Autophagy Inhibition Enhances CDK4/6 Inhibitor-Induced Apoptosis in t(8;21) Acute Myeloid Leukemia Cells

Acute myeloid leukemia (AML) is a genetically and clinically heterogeneous disease. Although t(8;21) AML patients have a more favorable prognosis than other cytogenetic subgroups, nearly 40% of t(8;21) AML patients experience relapse. Therefore, novel therapeutic approaches based on a better understanding of the biology of t(8;21) AML need to be developed.

In this study, at first, we re-analyzed the sequencing data of 149 pediatric t(8;21) AML patients from St. Jude Children's Research Hospital tissue resource core facility and the JPLSG AML-05 study, and 134 adult t(8;21) AML patients from CALGB/Alliance trials and the University Hospital of Ulm. In pediatric patients, 13 CCND2 mutations were detected in 11 patients (11/149, 7.4%), and in adult patients, 14 CCND2 mutations were detected in 12 patients (12/134, 9.0%). In both cohorts, CCND2 mutations were located on the PEST domain, suggesting that the mutations stabilize the cyclin D2 protein.

Next, we compared CCND2 mRNA expression between t(8;21) AML patients (n=24) and non-t(8;21) AML patients (n=163) using the TARGET AML cohort. In non-t(8;21) AML patients, CCND2 expression varied from low to high levels, whereas in t(8;21) AML patients, CCND2 expression was restricted to higher levels. Consistently, CCND2 expression was higher in t(8;21) AML cell lines (n=2: Kasumi-1 and SKNO-1), compared with non-t(8;21) AML cell lines (n=32). Kasumi-1 cells transfected with shCCND2 showed cell cycle arrest at G1 phase and impaired cell proliferation. These results suggest that the frequency of CCND2 mutations and CCND2 expression are increased in t(8;21) AML, and high CCND2 expression plays an important role in t(8;21) AML cell proliferation.

Because CCND2 is not a druggable target, we examined the effect of CDK4/6 inhibitors (palbociclib and abemaciclib) on t(8;21) AML cells. Analysis of 19 AML cell lines showed that t(8;21) AML cells had lower IC50 values for CDK4/6 inhibitors than non-t(8;21) AML cells. CDK4/6 inhibitors caused cell cycle arrest at G1 phase and impaired cell proliferation in t(8;21) AML cells.

To identify potential therapeutic approaches in combination with CDK4/6 inhibitors in t(8;21) AML, we performed microarray analysis and examined the effects of CDK4/6 inhibition. In addition to the pathways associated with the cell cycle (regulation of sister chromatid separation, retinoblastoma gene, and cell cycle), the MAP-ERK and PI3K-AKT-mTOR signaling pathways were downregulated by CDK4/6 inhibition. Because these pathways are involved in autophagy regulation via mTOR, we focused on examining autophagy in subsequent experiments.

Assessment of the effect of CDK4/6 inhibition on autophagy in t(8;21) AML cells showed that the CDK4/6 inhibitor (abemaciclib) treatment induced LC3B-I to LC3B-II conversion in both Kasumi-1 and SKNO-1 cells. Transmission electron microscopic examination of autophagosome formation detected a large number of autophagosomes in the cytoplasm of Kasumi-1 and SKNO-1 cells treated with abemaciclib, whereas few autophagosomes were detected in control samples. These results suggest that autophagy is induced by CDK4/6 inhibition in t(8;21) AML cells.

Autophagy is involved in the resistance to chemotherapy in cancer cells, therefore, we hypothesized that autophagy inhibition may be a promising therapeutic approach. Treatment of t(8;21) AML cells with the autophagy inhibitors chloroquine (CQ) or LY294002 in combination with abemaciclib significantly increased the frequency of apoptotic (Annexin V positive) cells compared with that in untreated cells, whereas CQ or LY294002 single treatment had no significant effect on apoptosis. Consistently, combinatorial inhibition of CDK4/6 and autophagy upregulated cleaved caspase 3 expression. The combinatorial effect was confirmed by silencing the autophagy-related protein ATG7 using small interfering RNA in abemaciclib-treated t(8;21) AML cells. These results suggest that autophagy inhibition enhances CDK4/6 inhibitor-induced apoptosis in t(8;21) AML cells.

In conclusion, the present results indicate that inhibition of CDK4/6 and autophagy may be a novel and promising biomarker-driven therapeutic strategy for the treatment of t(8;21) AML.