Prosurvival autophagy is regulated by protein kinase CK1 alpha in multiple myeloma

Marilena Carrino,Sara Canovas Nunes,Sabrina Manni,Gianpietro Semenzato,Laura Quotti Tubi,Francesco Piazza,Gregorio Barilà,Livio Trentin,Renato Zambello,Anna Fregnani

doi:10.1038/s41420-019-0179-1

Abstract

Multiple myeloma (MM) is a tumor of plasma cells (PCs). Due to the intense immunoglobulin secretion, PCs are prone to endoplasmic reticulum stress and activate several stress-managing pathways, including autophagy. Indeed, autophagy deregulation is maladaptive for MM cells, resulting in cell death. CK1α, a pro-survival kinase in MM, has recently been involved as a regulator of the autophagic flux and of the transcriptional competence of the autophagy-related transcription factor FOXO3a in several cancers. In this study, we investigated the role of CK1α in autophagy in MM. To study the autophagic flux we generated clones of MM cell lines expressing the mCherry-eGFP-LC3B fusion protein. We observed that CK1 inhibition with the chemical ATP-competitive CK1 α/δ inhibitor D4476 resulted in an impaired autophagic flux, likely due to an alteration of lysosomes acidification. However, D4476 caused the accumulation of the transcription factor FOXO3a in the nucleus, and this was paralleled by the upregulation of mRNA coding for autophagic genes. Surprisingly, silencing of CK1α by RNA interference triggered the autophagic flux. However, FOXO3a did not shuttle into the nucleus and the transcription of autophagy-related FOXO3a-dependent genes was not observed. Thus, while the chemical inhibition with the dual CK1α/δ inhibitor D4476 induced cell death as a consequence of an accumulation of ineffective autophagic vesicles, on the opposite, CK1α silencing, although it also determined apoptosis, triggered a full activation of the early autophagic flux, which was then not supported by the upregulation of autophagic genes. Taken together, our results indicate that the family of CK1 kinases may profoundly influence MM cells survival also through the modulation of the autophagic pathway.

Highlights

Multiple myeloma (MM) is a tumor of plasma cells (PCs) that accumulate in the bone marrow (BM) causing BM insufficiency, osteolytic bone lesions and hypercalcemia[1]
D4476 treatment impaired the autophagic flux after lysosome fusion, while CK1α silencing did not promote the nuclear localization and the transcriptional activity of FOXO3a, with the final result of de-fueling the autophagic process. Since both D4476 treatment and CK1α silencing culminate in MM cell death[13], our findings suggest that the deregulation of autophagy upon CK1α inactivation may be deleterious for MM cells, pointing to a role for this kinase as a master regulator of stress signaling in malignant PCs
We speculated that the observed accumulation of both LC3B-II and p62 in D4476-treated cells could depend on different mechanisms: (i) induction by the autophagic stimuli of new p62 synthesis, replacing the p62 protein being degraded inside the autophagosomes; (ii) blockage of the autophagic flux without consequent degradation of the autophagosomes

Summary

Introduction

Multiple myeloma (MM) is a tumor of plasma cells (PCs) that accumulate in the bone marrow (BM) causing BM insufficiency, osteolytic bone lesions and hypercalcemia[1]. MM cells massively secrete immunoglobulins, provoking blood hyperviscosity, renal insufficiency and amyloidosis[2], and causing endoplasmic reticulum (ER). In MM, silencing of autophagic molecules, such as ATG7 and SQSTM1/p62, induces cells death[5]. Compounds such as 3methyladenine (3-MA) that prevents autophagy at the earliest stage of autophagosome formation, and chloroquine that disrupts lysosomal acidification preventing autophagosomes fusion and degradation, produce cytotoxic effects to MM cells[6]. The hyperactivation of autophagy is maladaptive for MM cells. Official journal of the Cell Death Differentiation Association

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