Distinct CDK6 complexes determine tumor cell response to CDK4/6 inhibitors and degraders
Ontology highlight
ABSTRACT:
INSTRUMENT(S): Q Exactive HF-X
ORGANISM(S): Homo Sapiens (ncbitaxon:9606)
SUBMITTER: 
Beatrix Ueberheide
PROVIDER: MSV000086571 | MassIVE | Wed Dec 09 13:44:00 GMT 2020
REPOSITORIES: MassIVE
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Project description:Cyclin dependent kinase 4 and 6 (CDK4/6) in complex with D-type cyclins promote cell cycle entry, at least in part through phosphorylation of the retinoblastoma tumor suppressor protein (Rb). The CDK4/6-cyclin D-Rb pathway is commonly deregulated in human cancer, often through CDK4/6 or D-type cyclin overexpression, or inactivation of the CDK4/6 antagonist p16/CDKN2A. Importantly, a substantial fraction of cancers depend on continuous CDK4/6-cyclin D kinase activity and are sensitive to CDK4/6-specific inhibitors. Here, we investigate critical CDK4/6-cyclin D functions that may determine the sensitivity to CDK4/6 inhibitors, making use of the essential roles of CDK4/6 (CDK-4) and cyclin D (CYD-1) in the nematode C. elegans. In an unbiased screen, we found that simultaneous loss of C. elegans Rb (lin-35) and down-regulation of the APC/C substrate specificity factor FZR1/Cdh1 completely overcomes CDK-4/CYD-1 requirement. Furthermore, CDK-4/CYD-1 phosphorylates specific residues in the LIN-35 Rb spacer domain and FZR-1 N-terminus that correspond to inactivating phosphorylations of the human homologs. Thus, CDK-4/CYD-1 appears to promote cell cycle entry by antagonizing not only transcriptional repression by LIN-35 Rb but also protein degradation by APC/CFZR-1. Simultaneous knockdown of Rb and FZR1 in human breast cancer cells synergistically overcomes arrest by the CDK4/6-specific inhibitor PD 00332991. These results reveal APC/CFZR1 as a putative CDK4/6-Cyclin D target and important contributing factor in the response to CDK4/6-inhibitor treatment.
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Project description:Malignant peripheral nerve sheath tumors (MPNSTs) are lethal, Ras-driven sarcomas that lack effective therapies. Ras effectors, CDK4/6 and MEK, were identified as rational targets for combination therapy from patient tumor analyses and preclinical drug studies. In MPNST cells, low-dose combinations of CDK4/6 and MEK inhibitors synergistically reactivated the retinoblastoma (RB1) tumor suppressor, induced cell death, and decreased clonogenic survival. In immune-deficient mice, dual CDK4/6-MEK inhibition slowed growth but did not shrink MPNSTs in 4 of 5 patient-derived xenografts (PDXs). By comparison, combination therapy of de novo MPNSTs in immunocompetent mice caused tumors to regress, delayed resistant tumor outgrowth, and improved survival relative to monotherapies. While drug-resistant tumors adopted an immunosuppressive microenvironment enriched with MHC II-low macrophages, drug-sensitive tumors that regressed contained plasma cells and increased T cell clustering. In other cancers, intratumoral plasma cells are associated with better response to immune checkpoint blockade (ICB). Impressively, CDK4/6-MEK inhibition sensitized MPNSTs to anti-PD-L1 ICB therapy with some mice displaying complete tumor ablation. These results reveal a novel plasma cell-associated immune response and extended antitumor activity of combined CDK4/6-MEK inhibition in MPNSTs, which dramatically enhanced the efficacy of ICB therapy. This work highlights promising, potentially curative treatment options for MPNSTs.
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