Proteasome inhibition-induced p38 MAPK/ERK signaling regulates autophagy and apoptosis through the dual phosphorylation of glycogen synthase kinase 3β

► MG132 induces the phosphorylation of GSK3βSer9 and, to a lesser extent, of GSK3βThr390. ► MG132 induces dephosphorylation of p70S6KThr389 and phosphorylation of p70S6KThr421/Ser424. ► Inactivation of p38 dephosphorylates GSK3βSer9 and phosphorylates GSK3βThr390. ► Inactivation of p38 phosphorylates p70S6KThr389 and increases the phosphorylation of p70S6KThr421/Ser424. ► Inactivation of p38 decreases autophagy and increases apoptosis induced by MG132. Proteasome inhibition is a promising approach for cancer treatment; however, the underlying mechanisms involved have not been fully elucidated. Here, we show that proteasome inhibition-induced p38 mitogen-activated protein kinase regulates autophagy and apoptosis by modulating the phosphorylation status of glycogen synthase kinase 3β (GSK3β) and 70kDa ribosomal S6 kinase (p70S6K). The treatment of MDA-MB-231 cells with MG132 induced endoplasmic reticulum stress through the induction of ATF6a, PERK phosphorylation, and CHOP, and apoptosis through the cleavage of Bax and procaspase-3. MG132 caused the phosphorylation of GSK3β at Ser9 and, to a lesser extent, Thr390, the dephosphorylation of p70S6K at Thr389, and the phosphorylation of p70S6K at Thr421 and Ser424. The specific p38 inhibitor SB203080 reduced the p-GSK3βSer9 and autophagy through the phosphorylation of p70S6KThr389; however, it augmented the levels of p-ERK, p-GSK3βThr390, and p-70S6KThr421/Ser424 induced by MG132, and increased apoptotic cell death. The GSK inhibitor SB216763, but not lithium, inhibited the MG132-induced phosphorylation of p38, and the downstream signaling pathway was consistent with that in SB203580-treated cells. Taken together, our data show that proteasome inhibition regulates p38/GSKSer9/p70S6KThr380 and ERK/GSK3βThr390/p70S6KThr421/Ser424 kinase signaling, which is involved in cell survival and cell death.