Cyclin-dependent kinase 4 (CDK4)–Cyclin D complex (CDK4–Cyclin D)

The Cyclin-dependent kinase 4 (CDK4)–Cyclin D complex is a critical regulator of the eukaryotic cell cycle, specifically governing the transition from the G1 phase to the S phase [1]. Upon activation by mitogenic signals, D-type cyclins (D1, D2, or D3) bind to CDK4, forming an active holoenzyme that phosphorylates the Retinoblastoma (Rb) protein [2]. This phosphorylation event releases E2F transcription factors, which then trigger the expression of genes required for DNA replication [1, 2]. Dysregulation of this complex, often through Cyclin D1 amplification or loss of the endogenous inhibitor p16INK4a, is a hallmark of various malignancies, including hormone receptor-positive breast cancer and mantle cell lymphoma [3]. Therapeutic targeting of the complex with selective CDK4/6 inhibitors has become a standard of care, effectively inducing cell cycle arrest and inhibiting tumor growth [4]. However, clinical use is often limited by side effects such as myelosuppression and gastrointestinal distress, and resistance can develop through bypass mechanisms like CDK2 activation or Rb loss [5]. Sources: [1] UniProt (P11802); [2] StatPearls (CDK4/6 Inhibitors); [3] PubMed (PMID: 28273103); [4] FDA Label (Ibrance); [5] PubMed (PMID: 30333116).

Other names

CDK4/Cyclin D complexCCND1-CDK4 complexCyclin D1-CDK4 complexCDK4-CCND complex

Competitive inhibition of the ATP-binding site of the CDK4 enzyme within the complex, which prevents the phosphorylation of the Retinoblastoma (Rb) protein. This maintains Rb in its active, hypophosphorylated state, sequestering E2F transcription factors and inducing G1-phase cell cycle arrest [2, 4].