ABSTRACT
Cyclin-dependent kinase-6 (CDK6) is required for early thymocyte development and tumorigenesis. To mechanistically dissect the role of CDK6 in thymocyte development, we generated and analyzed mutant knock-in mice and found that mice expressing a kinase-dead Cdk6 allele (Cdk6(K43M)) had a pronounced reduction in thymocytes and hematopoietic stem cells and progenitor cells (Linâ»Sca-1âºc-Kit⺠[LSK]). In contrast, mice expressing the INK4-insensitive, hyperactive Cdk6(R31C) allele displayed excess proliferation in LSK and thymocytes. However, this is countered at least in part by increased apoptosis, which may limit progenitor and thymocyte expansion in the absence of other genetic events. Our mechanistic studies demonstrate that CDK6 kinase activity contributes to Notch signaling because inactive CDK6 kinase disrupts Notch-dependent survival, proliferation, and differentiation of LSK, with concomitant alteration of Notch target gene expression, such as massive up-regulation of CD25. Further, knockout of CD25 in Cdk6(K43M) mice rescued most defects observed in young mice. These results illustrate an important role for CDK6 kinase activity in thymocyte development that operates partially through modulating Notch target gene expression. This role of CDK6 as a downstream mediator of Notch identifies CDK6 kinase activity as a potential therapeutic target in human lymphoid malignancies.
Subject(s)
Cell Differentiation/physiology , Cell Proliferation , Cyclin-Dependent Kinase 6/biosynthesis , Gene Expression Regulation, Enzymologic/physiology , Signal Transduction/physiology , Thymus Gland/enzymology , Alleles , Animals , Cell Survival/physiology , Cyclin-Dependent Kinase 6/genetics , Gene Knock-In Techniques , Hematologic Neoplasms/genetics , Hematologic Neoplasms/metabolism , Hematologic Neoplasms/therapy , Humans , Interleukin-2 Receptor alpha Subunit/biosynthesis , Interleukin-2 Receptor alpha Subunit/genetics , Mice , Mice, Knockout , Receptors, Notch/genetics , Receptors, Notch/metabolismABSTRACT
Cyclin-dependent kinase 6 (CDK6) promotes cell cycle progression and is overexpressed in human lymphoid malignancies. To determine the role of CDK6 in development and tumorigenesis, we generated and analyzed knockout mice. Cdk6-deficient mice show pronounced thymic atrophy due to reduced proliferative fractions and concomitant transitional blocks in the double-negative stages. Using the OP9-DL1 system to deliver temporally controlled Notch receptor-dependent signaling, we show that CDK6 is required for Notch-dependent survival, proliferation, and differentiation. Furthermore, CDK6-deficient mice were resistant to lymphomagenesis induced by active Akt, a downstream target of Notch signaling. These results show a critical requirement for CDK6 in Notch/Akt-dependent T-cell development and tumorigenesis and strongly support CDK6 as a specific therapeutic target in human lymphoid malignancies.
