Abemaciclib induces atypical cell death in cancer cells characterized by formation of cytoplasmic vacuoles derived from lysosomes

Abstract

In the cell cycle, the G1/S transition is controlled by the cyclin‐dependent kinase (CDK) 4/6‐cyclin D complex. Constitutive activation of CDK4/6 dysregulates G1/S transition, leading to oncogenic transformation. We found that 3 CDK4/6 inhibitors, abemaciclib, ribociclib, and palbociclib, exerted a cytocidal effect as well as a cytostatic effect at the G1 phase in cancer cell lines, including A549 human non–small cell lung cancer cells. Among these inhibitors, abemaciclib exhibited the most potent cytotoxic effect. The cell‐death phenotype induced by abemaciclib, which entailed formation of multiple cytoplasmic vacuoles, was not consistent with apoptosis or necroptosis. Abemaciclib blocked autophagic flux, resulting in accumulation of autophagosomes, however vacuole formation and cell death induced by abemaciclib were independent of autophagy. In addition, methuosis, a cell‐death phenotype characterized by vacuole formation induced by excessive macropinocytosis, was excluded because the vacuoles did not incorporate fluorescent dextran. Of note, both formation of vacuoles and induction of cell death in response to abemaciclib were inhibited by vacuolar‐type ATPase (V‐ATPase) inhibitors such as bafilomycin A1 and concanamycin A. Live‐cell imaging revealed that the abemaciclib‐induced vacuoles were derived from lysosomes that expanded following acidification. Transmission electron microscopy revealed that these vacuoles contained undigested debris and remnants of organelles. Cycloheximide chase assay revealed that lysosomal turnover was blocked by abemaciclib. Furthermore, mTORC1 inhibition along with partial lysosomal membrane permeabilization occurred after abemaciclib treatment. Together, these results indicate that, in cancer cells, abemaciclib induces a unique form of cell death accompanied by swollen and dysfunctional lysosomes.

Abemaciclib, A Selective CDK4/6 Inhibitor, Restricts the Growth of Pediatric Ependymomas

“…These data suggest that abemaciclib causes caspase-independent cell death in ependymoma cells. Abemaciclib-induced caspase-independent cell death has also been found in multiple myeloma cells [32], and another study suggested that abemaciclib induces lysosomal swelling and dysfunction, resulting in atypical cell death [39]. Cytoplasmic vacuoles were also observed in SNU-EP2 and SNU-EP1203 cells after 24 h of abemaciclib treatment.…”

Cyclin-dependent kinase inhibitors exert distinct effects on patient-derived 2D and 3D glioblastoma cell culture models

“…Here, these characteristics superficially may resemble methuosis(-like) processes in addition to the appearance of apoptosis 43 . Recent data from the literature support this mechanism, as some antineoplastic agents, including abemaciclib promote vacuolization, which may lead to methuosis 24 , 44 , 45 .…”

“…CDKi-mediated apoptosis has been described in various tumor models 19 – 23 . By contrast, autophagy was only recently described for abemaciclib 24 and just one publication described the cytoprotective role of autophagy under dinaciclib therapy in NSCLC 25 . Though not analyzed in detail here, LC3B-II aggregation and decreased p62/sequestosome1 expression levels are likely alterations as earlier shown for flavopiridol, another pan-CDKi 26 .…”

Targeting Autophagy in Breast Cancer

“…Also, Abemaciclib was reported to induce autophagy in multiple myeloma and renal cell carcinoma models [128,129]. However, it was recently reported that, in different cancer models, Abemaciclib induced an atypical cell death, independent from autophagy, accompanied by lysosomal dysfunction, block of autophagic flux, and accumulation of autophagosomes [130]. Recently, a relevant finding suggested that CDK4/6 kinases regulate lysosome biogenesis during cell cycle, through phosphorylation of TFEB/TFE3 in the nucleus, thereby promoting their shuttling to the cytoplasm.…”