The many faces of p57Kip2 : Acting in apoptosis, differentiation and cytoskeleton reorganisation

Abstract: p57Kip2 is a known cyclin dependent kinase inhibitor, which has been suggested to be a tumor suppressor gene. Indeed various human cancers show a reduction in p57Kip2 expression, indicating that this protein might be of relevance in tumorigenesis. Mutated forms of p57Kip2 have rarely been detected in human tumors; rather epigenetic mechanisms are involved in inactivating the expression of the gene. Inactivation of the p57 Kip2 gene correlates with disease progression and poor prognosis for the patient, indicating that reactivation of p57Kip2 in cancer cells could possible inhibit tumor cell growth. Prior research has primarily focused on p57Kip2´s role as cell-cycle regulator, whereas the aim of this thesis is to study its function during other cellular events such as apoptosis, cytoskeleton reorganisation and differentiation of neuronal cells. We report that selective expression of p57Kip2 sensitizes tumor cells to cell death induced by different drugs. This function is independent of its role as a cyclin dependent kinase inhibitor. Our studies also show that p57Kip2 primarily promotes the mitochondrial apoptotic pathway. In accordance, we found that Bcl-2 overexpression or VDAC inhibition were able to inhibit p57Kip2 cell death promoting effect. We also reveal that p57Kip2 is a direct target gene for p73?, which is known for its pro-apoptotic properties. Moreover, the p73?-induced p57Kip2 expression contributes to mitochondrial events related to apoptotic cell death. Furthermore, we establish that p57Kip2 expression promotes actin stress fiber formation in cells. It interacts with, and activates the actin cytoskeleton modifying enzyme, LIM-Kinase-1. This activation resulted into increase phosphorylation, inactivation of cofilin and in a reduction of actin protein mobile fraction, ultimately, affecting negatively cell mobility. Remodelling of actin cytoskeleton plays a key role in cell migration and has implication for invasion and metastasis, supporting the proposal that p57Kip2 can act as a tumor suppressor gene by affecting the cytoskeleton. We further investigated the role of p57Kip2 in neural stem cell differentiation. We show that p57Kip2 transiently accumulates in the nuclei of neural progenitors during early astrocyte differentiation and represses neuronal differentiation. It can interact with a subset of pro-neuronal bHLH factors, including Mash1. p57Kip2 inhibits the transcriptional activity of Mash1 and thereby represses neuronal differentiation, possibly to allow proper glial differentiation. These findings once again confirm the importance of p57Kip2 during development and differentiation. In conclusion, our work shows that p57Kip2 plays essential roles during several cellular processes besides acting as a cell-cycle regulator. This thesis contributes to the understanding of p57Kip2 s role in apoptosis, differentiation and cytoskeleton reorganisation.