Team 1 (A. Besson): to determine the roles of CDK inhibitors in tumorigenesis
Cyclin-dependent kinase (CDK) inhibitors (CKIs) of the Cip/Kip family are tumor suppressors by virtue of their ability to block cellular proliferation in the nucleus. However, increasing evidence indicate that CKIs also have cyclin-CDK independent functions that are potentially oncogenic. The main interest in the laboratory is to identify and characterize these novel functions and determine their importance in the context of tumorigenesis. The end goal is to use this knowledge to identify new prognostic markers and therapeutic targets or strategies for cancer diagnostic and treatment.
To study the cyclin-CDK independent functions of p27/Kip1 (p27) and p57/Kip2 (p57), we generated knock-in mouse models in which p27 or p57 can no longer bind to and regulate cyclin-CDK complexes. These animals provide a mean to genetically dissect the functions of these proteins. In the lab, we combine the use of genetics with cellular, molecular and biochemical approaches to study the novel functions of these CDK inhibitors and their underlying mechanisms and to determine their importance in vivo, particularly in the context of tumorigenesis.
Cell cycle progression is driven by the activities of cyclin/cyclin-dependent kinase (CDK) complexes. One level of regulation of these complexes is provided by cyclin-CDK inhibitors (CKIs) which modulate their activity. The CKIs p27 and p57 are crucial regulators of the transition from G0, through G1, into S phase.
The importance of p27 in the control of cell proliferation is underscored by the phenotype of p27-null mice, which display a marked increase in body size and multi-organ hyperplasia. p27 is a tumor suppressor and mice lacking p27 are predisposed to both spontaneous and carcinogen induced tumorigenesis. In humans, p27 is a significant prognostic marker in many types of cancers and decreased levels of nuclear p27 are commonly observed in tumors.
On the other hand, p57 plays a prominent role during embryonic development and p57-null animals die around birth due to several developmental anomalies and this phenotype is reminiscent of the features observed in Beckwith-Wiedemann syndrome (BWS) in human. In fact, p57 is inactivated by mutation or silencing in most BWS cases.
However, a more complex picture has recently emerged. Several lines of evidence indicate that p27’s role may not be limited to its tumor suppressive, cyclin-CDK inhibitor function. Unlike classical tumor suppressors, mutations of the p27 gene are exceedingly rare in tumors and p27 is preferentially inactivated by loss of nuclear p27 expression, either via increased proteolytic degradation and/or exclusion from the nucleus. In addition, in clinical studies, increased levels of p27 or its cytoplasmic localization in subsets of several types of high-grade tumors correlates with aggressive disease, poor prognosis and increased metastasis. Thus, evidence suggests that p27 may have roles in the cytoplasm and may actively participate in the pathology of tumors.
In fact, while investigating the cytoplasmic role of p27, we found that independently of its role in cyclin-CDK regulation, p27 could interact with RhoA, thereby inhibiting RhoA activation by interfering with the binding of RhoA to its GEFs. In this way, p27 regulates actin cytoskeleton dynamics and cell migration. This function is critical for neuronal migration during mouse development and in cancer cells for promoting invasion and metastasis.
Increasing evidence also indicates that p57 has cyclin-CDK independent roles and other partners. Moreover, a fraction of Beckwith-Wiedemann syndrome cases originates from point mutations in p57 located outside of the cyclin-CDK regulatory region, suggesting that the loss of cyclin-CDK independent roles of p57 may, at least in part, be responsible for BWS. To this end we generated a p57CK- mutant mouse line in which p57 no longer binds to cyclins and CDKs to determine the importance of the cyclin-CDK independent roles of p57 in vivo, both in the context of BWS and tumorigenesis. This mouse model is currently being characterized.
Overall, our goal is to determine whether and how CDK inhibitors of the Cip/Kip family may act both as tumor suppressors and oncogenes depending on the cellular context. Our work also brings insight into fundamental aspects of biology, such as the regulation of cytokinesis or the mechanisms governing stem cell fate. We use a vast array of approaches ranging from the generation of novel murine models, biochemistry, and cellular and molecular biology. Our findings may help identify novel therapeutic targets to treat cancers.
The main areas of interest in the lab are:
- To characterize the oncogenic roles of p27 in the cytoplasm, especially its relationship with Rho GTPases
- To study the cyclin-CDK independent role of p27 during mitosis and cytokinesis.
- To determine the roles of p27 in bronchio-alveolar stem cell regulation.
- To investigate the cyclin-CDK independent functions of p57Kip2.