CDK4 and CDK6 kinases: From basic science to cancer therapy

Abstract

Cyclin-dependent kinases 4 and 6 (CDK4 and CDK6) and their activating partners, D-type cyclins, link the extracellular environment with the core cell cycle machinery. Constitutive activation of cyclin D–CDK4/6 represents the driving force of tumorigenesis in several cancer types. Small-molecule inhibitors of CDK4/6 have been used with great success in the treatment of hormone receptor–positive breast cancers and are in clinical trials for many other tumor types. Unexpectedly, recent work indicates that inhibition of CDK4/6 affects a wide range of cellular functions such as tumor cell metabolism and antitumor immunity. We discuss how recent advances in understanding CDK4/6 biology are opening new avenues for the future use of cyclin D–CDK4/6 inhibitors in cancer treatment.

Fig. 1.. Molecular events governing progression through the G1 phase of the cell-cycle.

Mammalian-cell cycle can be divided into G1, S (DNA-synthesis), G2 and M (mitosis) phases. During G1 phase, cyclin D (CycD)-CDK4/6 kinases together with cyclin E (CycE)-CDK2 phosphorylate the retinoblastoma protein, RB1. This activates the E2F transcriptional program, and allows entry of cells into S-phase. INK family of inhibitors (p16^INK4A, p15^INK4B, p18^INK4C and p19^INK4D) inhibit cyclin D-CDK4/6; KIP/CIP proteins (p21^CIP1, p27^KIP1 and p57^KIP2) inhibit cyclin E-CDK2. Cyclin D-CDK4/6 complexes use p27^KIP1 and p21^CIP1 as ‘assembly factors’ and sequester them away from cyclin E-CDK2, thereby activating CDK2. Proteins that are frequently lost or downregulated in cancers are marked with green arrows, overexpressed proteins with red arrows.

Fig. 2.. CDK4 and CDK6 – more than cell cycle kinases.

While the role of CDK4 and CDK6 in cell-cycle progression has been well-documented, both kinases regulate several other functions that are only now starting to be unraveled. (A) Inhibition of CDK4/6 (CDK4/6i) affects lysosome and mitochondrial numbers, as well as oxidative phosphorylation. Cyclin D3-CDK6 phosphorylates glycolytic enzymes 6-phosphofruktokinase (PFKP) and pyruvate kinase M2 (PKM2), thereby controlling ROS levels via the pentose phosphate (PPP) and serine pathways. (B) Inhibition of CDK4/6 affects the anti-tumor immunity, acting both within cancer cells as well as on the immune system of the host. In tumor cells, inhibition of CDK4/6 impedes expression of an E2F-target, DNA methyltransferase (DNMT). DNMT-inhibition reduces methylation of endogenous retroviral genes (ERV), and increases intracellular levels of double-stranded (ds) RNA (114). In effector T-cells, inhibition of CDK4/6 stimulates NFAT transcriptional activity and enhances secretion of IFNγ and interleukin 2 (IL2) (115).

Fig. 3.. Mechanisms of cancer cell resistance to CDK4/6-inhibition.

Known mechanisms include loss of RB1, activation of pathways impinging on cycD-CDK4/6, amplification of the CDK4/6 genes and overexpression of CDK6 protein, activation of CycE-CDK2, lysosomal sequestration of CDK4/6-inhibitors. Blank pieces of the puzzle denote additional mechanisms that remain to be discovered.

Print Figure.. Targeting cyclin D-CDK4/6 for cancer treatment.

D-cyclins (CycD) activate CDK4 and CDK6 in G1 phase of the cell cycle. CycD-CDK4/6 contribute to cell-cycle progression by phosphorylating the retinoblastoma protein, RB1. RB1 inhibits E2F transcription-factors; phosphorylation of RB1 activates E2F-driven transcription. In many cancers CycD-CDK4/6 is constitutively activated and drives uncontrolled cell proliferation. The development of small-molecule CDK4/6-inhibitors provided a therapeutic tool to repress constitutive CycD-CDK4/6 activity and inhibit cancer cell proliferation. As with several targeted therapies, many tumors eventually develop resistance and resume cell proliferation despite CDK4/6-inhibition. New combination treatments, involving CDK4/6-inhibitors plus inhibition of other pathways, are being tested in the clinics to delay or overcome the resistance.