Cyclin-dependent kinase is an enzyme encoded by the CDK4 gene in humans. CDK4 is a member of the cyclin-dependent kinase family, which is highly similar to the gene products of Saccharomyces cerevisiae cdc28 and Schizosaccharomyces cerevisiae cdc2. It is the catalytic subunit of the protein kinase complex and is important for the G1 phase of the cell cycle. The kinase activity is restricted to the G1-S phase, which is controlled by the regulatory subunit D-type cyclin and the CDK inhibitor p16INK4a. This kinase has been shown to be responsible for phosphorylation of the retinoblastoma gene product (Rb). The Ser/Thr kinase component of the cyclin D-CDK4 (DC) complex can phosphorylate and inhibit members of the retinoblastoma (RB) protein family, including RB1, and regulate the cell cycle during the G1/S transition. Phosphorylation of RB1 dissociates the transcription factor E2F from the RB/E2F complex and transcribes subsequent E2F target genes that are responsible for progressing through the G1 phase. In early G1, RB1 is hypophosphorylated. The cyclin D-CDK4 complex is a major integrator of various mitotic and anti-mitotic signals. SMAD3 is also phosphorylated in a cell cycle-dependent manner and inhibits its transcriptional activity. The ternary complex component, cyclin D/CDK4/CDKN1B, is required for nuclide translocation and cyclin D-CDK4 complex activity.
Figure 1. Protein structure of CDK4.
CDK4 pathway
The mammalian cell cycle is regulated by two key classes of molecules-cyclin regulation and catalytic CDK formation of active heterodimers, leading to phosphorylation of target proteins. CDK4 and its close homolog CDK6 are serine / threonine kinases that form heterodimers with D-type cyclins and are the major regulators of the G1-S transition in the cell cycle (Figure 2).
Together with the cyclin E–CDK2 complex, the cyclin D–CDK4/6 complex in turn phosphorylates retinoblastoma protein (RB1), thereby reducing its ability to inhibit RNA polymerase I and III activity and gene transcription (Figure 3). Hypophosphorylated RB1 regulates gene transcription by inhibiting the role of the E2F transcription factor family and recruiting histone deacetylase to the promoter of the desired gene in the S phase. These inhibitory effects of phosphorylated RB1 result in the regulation of ribosome biogenesis, which affects protein synthesis and transcription factors that coordinate subsequent cell cycle processes, nucleotide biosynthesis, DNA replication, mitotic processes, and DNA damage repair. Therefore, CDK4/6-mediated RB1 inactivation is essential for cell cycle progression. This process is negatively regulated by the tumor suppressor gene p16INK4A, which specifically inhibits the assembly and activation of the cyclin D–CDK4/6 complex. Further control of this pathway is performed through a negative feedback loop, where CDK4/6 inactivation of RB1 reduces the inhibitory effect of RB1-mediated p16INK4A, leading to an increase in p16INK4A, followed by a decrease in CDK4/6 activity. Therefore, this negative feedback loop effectively plays a natural braking role in activating the path. In HPV-infected cells, due to the absence of RB1, p16INK4A expression is also increased, of which E7 protein causes RB1 degradation, and this expression is also present in cells that genetically have RB1. These data confirm that regulatory feedback from RB1 to p16INK4A exists in both physiological and pathological conditions. In these cases, RB1 is permanently lost, and this loss prevents the increased p16INK4A expression from inhibiting the cell cycle. In normal cells, p16INK4A levels are usually low, allowing the cell cycle to progress. Oncogenic signal transduction, DNA damage, and increased p16INK4A expression after physiological aging inhibit cell proliferation. These three processes also trigger cell senescence, suggesting that p16INK4A-mediated cell cycle arrest may be a necessary condition for inducing aging.
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