RB1/E2F regulatory axis

The RB1/E2F regulatory axis is a key molecular regulatory mechanism of the cell cycle that controls the transition from the G1 phase to the S phase and, thereby, the onset of DNA replication. It acts as a molecular cell cycle brake and prevents uncontrolled cell proliferation.

The main components of the RB1/E2F control axis

are:

• the tumor suppressor protein pRB (retinoblastoma protein), encoded by the RB1 gene

and

• the family of E2F transcription factors (in higher eukaryotes, E2F refers to an important family of transcription factors. These proteins regulate the cell cycle, DNA replication, and programmed cell death. The name is historically derived from “E2 promoter-binding factor,” as these factors were originally discovered as cellular proteins that bind to the E2 promoter of adenoviruses).

- RB1/pRB: In its active, minimally phosphorylated state, pRB binds to activating E2F transcription factors and suppresses their transcription-promoting effects.

- E2F: In particular, E2F1, E2F2, and E2F3 activate genes required for DNA synthesis, DNA replication, and cell cycle progression.

- Cyclin D/CDK4/6: Mitogenic growth signals activate this kinase complex, which phosphorylates pRB and attenuates its inhibitory effect on E2F.

- Cyclin E/CDK2: Enhances pRB phosphorylation and promotes the full activation of the E2F-dependent transcriptional program.

- G1/S transition: The cell crosses the restriction point and enters the DNA synthesis phase.

Significance in Merkel Cell Carcinoma

The RB1/E2F regulatory axis is important in Merkel cell carcinoma (MCC), as two distinct molecular mechanisms are at play here.

• MCPyV-positive Merkel cell carcinomas: The viral Large T antigen (LT) of the Merkel cell polyomavirus binds to pRB via its LXCXE motif and impairs its inhibitory function. This functionally releases E2F without requiring genetic inactivation of RB1.
• MCPyV-negative Merkel cell carcinomas: In this tumor variant, inactivation of the RB1/E2F regulatory axis frequently occurs through somatic, UV-induced mutations in the RB1 gene. Mutations in TP53 are typically found in parallel.

Significance in Tumor Biology

A disruption of the RB1/E2F regulatory axis is one of the fundamental mechanisms of malignant transformation. Various changes are possible in this context:

• Inactivating RB1 mutation: Loss of pRB-mediated E2F inhibition
• Deletion of CDKN2A (p16INK4A): Increased CDK4/6 activity
• Amplification of CCND1 (Cyclin D1): Increased pRB phosphorylation
• Amplification of CDK4: Functional inactivation of pRB
• Excessive E2F activity: Uncontrolled expression of cell cycle genes

1. Chen M et al. (2022) The E2F1/CKS2/PTEN signaling axis regulates malignant phenotypes in pediatric retinoblastoma. Cell Death Dis 13:784.
2. Liu F et al. (2020) MicroRNA-598 acts as an inhibitor in retinoblastoma by targeting E2F1 and regulating the AKT pathway. J Cell Biochem 121:2294–2302.
3. Qin H et al. (2021) miR-1258 attenuates tumorigenesis by targeting E2F1 to inhibit PCNA and MMP2 transcription in glioblastoma. Front Oncol 11:671144.