CRCT - Cancer Research Center of Toulouse
ResearchResearch Teams01 A. Besson: CDK inhibitors in oncogenesis & tumor suppression

01 A. Besson: CDK inhibitors in oncogenesis & tumor suppression

Team Research Project

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.



  • Cell cycle: the cell division cycle is a succession of phases during which a cell duplicates its contents (DNA, organelles…) and divide into two daughter cells genetically identical. During the interphase (composed of the G1, S and G2 phases), the cell duplicates its content and during mitosis the cell divides into two cells and physically separate during cytokinesis. The cell cycle is tightly regulated and among critical regulators of cell cycle progession are the Cyclin-dependent kinase complexes (cyclin-CDKs) which are heterodimers of a cyclin (the regulatory, substrate recognition subunit) and a cyclin-dependent kinase (CDK) which are the catalytic subunits that can phosphorylate specific substrates, thereby promoting cell cycle progression. Cyclin-CDKs are themselves tightly regulated at multiple levels, one being their inhibition by CDK inhibitors suchas as p27/Kip1 and p57/Kip2. Deregulated cell division is one of the hallmarks of cancer and virtually all cancer cells present a deregulation in the signaling pathways controlling cell cycle progression (hyperactivation of growth promoter or loss of growth inhibitors).
  • p27/Kip1: Kinase inhibitory protein 1 of molecular weight of 27 kDa encoded by the CDKN1B gene (cyclin-dependent kinase inhibitory protein 1B). p27 belongs to the Cip/Kip family of cyclin dependent kinase inhibitor with p21/Cip1 and p57/Kip2. The main function of these proteins is to inhibit the activities of cyclin-CDK complexes which drive progression through the cell division cycle. P27 has mainly been involved in the inhibition of CyclinE-CDK2 and Cyclin D-Cdk4 or CDK6 complexes and is therefore a major regulator of the G1/S transition and can cause a cell cycle arrest in G1. This cell cycle inhibitory function confers tumor suppressor properties to p27.
  • p57/Kip2: another member of the Cip/Kip family of CDK inhibitor, with p21/Cip1 and p27/Kip2 with whom it shares the ability to block cell cycle progression in G1 via the inhibition of cyclin-CDK complexes. p57 is encoded by the CDKN1C gene. Unlike other CDK inhibitors however, p57 plays a major role during embryonic development and the knockout of the gene in mice is lethal at birth. In fact, the p57 gene is one of the most frequently mutated or silenced gene involved in the Beckwith-Wiedemann syndrome in human, characterized by multiple developmental anomalies and increased risk of tumor development during childhood.
  • Oncogenesis (or carcinogenesis) is the progressive process of transformation of normal cells into cancer cells.
  • Mitosis: the process in which a cell divides its nuclear content into two nuclei. However, the term is often used to refer to the whole process of cell division (mitosis and cytokinesis). Mitosis is divided into several phases during which the nuclear DNA condenses into chromosomes (prophase), chromosomes align on an equatorial plane (metaphase), the two chromatids of each chromosome separate and migrate towards the poles of the cell and start constricting at the cell equator (anaphase) and the cell reforms two nuclei around the two pools of chromatids (telophase). When telophase initiates, it is also the beginning of cytokinesis, which will cleave the cell into two daughter cells.
  • Cytokinesis: Division of the cell cytoplasm into two daughter cells. This step takes place at the end of mitosis (which is the division of the nuclear material, i.e. DNA) and results in the physical separation of the cell into two daughter cells during the final step of cytokinesis: abscission. During cytokinesis, a contractile ring forms on the cell membrane, at the equator of the cell and progressively constrict, dividing the cytoplasm into the two daughter cell linked by an intercellular bridge which is severed during the final step of abscission.




 12th GERLI international Lipidomics Meeting - Microbe and Host Lipids
12th GERLI international Lipidomics Meeting - Microbe and Host Lipids

 Cellular origin and mechanisms of pancreatic cancer
23/05/16 - Dr Axel Behrens - Auditorium IUCT-O - 10:00 - 11:00
Inserm Université Toulouse 3 - Paul Sabatier CNRS