ATF4 and glioblastoma: TMZ-resistant cell model was further established by using the high-level drug-resistant GBM cells (LN229, U87MG, and T98G) as well as the LN229 cells transfected with pcDNA3.1-ATF4 overexpression plasmid, named ATF4 overexpression LN229 cells (LN229ATF4O cells).