The T197A Knock-in Model of Cdkn1b Gene to Study the Effects of p27 Restoration In Vivo

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.

Regulation of GDF9 and CDKN1B expression in Tibetan sheep testes during different stages of maturity

Normal spermatogenesis is heavily dependent on the balance of germ cell proliferation, differentiation and apoptosis. Growth differentiation factor 9 (GDF9) and cyclin-dependent kinase inhibitor 1 B (CDKN1B) are strongly associated with cell cycle transition from G0/G1 to S and G2/M phase and hence regulating the growth and development of testicular germ cells and somatic cells. The current study was aimed at seeking out scientific evidence to determine if GDF9 and CDKN1B gene expression functions in the development of Tibetan sheep testes. To this end, developmental testes were derived from three-month-old (pre-puberty), one-year-old (sexual maturity), and three-year-old (adult) Tibetan sheep and then the expression and localization patterns of GDF9 and CDKN1B in these testes were evaluated using quantitative real-time PCR (qRT-PCR), Western blot and immunofluorescence. qRT-PCR and Western blot results showed that GDF9 and CDKN1B were detected in the testes throughout the different developmental stages. The abundance of GDF9 mRNA and protein in the testes of one- and three-year-old Tibetan sheep were higher than that in the testes of three-month-old Tibetan sheep; the mRNA and protein abundance of the CDKN1B gene in three-month-old Tibetan sheep testes were higher than that in the testes of the one-and three-year-old sheep. Moreover, immunofluorescence results suggested that the GDF9 protein was expressed in spermatogonia and Leydig cells, and that the CDKN1B protein was localized mainly in Leydig cells with some in the seminiferous epithelium throughout developmental stages. This indicated a novel role of the GDF9 and CDKN1B genes in Leydig cell development over and above their known roles in germ cell development. These findings have significant implications for our understanding of the molecular mechanisms of GDF9 and CDKN1B genes in Tibetan sheep spermatogenesis.

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.