Hypomorphic mTOR Downregulates CDK6 and Delays Thymic Pre-T LBL Tumorigenesis

Abstract

◥PI3K/AKT/mTOR pathway hyperactivation is frequent in T-cell acute lymphoblastic leukemia/lymphoma (T-ALL/LBL). To model inhibition of mTOR, pre-T-cell lymphoblastic leukemia/lymphoma (pre-T LBL) tumor development was monitored in mice with T lymphocyte-specific, constitutively active AKT (Lck-MyrAkt2) that were either crossed to mTOR knockdown (KD) mice or treated with the mTOR inhibitor everolimus. Lck-MyrAkt2;mTOR KD mice lived significantly longer than Lck-MyrAkt2;mTOR wild-type (WT) mice, although both groups ultimately developed thymic pre-T LBL. An increase in survival was also observed when Lck-MyrAkt2;mTOR WT mice were treated for 8 weeks with everolimus. The transcriptional profiles of WT and KD thymic lymphomas were compared, and Ingenuity Pathway Upstream Regulator Analysis of differentially expressed genes in tumors from mTOR WT versus KD mice identified let-7 and miR-21 as potential regulatory genes. mTOR KD mice had higher levels of let-7a and miR-21 than mTOR WT mice, and rapamycin induced their expression in mTOR WT cells. CDK6 was one of the most downregulated targets of both let-7 and miR21 in mTOR KD tumors. CDK6 overexpression and decreased expression of let-7 in mTOR KD cells rescued a G 1 arrest phenotype. Combined mTOR (rapamycin) and CDK4/6 (palbociclib) inhibition decreased tumor size and proliferation in tumor flank transplants, increased survival in an intravenous transplant model of disseminated leukemia compared with single agent treatment, and cooperatively decreased cell viability in human T-ALL/LBL cell lines. Thus, mTOR KD mice provide a model to explore drug combinations synergizing with mTOR inhibitors and can be used to identify downstream targets of inhibition.

Overexpression of wild-type IL-7Rα promotes T-cell acute lymphoblastic leukemia/lymphoma

“…These results are in line with other studies providing evidence of the potential of Bcl-2 inhibitors against T-ALL. 73-75 In addition, in agreement with IL-7 induction of cell cycle progression in human T-ALL cells, 16 we demonstrated the efficacy of the Cdk4/6 inhibitor palbociclib 76 in our mouse T-ALLs.…”

MicroRNAs as Potential Predictors of Response to CDK4/6 Inhibitor Treatment

“…Overall, 16 articles were identified and screened. After removal of 2 irrelevant studies [38,39] and 1 non-eligible article [37], 13 articles were considered eligible for our study [40][41][42][43][44][45][46][47][48][49][50][51][52]. A thorough search in ClinicalTrials.gov (access date on 1 January 2021) for additional studies retrieved no eligible studies.…”

“…Thymic tumors of mTOR knock-down mice were characterized by upregulation of miR-21 and let-7a miRNAs and undetectable levels of the CDK6 protein. These tumor cells with reduced mTOR activity demonstrated greater resistance to treatment with palbociclib than tumor cells from mTOR wild-type mice [45]. Collectively, genetic or pharmacological downregulation of mTOR resulted in let-7 and miR21 increase and, subsequently, in downregulation of CDK6.…”

“…Collectively, genetic or pharmacological downregulation of mTOR resulted in let-7 and miR21 increase and, subsequently, in downregulation of CDK6. Accordingly, simultaneous inhibition of the mTOR pathway and CDK6 activity by palbociclib exerted greater antitumor activity than either treatment alone, both in vitro in human T cell acute lymphoblastic leukemia/lymphoma and in vivo [45].…”

“…let-7a and miR-21: This study describes an mTOR-let-7/miR21-CDK6 axis in murine thymic T-cell acute lymphoblastic leukemia/lymphoma tumors (T-ALL/LBL). mTOR inhibition resulted in let-7 and miR21 upregulation and consequent repression of CDK6 [45]. Thymic tumors of mTOR knock-down mice were characterized by upregulation of miR-21 and let-7a miRNAs and undetectable levels of the CDK6 protein.…”