Another phase I/II trial in adults with AML is designed to evaluate the safety, tolerability, and efficacy of palbociclib in combination with CPX-351 (an U.S. is compatible with life but leads to defects in hematopoietic cell proliferation and mild anemia [7,8]. There is increasing evidence for additional substrates and NBI-42902 functional differences between these two kinases that go beyond the control of cell cycle  (Figure 1). Contrary to gene is frequently amplified or overexpressed in a variety of human lymphomas and leukemias [7,10,11,12,13,14,15,16,17,18,19,20,21]. During the last years, it has been shown that CDK6 but not CDK4 is a direct regulator of transcription in a kinase-dependent and -independent manner, interacting with a range of transcription factors including members of the signal transducer and activator of transcription (STAT) and activator protein 1 (AP-1) family [8,22,23,24,25,26,27]. Besides inducing the transcription of the tumor suppressor p16INK4A as an endogenous feed-back loop, CDK6 also mediates the transcription of vascular endothelial growth factor A (VEGF-A), a well-characterized angiogenic factor and tumor promoter, thereby linking two hallmark cancer features [28,29]. In addition, CDK6 stabilizes the cytoskeletal integrity of erythroid cells on a transcriptional and structural level  (Figure 1). Open in a separate window Figure 1 CDK6 promotes cell-cycle progression and phosphorylates various substrates in a kinase-dependent manner and regulates transcription kinase-dependent as well NBI-42902 as kinase-independent. Recently, CDK6 was assigned a counter-regulatory function during oncogene-induced stress. Throughout transformation, CDK6 is needed to antagonize p53 responses by phosphorylating its DNA binding partners, nuclear transcription factor Y (NFY) and specific protein 1 (SP1), at promoters of p53 antagonizing genes. This finding is reflected in human gene expression signatures from patients with acute lymphoid leukemia (ALL) and myelodysplastic syndrome (MDS). Moreover, an enrichment of p53 negative regulators and NFY target genes showed a positive correlation with CDK6 NBI-42902 across myeloid and lymphoid disease entities. These data Mouse monoclonal to ITGA5 point at the requirement of additional mutations in the p53 pathway to overcome oncogenic stress when CDK6 kinase activity is blocked. (janus kinase 2; show a significantly prolonged latency with mitigated clinical symptoms, including increased red blood cell and platelet counts . In line with data from untransformed HSCs , CDK6 is needed to release the most dormant JAK2-V617F+ HSCs from quiescence which is shown in increased long- and short-term HSC numbers in mice. The underlying mechanism includes an altered cytokine secretion and malignant stem cell activation which is regulated by CDK6 in a largely kinase-independent manner. Moreover, apoptotic players are regulated by CDK6 (e.g., ((((show enhanced apoptosis. ((. In an attempt to clarify the requirement of CDK6 kinase activity, RNA-Seq experiments have been performed using the CDK4/6 inhibitor palbociclib. These data reveal a predominant kinase-independent role of CDK6 in JAK2-V617F+ stem/progenitor cells including the altered apoptosis signaling . Further support for a predominant kinase-independent role of CDK6 in JAK2-V617F+ disease stems from studies with human patient samples: primary mononuclear cells from the bone marrow of JAK2-V617F-positive MPN patients treated with palbociclib fail to show increased signs of apoptosis . These data suggest that fine-tuning CDK6 levels may be beneficial for the management of MPN and provides a rationale for the development and implication of CDK6-specific degraders. 3. The Role of CDK6 in AML 3.1. CDK6 as Driver and Therapeutic Target in MLL Rearrangements The ((gene occur in 80% of infant ALL cases but are less common in older children and adults (5C10%; primarily AML) . A key functional feature of MLL translocations is their ability to lead to aberrant expression of stem cell gene programs and thus to confer leukemia-initiating activity to hematopoietic stem/progenitor cells (HSPCs) . Recently, CDK6 but not CDK4 was found to be a direct target of MLL-fusion proteins in infant MLL-AF4+ (MLL-ALL1-fused gene from chromosome 4 protein) ALL  and in MLL-AF9+ (MLL-ALL1-fused gene from chromosome 9 protein) AML . MLL-AF9 binds the CDK6 locus and its forced expression in wildtype cells elevates levels of CDK6 (Figure 3). It is postulated that CDK6 drives MLL-AF9-mediated disease by inhibiting myeloid differentiation based on the observation that small hairpin RNA (shRNA)-mediated depletion of CDK6 induced myeloid differentiation in MLL-rearranged (MLLr) AML cells. In this system, cell-cycle progression remained unaffected. These effects are specific for CDK6 as rescue experiments with wildtype CDK6 reconstitute myeloid differentiation, a feature not shared by wildtype CDK4. This differentiation phenotype requires the catalytic activity of CDK6 as inhibition of CDK6 by palbociclib mimicked the results obtained with shRNA-mediated knockdown. Palbociclib exposure increases the differentiation of MLLr AML cell lines and mononuclear cells from patient-derived AML cells. In vivo proof of concept.