p27kip1 controls H-Ras/MAPK activation and cell cycle entry via modulation of MT stability.

The cyclin-dependent kinase (CDK) inhibitor p27(kip1) is a critical regulator of the G1/S-phase transition of the cell cycle and also regulates microtubule (MT) stability. This latter function is exerted by modulating the activity of stathmin, an MT-destabilizing protein, and by direct binding to MTs. We recently demonstrated that increased proliferation in p27(kip1)-null mice is reverted by concomitant deletion of stathmin in p27(kip1)/stathmin double-KO mice, suggesting that a CDK-independent function of p27(kip1) contributes to the control of cell proliferation. Whether the regulation of MT stability by p27(kip1) impinges on signaling pathway activation and contributes to the decision to enter the cell cycle is largely unknown. Here, we report that faster cell cycle entry of p27(kip1)-null cells was impaired by the concomitant deletion of stathmin. Using gene expression profiling coupled with bioinformatic analyses, we show that p27(kip1) and stathmin conjunctly control activation of the MAPK pathway. From a molecular point of view, we observed that p27(kip1), by controlling MT stability, impinges on H-Ras trafficking and ubiquitination levels, eventually restraining its full activation. Our study identifies a regulatory axis controlling the G1/S-phase transition, relying on the regulation of MT stability by p27(kip1) and finely controlling the spatiotemporal activation of the Ras-MAPK signaling pathway.

Splicing factor proline- and glutamine-rich (SFPQ) protein regulates platinum response in ovarian cancer-modulating SRSF2 activity.

In epithelial ovarian cancer (EOC), response to platinum (PT)-based chemotherapy dictates subsequent treatments and predicts patients' prognosis. Alternative splicing is often deregulated in human cancers and can be altered by chemotherapy. Whether and how changes in alternative splicing regulation could impact on the response of EOC to PT-based chemotherapy is still not clarified. We identified the splicing factor proline and glutamine rich (SFPQ) as a critical mediator of response to PT in an unbiased functional genomic screening in EOC cells and, using a large cohort of primary and recurrent EOC samples, we observed that it is frequently overexpressed in recurrent PT-treated samples and that its overexpression correlates with PT resistance. At mechanistic level, we show that, under PT treatment, SFPQ, in complex with p54nrb, binds and regulates the activity of the splicing factor SRSF2. SFPQ/p54nrb complex decreases SRSF2 binding to caspase-9 RNA, favoring the expression of its alternative spliced antiapoptotic form. As a consequence, SFPQ/p54nrb protects cells from PT-induced death, eventually contributing to chemoresistance. Overall, our work unveils a previously unreported SFPQ/p54nrb/SRSF2 pathway that in EOC cells plays a central role in regulating alternative splicing and PT-induced apoptosis and that could result in the design of new possible ways of intervention to overcome PT resistance.

Identification and Characterization of a New Platinum-Induced TP53 Mutation in MDAH Ovarian Cancer Cells.

Platinum-based chemotherapy is the therapy of choice for epithelial ovarian cancer (EOC). Acquired resistance to platinum (PT) is a frequent event that leads to disease progression and predicts poor prognosis. To understand possible mechanisms underlying acquired PT-resistance, we have recently generated and characterized three PT-resistant isogenic EOC cell lines. Here, we more deeply characterize several PT-resistant clones derived from MDAH-2774 cells. We show that, in these cells, the increased PT resistance was accompanied by the presence of a subpopulation of multinucleated giant cells. This phenotype was likely due to an altered progression through the M phase of the cell cycle and accompanied by the deregulated expression of genes involved in M phase progression known to be target of mutant TP53. Interestingly, we found that PT-resistant MDAH cells acquired in the TP53 gene a novel secondary mutation (i.e., S185G) that accompanied the R273H typical of MDAH cells. The double p53S185G/R273H mutant increases the resistance to PT in a TP53 null EOC cellular model. Overall, we show how the selective pressure of PT is able to induce additional mutation in an already mutant TP53 gene in EOC and how this event could contribute to the acquisition of novel cellular phenotypes.

Common biological phenotypes characterize the acquisition of platinum-resistance in epithelial ovarian cancer cells.

Standard of care for Epithelial Ovarian Cancer (EOC) patients relies on platinum-based therapy. However, acquired resistance to platinum occurs frequently and predicts poor prognosis. To understand the mechanisms underlying acquired platinum-resistance, we have generated and characterized three platinum-resistant isogenic EOC cell lines. Resistant cells showed 3-to 5- folds increase in platinum IC50. Cross-resistance to other chemotherapeutic agents commonly used in the treatment of EOC patients was variable and dependent on the cell line utilized. Gene expression profiling (GEP) of coding and non-coding RNAs failed to identify a common signature that could collectively explain the mechanism of resistance. However, we observed that all resistant cell lines displayed a decreased level of DNA platination and a faster repair of damaged DNA. Furthermore, all platinum resistant cell lines displayed a change in their morphology and a higher ability to grown on mesothelium. Overall, we have established and characterized three new models of platinum-resistant EOC cell lines that could be exploited to further dissect the molecular mechanisms underlying acquired resistance to platinum. Our work also suggests that GEP studies alone, at least when performed under basal culture condition, do not represent the optimal way to identify molecular alterations linked to DNA repair pathway defects.

CDK6 protects epithelial ovarian cancer from platinum-induced death via FOXO3 regulation.

Epithelial ovarian cancer (EOC) is an infrequent but highly lethal disease, almost invariably treated with platinum-based therapies. Improving the response to platinum represents a great challenge, since it could significantly impact on patient survival. Here, we report that silencing or pharmacological inhibition of CDK6 increases EOC cell sensitivity to platinum. We observed that, upon platinum treatment, CDK6 phosphorylated and stabilized the transcription factor FOXO3, eventually inducing ATR transcription. Blockage of this pathway resulted in EOC cell death, due to altered DNA damage response accompanied by increased apoptosis. These observations were recapitulated in EOC cell lines in vitro, in xenografts in vivo, and in primary tumor cells derived from platinum-treated patients. Consistently, high CDK6 and FOXO3 expression levels in primary EOC predict poor patient survival. Our data suggest that CDK6 represents an actionable target that can be exploited to improve platinum efficacy in EOC patients. As CDK4/6 inhibitors are successfully used in cancer patients, our findings can be immediately transferred to the clinic to improve the outcome of EOC patients.

p27kip1 expression limits H-Ras-driven transformation and tumorigenesis by both canonical and non-canonical mechanisms.

The tumor suppressor protein p27Kip1 plays a pivotal role in the control of cell growth and metastasis formation.Several studies pointed to different roles for p27Kip1 in the control of Ras induced transformation, although no explanation has been provided to elucidate these differences. We recently demonstrated that p27kip1 regulates H-Ras activity via its interaction with stathmin.Here, using in vitro and in vivo models, we show that p27kip1 is an important regulator of Ras induced transformation. In H-RasV12 transformed cells, p27kip1 suppressed cell proliferation and tumor growth via two distinct mechanisms: 1) inhibition of CDK activity and 2) impairment of MT-destabilizing activity of stathmin. Conversely, in K-Ras4BV12 transformed cells, p27kip1 acted mainly in a CDK-dependent but stathmin-independent manner.Using human cancer-derived cell lines and primary breast and sarcoma samples, we confirmed in human models what we observed in mice.Overall, we highlight a pathway, conserved from mouse to human, important in the regulation of H-Ras oncogenic activity that could have therapeutic and diagnostic implication in patients that may benefit from anti-H-Ras therapies.

Genetic characterization of p27(kip1) and stathmin in controlling cell proliferation in vivo.

The CDK inhibitor p27(kip1) is a critical regulator of cell cycle progression, but the mechanisms by which p27(kip1) controls cell proliferation in vivo are still not fully elucidated. We recently demonstrated that the microtubule destabilizing protein stathmin is a relevant p27(kip1) binding partner. To get more insights into the in vivo significance of this interaction, we generated p27(kip1) and stathmin double knock-out (DKO) mice. Interestingly, thorough characterization of DKO mice demonstrated that most of the phenotypes of p27(kip1) null mice linked to the hyper-proliferative behavior, such as the increased body and organ weight, the outgrowth of the retina basal layer and the development of pituitary adenomas, were reverted by co-ablation of stathmin. In vivo analyses showed a reduced proliferation rate in DKO compared to p27(kip1) null mice, linked, at molecular level, to decreased kinase activity of CDK4/6, rather than of CDK1 and CDK2. Gene expression profiling of mouse thymuses confirmed the phenotypes observed in vivo, showing that DKO clustered with WT more than with p27 knock-out tissue. Taken together, our results demonstrate that stathmin cooperates with p27(kip1) to control the early phase of G1 to S phase transition and that this function may be of particular relevance in the context of tumor progression.

p27kip1 controls cell morphology and motility by regulating microtubule-dependent lipid raft recycling.

p27(kip1) (p27) is an inhibitor of cyclin/cyclin-dependent kinase complexes, whose nuclear loss indicates a poor prognosis in various solid tumors. When located in the cytoplasm, p27 binds Op18/stathmin (stathmin), a microtubule (MT)-destabilizing protein, and restrains its activity. This leads to MT stabilization, which negatively affects cell migration. Here, we demonstrate that this p27 function also influences morphology and motility of cells immersed in three-dimensional (3D)matrices. Cells lacking p27 display a decrease in MT stability, a rounded shape when immersed in 3D environments, and a mesenchymal-amoeboid conversion in their motility mode. Upon cell contact to extracellular matrix, the decreased MT stability observed in p27 null cells results in accelerated lipid raft trafficking and increased RhoA activity. Importantly, cell morphology, motility, MT network composition, and distribution of p27 null cells were rescued by the concomitant genetic ablation of Stathmin, implicating that the balanced expression of p27 and stathmin represents a crucial determinant for cytoskeletal organization and cellular behavior in 3D contexts.