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

p27Kip1 and Tumors: Characterization of CDKN1B Variants Identified in MEN4 and Breast Cancer

p27Kip1 is a key cell cycle gatekeeper governing the timing of Cyclin-dependent kinase (CDK) activation/inactivation and, consequently, cell proliferation. Structurally, the protein is largely unfolded, a feature that strongly increases its plasticity and interactors and enhances the number of regulated cellular processes. p27Kip1, like other intrinsically unstructured proteins, is post-translationally modified on several residues. These modifications affect its cellular localization and address p27Kip1 for specific interactions/functions. Several germline or somatic CDKN1B (the p27Kip1 encoding gene) mutations have been demonstrated to be associated with multiple endocrine neoplasia type 4 (MEN4), hairy cell leukemia, small-intestine neuroendocrine tumors, and breast and prostate cancers. Here, we analyzed the effect of four CDKN1B missense and nonsense mutations found in patients affected by MEN4 or cancers, namely, c.349C>T, p.P117S; c.397C>A, p.P133T; c.487C>T, p.Q163*; and c.511G>T, p.E171*. By transfecting breast cancer cell lines, we observed increased growth and cell motility for all the investigated mutants compared to wild-type p27Kip1 transfected cells. Furthermore, we discovered that the mutant forms exhibited altered phosphorylation on key residues and different localization or degradation mechanisms in comparison to the wild-type protein and suggested a possible region as crucial for the lysosome-dependent degradation of the protein. Finally, the loss of p27Kip1 ability in blocking cell proliferation was in part explained through the different binding efficiency that mutant p27Kip1 forms exhibited with Cyclin/Cyclin-dependent Kinase complexes (or proteins involved indirectly in that binding) with respect to the WT.

p27kip1 expression and phosphorylation dictate Palbociclib sensitivity in KRAS-mutated colorectal cancer

In colorectal cancer, mutation of KRAS (RAS^MUT) reduces therapeutic options, negatively affecting prognosis of the patients. In this setting, administration of CDK4/6-inhibitors, alone or in combination with other drugs, is being tested as promising therapeutic strategy. Identifying sensitive patients and overcoming intrinsic and acquired resistance to CDK4/6 inhibition represent still open challenges, to obtain better clinical responses. Here, we investigated the role of the CDK inhibitor p27^kip1 in the response to the selective CDK4/6-inhibitor Palbociclib, in colorectal cancer. Our results show that p27^kip1 expression inversely correlated with Palbociclib response, both in vitro and in vivo. Generating a model of Palbociclib-resistant RAS^MUT colorectal cancer cells, we observed an increased expression of p27^kip1, cyclin D, CDK4 and CDK6, coupled with an increased association between p27^kip1 and CDK4. Furthermore, Palbociclib-resistant cells showed increased Src-mediated phosphorylation of p27^kip1 on tyrosine residues and low doses of Src inhibitors re-sensitized resistant cells to Palbociclib. Since p27^kip1 showed variable expression in RAS^MUT colorectal cancer samples, our study supports the possibility that p27^kip1 could serve as biomarker to stratify patients who might benefit from CDK4/6 inhibition, alone or in combination with Src inhibitors.

p27kip1 at the crossroad between actin and microtubule dynamics

The p27^kip1 protein, mainly known as a negative regulator of cell proliferation, has also been involved in the control of other cellular processes, including the regulation of cytoskeleton dynamics. Notably, these two functions involve distinct protein domains, residing in the N- and C-terminal halves, respectively. In the last two decades, p27^kip1 has been reported to interact with microtubule and acto-myosin cytoskeletons, both in direct and indirect ways, overall drawing a picture in which several factors play their role either in synergy or in contrast one with another. As a result, the role of p27^kip1 in cytoskeleton dynamics has been implicated in cell migration, both in physiologic and in neoplastic contexts, modulating cytokinesis, lipid raft trafficking, and neuronal development. Recently, two distinct papers have further reported a central role for p27^kip1 in the control of microtubule stability and post-translational modifications, dissecting the interaction between p27^kip1 and α-tubulin-acetyl-transferase (α-TAT), an enzyme involved in the stability of microtubules, and protein-regulator of cytokinesis 1 (PRC1), a nuclear regulator of the central spindle during mitosis. In light of these recent evidences, we will comment on the role of p27^kip1 on cytoskeleton regulation and its implication for cancer progression.

Landscape of CDKN1B Mutations in Luminal Breast Cancer and Other Hormone-Driven Human Tumors

The CDKN1B gene encodes for the p27Kip1 protein, firstly characterized as a cyclin dependent kinase (CDK)-inhibitor. Germline CDKN1B pathogenic variants have been described in hereditary tumors, such as multiple endocrine neoplasia (MEN)-like syndromes and familial prostate cancer. Despite its central role in tumor progression, for a long time it has been proposed that CDKN1B was very rarely somatically mutated in human cancer and that its expression levels were almost exclusively regulated at post-transcriptional level. Yet, the advent of massive parallel sequencing has partially subverted this general understanding demonstrating that, at least in some types of cancer, CDKN1B is mutated in a significant percentage of analyzed samples. Recent works have demonstrated that CDKN1B can be genetically inactivated and this occurs particularly in sporadic luminal breast cancer, prostate cancer and small intestine neuroendocrine tumors. However, a clear picture of the extent and significance of CDKN1B mutations in human malignances is still lacking. To fill this gap, we interrogated the COSMIC, ICGC, cBioPortal, and TRANSFAC data portals and current literature in PubMed, and reviewed the mutational spectrum of CDKN1B in human cancers, interpreting the possible impact of these mutations on p27Kip1 protein function and tumor onset and progression.

Loss of p27kip1 increases genomic instability and induces radio-resistance in luminal breast cancer cells

Genomic instability represents a typical feature of aggressive cancers. Normal cells have evolved intricate responses to preserve genomic integrity in response to stress, such as DNA damage induced by γ-irradiation. Cyclin-dependent kinases (CDKs) take crucial part to these safeguard mechanisms, but involvement of CDK-inhibitors, such as p27^Kip1, is less clear. We generated immortalized fibroblasts from p27^kip1 knock-out (KO) mouse embryos and re-expressed p27^kip1 WT, or its mutant forms, to identify the function of different domains. We γ-irradiated fibroblasts and observed that loss of p27^Kip1 was associated to accumulation of residual DNA damage, increased number of mitotic aberration and, eventually, to survival advantage. Nuclear localization and cyclin/CDK-binding of p27^Kip1 were critical to mediate proper response to DNA damage. In human luminal breast cancer (LBC) p27^kip1 is frequently down-modulated and CDKN1B, p27^Kip1 gene, sporadically mutated. We recapitulated results obtained in mouse fibroblasts in a LBC cell line genetically manipulated to be KO for CDKN1B gene. Following γ-irradiation, we confirmed that p27^kip1 expression was necessary to preserve genomic integrity and to recognize and clear-out aberrant cells. Our study provides important insights into mechanisms underlying radio-resistance and unveils the possibility for novel treatment options exploiting DNA repair defects in LBC.