Mcl-1 downregulation by pro-inflammatory cytokines and palmitate is an early event contributing to β-cell apoptosis

Abstract

Pancreatic b-cell apoptosis is a key feature of diabetes mellitus and the mitochondrial pathway of apoptosis is a major mediator of b-cell death. We presently evaluated the role of the myeloid cell leukemia sequence 1 (Mcl-1), an antiapoptotic protein of the Bcl-2 family, in b-cells following exposure to well-defined b-cell death effectors, for example, pro-inflammatory cytokines, palmitate and chemical endoplasmic reticulum (ER) stressors. All cytotoxic stresses rapidly and preferentially decreased Mcl-1 protein expression as compared with the late effect observed on the other antiapoptotic proteins, Bcl-2 and Bcl-xL. This was due to ER stress-mediated inhibition of translation through eIF2a phosphorylation for palmitate and ER stressors and through the combined action of translation inhibition and JNK activation for cytokines. Knocking down Mcl-1 using small interference RNAs increased apoptosis and caspase-3 cleavage induced by cytokines, palmitate or thapsigargin, whereas Mcl-1 overexpression partly prevented Bax translocation to the mitochondria, cytochrome c release, caspase-3 cleavage and apoptosis induced by the b-cell death effectors. Altogether, our data suggest that Mcl-1 downregulation is a crucial event leading to b-cell apoptosis and provide new insights into the mechanisms linking ER stress and the mitochondrial intrinsic pathway of apoptosis. Mcl-1 is therefore an attractive target for the design of new strategies in the treatment of diabetes.

Is ARE/poly(U)-binding factor 1 (AUF1) a new player in cytokine-mediated beta cell apoptosis?

“…1). This is paralleled by the decrease of two other important anti-apoptotic proteins: BCL-2 and MCL-1 [28], and by the increase in expression/activation of the BH3-only proteins PUMA and BIM [30,31]. Activation of the BH3-only proteins results in BAX translocation to the mitochondria, outer mitochondrial membrane permeabilisation, cytochrome c release, caspase activation and consequently beta cell apoptosis [9,28,32].…”

“…Activation of the intrinsic apoptotic pathway is a key event in cytokine-mediated beta cell apoptosis [7,32,33], and overproduction of BCL-2 [34], BCL-XL [35] and MCL-1 prevents cytokine-induced apoptosis in vitro [28]. In vivo experiments, however, have shown that BCL-XL overproduction induces beta cell dysfunction [35], while increased production of BCL-2 is not sufficient to prevent autoimmune destruction of beta cells [36].…”

“…Of note, although induction of UPR is related to NO production in rat beta cells [24], NO does not seem to be required in mouse islets [25]. The mechanisms by which ER stress contributes to beta cell apoptosis are not completely clear, but induction of the pro-apoptotic proteins C/EBP homologous protein (CHOP), and death protein 5 (also called hara-kiri) (DP5) [26,27], and downregulation of the anti-apoptotic protein myeloid cell leukaemia sequence 1 (MCL-1), probably play a role in this process [28].…”

“…Although a direct role of AUF1 in Bcl2 mRNA stability has been observed, it is not yet clear how AUF1 regulates MCL-1. MCL-1 has no ARE site and cytokines regulate MCL-1 expression post-translationally [28]. The effect of AUF1 on MCL-1 may thus result in its effects on NFκB activation and NO production [28].…”

“…MCL-1 is a member of the B cell leukaemia/lymphoma 2 (BCL-2) family of proteins, which regulates the intrinsic (mitochondrial) apoptosis pathway in beta cells [28]. The BCL-2 proteins can be divided into the anti-apoptotic proteins (BCL-2, BCL-XL, BCL-W, MCL-1 and A1), the multi-domain pro-apoptotic proteins (BCL2-associated [29].…”