The PCL1-p53 axis promotes cellular quiescence

Our recent work identified a PCL1-p53 regulatory axis that is essential for the maintenance of cellular quiescence.¹ In this short feature we describe these results, presenting them in the broader context of how this new information may allow us to develop a strategy to target the PCL1-p53 axis for improved clinical outcomes in cancer patients.

The process of cellular quiescence represents a reversible, yet stable exit from the cell cycle. It was originally thought to represent a default ‘resting’ state that cells enter due to a lack of mitogenic signaling. However, it is now clear that cellular quiescence is not merely a passive response, but rather an actively maintained cellular state. Indeed many cell types, including tissue specific stem cells, exist predominantly in a quiescent state in vivo.² Recent studies have begun to unravel the molecular pathways that maintain stem cell quiescence and have demonstrated that the ability of these cells to remain quiescent is essential for long-term stem cell function. The p53 tumor suppressor gene is essential for restraining cell cycle entry, and promoting cellular quiescence in both neural and haematopoietic stem cells (NSCs and HSCs) in vivo.^3,4 The loss of p53 in these cells leads to an expansion in stem cell number, which is thought to occur due to their reduced ability to remain quiescent and consequent entry into the cell cycle. Interestingly, the conditional deletion of the p53 target gene Cdkn1a, which encodes the cyclin-dependent kinase inhibitor p21, causes similar effects in NSCs and HSCs, ultimately leading to premature stem cell exhaustion.² Taken together these results demonstrate that the p53 pathway is essential for the maintenance of cellular quiescence in normal tissue specific stem cells.

Polycomb group proteins are chromatin associated transcriptional repressors with essential roles in maintaining cellular identity during development and differentiation. However, they are also key regulators of cellular proliferation, which is both dependent and independent of their role in repression of the INK4A-ARF tumor suppressor locus.⁵ Our recent discovery of a chromatin independent role of Polycomb Like 1 (PCL1; also known as PHF1) to stabilize p53 and promote cellular quiescence was the first study to integrate Polycomb group protein function into the molecular circuitry at the heart of cellular quiescence, thereby broadening our understanding of the relationship between Polycomb and the cell cycle.¹ We unraveled a PCL1-p53 regulatory axis, which is important for the maintenance of the quiescent state (Figure 1 – left panel). The PCL1 gene promoter is a direct transcriptional target of p53 in quiescent cells, and consistent with this, PCL1 expression is at its highest in these cells. We also found that the PCL1 protein is required to maintain cells in a quiescent state and that ectopic expression of PCL1 leads to a p53 dependent reduction in proliferation. Compared to its paralogous proteins, PCL2 and PCL3, PCL1 has evolved a divergent N-terminal PHD domain to facilitate its specific interaction with the C-terminal regulatory domain (CTD) of p53. This PHD domain is essential for the ability of PCL1 to stabilize p53 protein levels and thereby promote cellular quiescence. This stabilization occurs off-chromatin and leads to increased activation of downstream p53 target genes, including CDKN1A. These new insights point to the PCL1 N-terminal PHD domain as being a potential ‘Achilles heel’ within the molecular circuitry of cellular quiescence.

Figure 1.

Targeting the PCL1-p53 axis in quiescent cancer stem cells. Left panel – In quiescent cells, the p53 protein transactivates the PCL1 gene. Off chromatin, the PCL1 protein interacts with p53 and boosts its levels by blocking destabilization signals, leading to higher expression of p53 target genes. Right panel – Targeted disruption of the PCL1-p53 interaction as a means to sensitize quiescent cancer stem cells to chemotherapy.

A strategy to disrupt cellular quiescence might have significant clinical utility. For example, many cancers are known to be composed of an heterogenous mix of cells including so called, cancer stem cells. These cells share many features with normal stem cells, including the ability to self-renew, and they are believed to represent the sole repopulating unit for the cancer. Interestingly, recent work has demonstrated that cancer stem cell populations in several cancer types predominantly exist in a quiescent state in vivo, rendering them resistant to conventional chemotoxic treatments, which are more effective against dividing cells.^6,7 Strikingly, chemotherapy can actually result in the relative expansion of these quiescent cancer stem cells following the depletion of proliferating cells. Moreover, these surviving cancer stem cells are then thought to re-seed the cancer following cessation of the treatment. It is therefore clear that cellular quiescence within a cancer stem cell population is a significant clinical hurdle for achieving sustainable patient remissions. However, these studies also provide an interesting proof-of-concept, wherein simultaneously inducing quiescent cancer stem cells to enter the cell cycle, in combination with chemotherapy provides improved clinical benefits. We propose that new strategies to disrupt the PCL1-p53 interaction could provide a means to disrupt cellular quiescence, and that these may have utility in rendering cancer stem cells more sensitive to chemotherapy (Figure 1, right panel).