Although RIPK1 (receptor [TNFRSF]-interacting protein kinase 1) is emerging as a critical determinant of cell fate in response to cellular stress resulting from activation of death receptors and DNA damage, its potential role in cell response to endoplasmic reticulum (ER) stress remains undefined. Here we report that RIPK1 functions as an important prosurvival mechanism in melanoma cells undergoing pharmacological ER stress induced by tunicamycin (TM) or thapsigargin (TG) through activation of autophagy. While treatment with TM or TG upregulated RIPK1 and triggered autophagy in melanoma cells, knockdown of RIPK1 inhibited autophagy and rendered the cells sensitive to killing by TM or TG, recapitulating the effect of inhibition of autophagy. Consistently, overexpression of RIPK1 enhanced induction of autophagy and conferred resistance of melanoma cells to TM- or TG-induced cell death. Activation of MAPK8/JNK1 or MAPK9/JNK2, which phosphorylated BCL2L11/BIM leading to its dissociation from BECN1/Beclin 1, was involved in TM- or TG-induced, RIPK1-mediated activation of autophagy; whereas, activation of the transcription factor HSF1 (heat shock factor protein 1) downstream of the ERN1/IRE1-XBP1 axis of the unfolded protein response was responsible for the increase in RIPK1 in melanoma cells undergoing pharmacological ER stress. Collectively, these results identify upregulation of RIPK1 as an important resistance mechanism of melanoma cells to TM- or TG-induced ER stress by protecting against cell death through activation of autophagy, and suggest that targeting the autophagy-activating mechanism of RIPK1 may be a useful strategy to enhance sensitivity of melanoma cells to therapeutic agents that induce ER stress.
Keywords: 3-MA, 3-methyladenine; AMPK, AMP-activated protein kinase; ATF6, activating transcription factor 6; Baf A1, bafilomycin A1; CAMKK2, calcium/calmodulin-dependent protein kinase kinase 2: β; EIF2AK3/PERK, eukaryotic translation initiation factor 2-α kinase 3; ER, endoplasmic reticulum; ERN1/IRE1, endoplasmic reticulum to nucleus signaling 1; HSF1, heat shock transcription factor 1; HSPA5, heat shock 70kDa protein 5 (glucose-regulated protein: 78kDa); MAP2K1/MEK1, mitogen-activated protein kinase kinase 1; MAPK, mitogen-activated protein kinase; MAPK1/ERK2, mitogen-activated protein kinase 1; MAPK11/p38β, mitogen-activated protein kinase 11; MAPK12/p38γ, mitogen-activated protein kinase 12; MAPK13/p38δ, mitogen-activated protein kinase 13; MAPK14/p38α, mitogen-activated protein kinase 14; MAPK3/ERK1, mitogen-activated protein kinase 3; MAPK8/JNK1, mitogen-activated protein kinase 8; MAPK9/JNK2, mitogen-activated protein kinase 9; NFKB1, nuclear factor of kappa light polypeptide gene enhancer in B-cells 1; PRKAA1, protein kinase AMP-activated: α 1 catalytic subunit; RIPK1; RIPK1, receptor (TNFRSF)-interacting protein kinase 1; SQSTM1/p62, sequestosome 1; TG, thapsigargin; TM, tunicamycin; TNFRSF1A/TNFR1, tumor necrosis factor receptor superfamily: member 1A; UPR, unfolded protein response; XBP1, x-box binding protein 1; autophagy; cell death; endoplasmic reticulum stress; melanoma.