Cancer represents a pathological manifestation of uncontrolled cell division; therefore, it has long been anticipated that our understanding of the basic principles of cell cycle control would result in effective cancer therapies. In particular, cyclin-dependent kinases (CDKs) that promote transition through the cell cycle were expected to be key therapeutic targets because many tumorigenic events ultimately drive proliferation by impinging on CDK4 or CDK6 complexes in the G1 phase of the cell cycle. Moreover, perturbations in chromosomal stability and aspects of S phase and G2/M control mediated by CDK2 and CDK1 are pivotal tumorigenic events. Translating this knowledge into successful clinical development of CDK inhibitors has historically been challenging, and numerous CDK inhibitors have demonstrated disappointing results in clinical trials. Here, we review the biology of CDKs, the rationale for therapeutically targeting discrete kinase complexes and historical clinical results of CDK inhibitors. We also discuss how CDK inhibitors with high selectivity (particularly for both CDK4 and CDK6), in combination with patient stratification, have resulted in more substantial clinical activity.

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The history and future of targeting cyclin-dependent kinases in cancer therapy

Cancer is characterized by uncontrolled tumour cell proliferation resulting from aberrant activity of various cell cycle proteins. Therefore, cell cycle regulators are considered attractive targets in cancer therapy. Intriguingly, animal models demonstrate that some of these proteins are not essential for proliferation of non-transformed cells and development of most tissues. By contrast, many cancers are uniquely dependent on these proteins and hence are selectively sensitive to their inhibition. After decades of research on the physiological functions of cell cycle proteins and their relevance for cancer, this knowledge recently translated into the first approved cancer therapeutic targeting of a direct regulator of the cell cycle. In this Review, we focus on proteins that directly regulate cell cycle progression (such as cyclin-dependent kinases (CDKs)), as well as checkpoint kinases, Aurora kinases and Polo-like kinases (PLKs). We discuss the role of cell cycle proteins in cancer, the rationale for targeting them in cancer treatment and results of clinical trials, as well as the future therapeutic potential of various cell cycle inhibitors.

Cell Cycle Proteins as Promising Targets in Cancer Therapy

Uncontrolled cellular proliferation, mediated by dysregulation of the cell-cycle machinery and activation of cyclin-dependent kinases (CDKs) to promote cell-cycle progression, lies at the heart of cancer as a pathological process. Clinical implementation of first-generation, nonselective CDK inhibitors, designed to inhibit this proliferation, was originally hampered by the high risk of toxicity and lack of efficacy noted with these agents. The emergence of a new generation of selective CDK4/6 inhibitors, including ribociclib, abemaciclib and palbociclib, has enabled tumour types in which CDK4/6 has a pivotal role in the G1-to-S-phase cell-cycle transition to be targeted with improved effectiveness, and fewer adverse effects. Results of pivotal phase III trials investigating palbociclib in patients with advanced-stage oestrogen receptor (ER)-positive breast cancer have demonstrated a substantial improvement in progression-free survival, with a well-tolerated toxicity profile. Mechanisms of acquired resistance to CDK4/6 inhibitors are beginning to emerge that, although unwelcome, might enable rational post-CDK4/6 inhibitor therapeutic strategies to be identified. Extending the use of CDK4/6 inhibitors beyond ER-positive breast cancer is challenging, and will likely require biomarkers that are predictive of a response, and the use of combination therapies in order to optimize CDK4/6 targeting.

Treating cancer with selective CDK4/6 inhibitors

The pattern of DNA methylation at cytosine bases in the genome is tightly linked to gene expression, and DNA methylation abnormalities are often observed in diseases. The ten eleven translocation (TET) enzymes oxidize 5-methylcytosines (5mCs) and promote locus-specific reversal of DNA methylation. TET genes, and especially TET2, are frequently mutated in various cancers, but how the TET proteins contribute to prevent the onset and maintenance of these malignancies is largely unknown. Here, we highlight recent advances in understanding the physiological function of the TET proteins and their role in regulating DNA methylation and transcription. In addition, we discuss some of the key outstanding questions in the field.

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Role of TET enzymes in DNA methylation, development, and cancer

Cells of the osteoblast lineage affect the homing and the number of long-term repopulating haematopoietic stem cells, haematopoietic stem cell mobilization and lineage determination and B cell lymphopoiesis. Osteoblasts were recently implicated in pre-leukaemic conditions in mice. However, a single genetic change in osteoblasts that can induce leukaemogenesis has not been shown. Here we show that an activating mutation of β-catenin in mouse osteoblasts alters the differentiation potential of myeloid and lymphoid progenitors leading to development of acute myeloid leukaemia with common chromosomal aberrations and cell autonomous progression. Activated β-catenin stimulates expression of the Notch ligand jagged 1 in osteoblasts. Subsequent activation of Notch signalling in haematopoietic stem cell progenitors induces the malignant changes. Genetic or pharmacological inhibition of Notch signalling ameliorates acute myeloid leukaemia and demonstrates the pathogenic role of the Notch pathway. In 38% of patients with myelodysplastic syndromes or acute myeloid leukaemia, increased β-catenin signalling and nuclear accumulation was identified in osteoblasts and these patients showed increased Notch signalling in haematopoietic cells. These findings demonstrate that genetic alterations in osteoblasts can induce acute myeloid leukaemia, identify molecular signals leading to this transformation and suggest a potential novel pharmacotherapeutic approach to acute myeloid leukaemia.

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Leukaemogenesis induced by an activating β-catenin mutation in osteoblasts

https://www.cell.com/cancer-cell/pdf/S1535-6108(12)00396-0.pdf

Therapeutic Targeting of the Cyclin D3:CDK4/6 Complex in T Cell Leukemia

Notch signaling pathway activation is known to contribute to the pathogenesis of a spectrum of human malignancies, including T cell leukemia. However, recent studies have implicated the Notch pathway as a tumor suppressor in myeloproliferative neoplasms and several solid tumors. Here we report a novel tumor suppressor role for Notch signaling in acute myeloid leukemia (AML) and demonstrate that Notch pathway activation could represent a therapeutic strategy in this disease. We show that Notch signaling is silenced in human AML samples, as well as in AML-initiating cells in an animal model of the disease. In vivo activation of Notch signaling using genetic Notch gain of function models or in vitro using synthetic Notch ligand induces rapid cell cycle arrest, differentiation, and apoptosis of AML-initiating cells. Moreover, we demonstrate that Notch inactivation cooperates in vivo with loss of the myeloid tumor suppressor Tet2 to induce AML-like disease. These data demonstrate a novel tumor suppressor role for Notch signaling in AML and elucidate the potential therapeutic use of Notch receptor agonists in the treatment of this devastating leukemia.

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Jacquelyn Freund

Papers

Therapeutic Targeting of the Cyclin D3:CDK4/6 Complex in T Cell Leukemia

Notch pathway activation targets AML-initiating cell homeostasis and differentiation