, and LIF		Bredel M et al.40
SNB-19	Incremental concentrations of TMZ (3, 5, 10, 20, 30, 60, 150 μM)	Maintain the TMZ resistant cells in TMZ free cell culture medium	SNB-19: 1.03 μM SNB-19A4: 101 μM SNB-19C1: 55 μM	No detectable MGMT expression in TMZ resistant cells Gene alteration (loss of 2p16.1-2p25.3, loss of partial amplification of the 4p14.4-4q21.22, loss of amplification of the 16q12.1–16q22.1 and 1p13.2-1q21.1)		Auger N et al.41
U87, U251, M059K, M059J	100 μM TMZ for 2 weeks		U251: ∼50 μM U251_R: >300 μM	MGMT expression is not involved in the acquisition of TMZ resistance in U251_R cells Up-regulated microRNA such as miR-10a, miR-195, miR-455-3p	miR-195 knockdown showed moderate growth inhibition to U251_R cells combined treatment with both miR-10a or miR-455-3p inhibitors with TMZ showed better cytotoxicity	Ujifuku K et al.19
SNB19, U373	Stepwise increment TMZ concentrations (1, 2, 5, 10, 20, 50, 100 μM) for 6 months	Maintain the TMZ resistant cells in 100 μM TMZ	SNB19: 36 μM U373: 68 μM SNB19_R: 280 μM U373_R: 289 μM	SNB19_R & U373_R cells acquired TMZ resistance via distinct mechanisms. SNB19_R: down-regulation of MSH6 message and protein (under continued presence of TMZ), up-regulation of BER gene NTL1 U373_R: MGMT expression, but its expression requires the selective pressure of continued TMZ presence		Zhang J et al.42
U251	Stepwise 2 fold increase of TMZ concentration from 2.5 μM to 1 mM for 6 months	Maintain the TMZ resistant cells in 160 μM TMZ. For over 50 passages, the resistance to TMZ was retained		Decreased MGMT expression Decreased mitochondrial DNA copy number Large heteroplasmic mtDNA deletions Remodeling of the entire electron transport chain (Significant decreases of complexes I and V and increases of complexes II/III and IV)		Oliva CR et al.43
U251, U373, T98G	TMZ concentration is 2 fold increased every two passages from 12.5 μM to 800 μM for 2 months		U251: 100 μM U373: 50 μM U251_R: 1000 μM U373_R: 800 μM	TMZ resistant cells do not have altered MGMT expression, but have upregulation of STAT3 and pSTAT3 (Ser727) while pSTAT3 (Tyr705) was decreased	STAT3 siRNA sensitize TMZ resistant cells	Lee E-S et al.44
LN-18, LNT-229, LN-308	24 h TMZ exposure every 2 weeks. Increasing concentration of TMZ for 6 months		LN-308: <40 μM LN-18: ∼400 μM LN-308_R: >400 μM LN-18_R: ∼800 μM	LN-18_R: up-regulation of MGMT LNT-229_R: down-regulation of DNA mismatch-repair protein LN-308_R: reduced methylation of LINE-1 repetitive elements		Happold C et al.25
U87	Culture the cells for 3 weeks with a low dose of TMZ		U87: <40 μM (growth inhibition) or <10 μM (clonogenic assay) U87_R: 150 μM (growth inhibition) or >400 μM (clonogenic assay)	Upregulation of MGMT and STAT3	STAT3 inhibitor	Kohsaka S et al.26
U343	Culture the cells with 200 μM of TMZ for 1 month. Continuous TMZ (150 μM) treatment for at least 5 months.		U343: <50 μM U343_R: 280.63 μM	Increased invasiveness Increased JNK signaling pathway Activation of known JNK effector paxillin	JNK inhibitor SP600125 or JNK siRNA suppressed up-regulation of invasiveness	Ueno H et al.45
U373	At every two passages, TMZ concentration is increased subsequently from 12.5 μM to 500 μM for months			TMZ resistant cells have upregulation of glucose, citrate, and isocitrate levels TMZ sensitive cells have upregulation of alanine, choline, creatine, and phosphorylcholine	A glucose analog (2-Deoxy-d-glucose) alone or with TMZ is cytotoxic to TMZ resistant cells	St-Coeur P-D et al.29
U251	Stepwise 2 fold increase of TMZ concentration from 1.25 μM to 160 μM for 10 months		U251: 58 μM U251_R: 271 μM	Upregulation of MGMT and phosphorylated-p65	IkBα inhibitor BAY 11-7082 sensitize TMZ resistant cells growth Combination effect of IkBα inhibitor and TMZ on TMZ resistant cells	Wang X et al.46
A172, LN229, U87	Culture the cells with 20 μM of TMZ → Culture the survival cells with 40 μM of TMZ → Culture the survival cells with 40 μM of TMZ			TMZ resistance is not due to increased repair of O6-methylguanin but is correlated to decreased mismatch repair (MMR) activity such as MSH2 and MSH6		McFaline-Figueroa JL et al.47
A172, GBM cancer stem cells	Culture the cells with continuous TMZ (200 or 400 μM) for 30 days		GBM3: 98 μM GBM3_R: >1000 μM GBM5: 634 μM GBM5_R: 1115 μM	TMZ resistant cells grow slowly Increased histone lysine demethylase (KDMs) gene expression, especially KDM5A No change in MGMT and drug efflux mechanisms	TMZ resistant cells are sensitive to the histone deacetylase (HDAC) inhibitor (SAHA 1 μM) and TMZ (200 μM) combination treatment	Banelli B et al.48

AGT: O⁶-methylguanine-DNA alkyltransferase; C8orf4: Chromosome 8 open reading frame 4; HDAC: histone deacetylase; KDM: Histone lysine demethylase; LIF: Leukemia inhibitory factor; MGMT: O⁶-methylguanine-DNA methyltransferase; MMR: Mismatch repair; O6-BG: O6-benzylguanine; SAHA: N-hydroxy-N′-phenyl-octanediamide; STAT3: Signal transducers and activators of transcription 3; TNFAIP3: Tumor necrosis factor-α-induced protein 3.

^aBold indicates that these cell lines were commonly used as host cells to generate acquired TMZ resistant cells.