From a simplified perspective, cancer can be described as a disorder of the cell cycle. During tumor formation, cells propagate with little or no control. Not surprisingly, cell cycle genes are frequently found to be deregulated during tumorigenesis. Cyclin-dependent kinases (CDKs) are positive regulators of cell cycle progression while cyclin-dependent kinase inhibitors (CKIs) work as brakes to stop cell division. Tumorigenesis results from abnormal expression or activation of positive regulators and/or practical suppression of negative regulators. Due to their critical roles in cell cycle, CDKs and CKIs have potential as druggable targets to stop or slow down tumorigenesis.
We utilize mouse models to study roles of CDKs and CKIs in tumorigenesis. Liver tumors can be very efficiently developed in rodents by using the hydrodynamic tail-vein injection protocol that introduces an oncogenic plasmid DNA cocktail including the Sleeping Beauty transposase, into the hepatocytes. Previously, we have shown that Cdk1 deficient livers are fully resistant to tumorigenesis. We aim to investigate the CDKs and CKIs as potential therapeutic targets in various tumors utilizing transgenic mouse models.
Progress through different phases of the cell cycle is regulated by concerted actions of CDKs in complexes with their partner cyclin molecules. There is a wide scale of compensation between CDKs, such that, none of the CDKs is individually essential for cell division. The only exception is Cdk1, which makes complexes with Cyclin B1 to initiate and orchestrate mitotic events. Genetic deletion of Cdk1 in mice results in very early embryonic lethality due to the inability of the Cdk1 deficient cells to enter mitosis.
Genetic deletion of the Cdk1 gene in mice is lethal due to a halt of cellular proliferation. Histological sections of the small intestine show highly proliferative intestinal crypt cells in control animals (left) and Cdk1 KO animals (right). Cdk1 deficient crypt cells with enlarged nuclei cannot proliferate and repopulate the microvilli, resulting in degeneration of the intestinal lining.
Mitosis involves a number of irreversible major cellular events such as nuclear envelope breakdown and chromosome condensation. Therefore, mitotic entry has to be regulated very tightly by a variety of regulatory mechanisms, whose aim is to ensure a total inhibition of Cdk1 kinase activity prior to mitosis. During mitotic entry, Cdk1 activity rises very sharply in a switch like mechanism. Cdk1 utilizes several effector kinases to phosphorylate mitotic substrates. On the other hand, dephosphorylation by phosphatases have to be prevented for an efficient accumulation of phosphorylations on these substrates. We study Cdk1 and its effector mitotic kinases utilizing transgenic animal models and cell lines derived from them.
There are several mechanisms that regulate Cdk1 kinase activity. First and foremost, both Cdk1 and Cyclin B1 are E2F target genes that get expressed upon reception of mitogenic stimuli. Inhibitory phosphorylations (introduced by the inhibitory kinases Wee1 and Myt1) have to be removed by Cdc25 phosphatase and an activating phosphorylation has to be introduced by Cdk-activating kinase for full Cdk1 activity (left panel). These regulatory mechanisms ensure that Cdk1 activity rises and falls steeply, in a switch-like manner during mitosis (right panel).
Greatwall/Mastl kinase is one of the several effector kinases of Cdk1 in mitosis. It is activated by Cdk1 during mitosis and in turn, it phosphorylates two small inhibitory proteins Arpp19 and Ensa that sterically inhibit PP2A phosphatase activity. This regulatory pathway ensures accumulation of mitotic phosphorylations rapidly.We have generated a conditional knockout mouse model of the Mastl kinase. Mastl KO embryos die very early during development. Conditional deletion of Mastl prior to meiosis resulted in an arrest before metaphase II. In somatic cells, deletion of Mastl resulted in chromosome segregation defects during anaphase. We are investigating the molecular mechanisms underlying these phenotypes using transgenic mouse models and cell lines derived from them.