AMBRA1-mediated degradation of p21 and p27 ensures G1/S progression and limits replication stress

The cell cycle is a fundamental process orchestrating cell replication and division, with precise regulation crucial for preventing uncontrolled growth. The G1-S phase transition is a critical checkpoint, tightly controlled by cyclins and cyclin-dependent kinase inhibitors (CKIs) like p21Waf1/Cip1 and p27Kip1. AMBRA1 is known to interact with the CRL4DDB1 E3 ubiquitin ligase complex to regulate D-type cyclins, facilitating G1-S progression. However, its specific role in the subsequent S phase and its broader impact on replication fidelity remained less understood, representing a key gap this research addresses.

Researchers investigated AMBRA1's role in cell cycle progression by examining its impact on CKI stability. They focused on AMBRA1-depleted cellular models to observe the consequences of its absence. The study specifically analyzed the turnover of p21Waf1/Cip1 and p27Kip1 proteins, assessing their stability and accumulation. Mechanistic studies explored the interaction between p21Waf1/Cip1, PCNA, and FEN1 during the S phase. Furthermore, they tested the sensitivity of AMBRA1-depleted cells to FEN1 inhibition, providing insights into the functional consequences of altered protein dynamics. The study also correlated AMBRA1 and p21Waf1/Cip1 levels in Sonic Hedgehog-type medulloblastomas.

AMBRA1 was found to be crucial for the ubiquitin-dependent degradation of both p21Waf1/Cip1 and p27Kip1, coupling the CRL4DDB1 complex to these key cell cycle inhibitors. In the absence of AMBRA1, the stability of p21Waf1/Cip1 significantly increased, leading to its accumulation, which was a more prominent effect than that observed for p27Kip1. This excess p21Waf1/Cip1 during the S phase resulted in a greater proportion of PCNA-bound p21Waf1/Cip1. This binding negatively affected the interaction of FEN1 with PCNA, a critical step for processing Okazaki fragments and ensuring complete DNA replication. Consequently, this interference left under-replicated DNA, leading to the accumulation of replication stress. Cells depleted of AMBRA1 were notably sensitive to FEN1 inhibition, confirming the functional importance of this pathway. Aberrantly low levels of AMBRA1 observed in Sonic Hedgehog-type medulloblastomas correlated with high levels of p21Waf1/Cip1 and were associated with a worse prognosis for patients.