The AMBRA1 E3 ligase adaptor regulates Cyclin D protein stability [U2OS cells]

The initiation of cell division is a fundamental process that integrates a large number of intra- and extra-cellular inputs. In mammalian cells, D-type cyclins (Cyclin D) couple these inputs to the decision to initiate DNA replication. Increased levels of Cyclin D promote cell cycle progression by activating cyclin-dependent kinases 4 and 6 (CDK4/6). Accordingly, increased levels and activity of Cyclin D and their associated kinases are strongly linked to unchecked cell proliferation and tumor development. Despite this central role of Cyclin D in cell cycle progression, the mechanisms regulating their levels remain incompletely understood. Here we describe AMBRA1 as the main regulator of Cyclin D protein degradation. We first identify AMBRA1 as the top candidate in a genome-wide CRISPR/Cas9 loss-of-function screen investigating the genetic basis of resistance to chemical CDK4/6 inhibition. AMBRA1 loss results in high protein levels of Cyclin D in cells and in mice. AMBRA1 loss further promotes lung cancer development in a mouse model, and low levels of AMBRA1 correlate with worse survival in lung cancer patients. Mechanistically, AMBRA1 acts as a substrate receptor for the Cullin 4 E3 ligase complex to promote ubiquitylation and proteasomal degradation of the three Cyclin D family members. Thus, AMBRA1 regulates Cyclin D protein levels and contributes to the development of cancer as well as the response of cancer cells to CDK4/6 inhibitors. Overall design: Two determine whether AMBRA1 loss impacts transcriptional programs, control and AMBRA1 knockout U2OS cells were examined by RNA sequencing. The transcriptomes from populations of cells expressing either non-targeting sgRNA or AMBRA1 sgRNAs were sequenced.