RBL1/p107 Expression Levels Are Modulated by Multiple Signaling Pathways

SIMPLE SUMMARY: The retinoblastoma-like (RBL)1/p107 protein is a member of the retinoblastoma (RB) family including the retinoblastoma (RB)1/p105 protein and RBL2/p130. They are key regulators of the cell cycle and play a central role in regulating cell proliferation. RB proteins act as tumor suppressors and their dysregulation is associated with tumor initiation. However, the mechanisms regulating RB protein actions are still very poorly characterized. We previously demonstrated that RBL2/p130 is a direct target of AKT and it is a key mediator of apoptosis associated with AKT inhibition. Here we demonstrated that RBL1/p107 levels are instead not directly modulated by AKT and discovered that RBL1/p107 levels are regulated by multiple pathways linked directly or indirectly to Ca(2+)-dependent signaling. These novel observations suggest a complex regulation of RBL1/p107 expression involving different components of signaling pathways controlled by Ca(2+) levels, pointing out a significant difference with the mechanisms modulating the close family member RBL2/p130. ABSTRACT: The members of the retinoblastoma (RB) protein family, RB1/p105, retinoblastoma-like (RBL)1/p107 and RBL2/p130 are critical modulators of the cell cycle and their dysregulation has been associated with tumor initiation and progression. The activity of RB proteins is regulated by numerous pathways including oncogenic signaling, but the molecular mechanisms of these functional interactions are not fully defined. We previously demonstrated that RBL2/p130 is a direct target of AKT and it is a key mediator of the apoptotic process induced by AKT inhibition. Here we demonstrated that RBL1/p107 levels are only minorly modulated by the AKT signaling pathway. In contrast, we discovered that RBL1/p107 levels are regulated by multiple pathways linked directly or indirectly to Ca(2+)-dependent signaling. Inhibition of the multifunctional calcium/calmodulin-dependent kinases (CaMKs) significantly reduced RBL1/p107 expression levels and phosphorylation, increased RBL1/p107 nuclear localization and led to cell cycle arrest in G0/G1. Targeting the Ca(2+)-dependent endopeptidase calpain stabilized RBL1/p107 levels and counteracted the reduction of RBL1/p107 levels associated with CaMKs inhibition. Thus, these novel observations suggest a complex regulation of RBL1/p107 expression involving different components of signaling pathways controlled by Ca(2+) levels, including CaMKs and calpain, pointing out a significant difference with the mechanisms modulating the close family member RBL2/p130.