p57Kip2, a glucocorticoid-induced CDK inhibitor, involved in cell proliferation, apoptosis and differentiation

Abstract

Glucocorticoids (GCs), are steroid hormones with several important physiological implications, of which some are connected with the ability of GCs to inhibit cell proliferation. Although GC treatment normally accumulates cells in the GI phase of the cell cycle, the mechanisms behind this phenomenon are not well understood. Considering that GCs exert their effects mainly by regulating the expression of genes via binding to and activating a ligand-dependent transcription factor, the glucocorticoid receptor (GR), the main aim of this thesis was to identify novel target genes for the GR, linked to this anti- proliferative effect.

Studies presented in this thesis identified the cyclin dependent kinase inhibitor, p57Kip2, to be directly induced by the GR by means of transcriptional induction. Furthermore, p57Kip2 was shown to be sufficient to reconstitute the anti -proliferative effect seen after GC treatment, indicating an important role for this protein in the GC-induced inhibition of cell proliferation. Further investigation also identified a functional glucocorticoid response element (GRE) in the human p5 7Kip2 promoter, located 5 kb upstream of the transcriptional start site. The GRE was also shown to be well conserved in the mouse both regarding sequence similarity and function, indicating a high biological relevance for this element.

GCs are known for their ability to promote lung maturation in the fetus. Since p57Kip2 has been implicated in mouse embryonal lung development, we hypothesized that GC administration could increase p57Kip2 expression in this tissue. Indeed, GC treatment of pregnant mice increased the expression of p57Kip2 in the proximal pseudostratified columnar epithelial cells of the embryonic lung. p57Kip2 expression and induction was temporally restricted to a period when transition from the pseudoglandular to the canalicular phases occurs during lung development. Furthermore, embryos lacking p57Kip2 presented a phenotype indicating an aberrant differentiation of the distal cuboidal epithelial cells. This study identified a potential in vivo model for GC-induced p57Kip2 expression, and also suggested a novel function for p57Kip2 in distal-proximal airway differentiation.

Apoptosis is a process of programmed cell death involved in various biological events, including tumourigenesis. Interestingly, connections between cell proliferation and apoptosis have been suggested, for instance through the activity of CDK2. CDK2, which is necessary both for progression through the G1-phase of the cell cycle and in apoptosis, is activated by several apoptotic stimuli, including treatment with the cytotoxic agent staurosporine. Since p57Kip2 is a known inhibitor of CDK2, we hypothesized that this protein could affect staurosporine-induced apoptosis in HeLa cells. Results presented in this thesis show that selective expression of p57Kip2 potentiates this apoptotic process. The stimulation of staurosporine-induced apoptosis also included increased activation of caspase-3 and a reactivation of CDK2, potentially mediated by a caspase-dependent cleavage of p57Kip2. In summary, we suggest a role for p57Kip2 in the response of tumour cells to cytotoxic drugs.

Finally, we propose a model, where the GC-induced cyclin dependent kinase inhibitor, p57Kip2, participates in the processes of anti-proliferation, differentiation and apoptosis.