MYC is stabilized by MALT1 function. (A) Primary mouse splenocytes expressing either wild-type MALT1 (^+/+) or a catalytically inactive MALT1 mutant (ki/ki) were stimulated with phorbol myristate acetate (PMA) and ionomycin for the indicated time points. Stimulation efficiency and MALT1 activation were assessed by western blotting using anti-p-extracellular signal-regulated kinase (p-ERK) and anti-CYLD antibodies, respectively. (B) Rec-1 and Mino cells were first treated with z-VRPR-fmk or DMSO for 24 hours and subsequently with cycloheximide (CHX). MYC protein expression was assessed by western blot using samples collected at the indicated time points. In both cell lines, MALT1 inhibition resulted in a reduced half-life of MYC protein. (C) Mino, Rec-1, and SP53 cells were treated with z-VRPR-fmk or DMSO and subsequently with MG132 or DMSO. MYC protein levels were increased by MG132 treatment as evaluated by western blotting. (D) In Rec-1, SP53, Maver-1, and Z-138 cells, either a control shRNA against MSMO1 or 1 of the 2 MALT1 shRNAs were induced with doxycycline for 24 hours. Subsequently, cells were treated with MG132 or DMSO. MYC protein levels were increased by MG132 treatment in MALT1-activated MCLs (Rec-1 and SP53), but not in MALT1-inactive MCLs (Maver-1 and Z-138) as evaluated by western blotting.