A novel nuclear function for the centrosomal serine/threonine kinase Nek2

Protein kinases that regulate the centrosome cycle are often aberrantly regulated in neoplastic cells. Changes in their expression or activity can lead to perturbations in centrosome duplication and aneuploidy. In addition, many centrosomal protein kinases participate to other aspects of cell cycle progression. Testicular germ cell tumors (TGCTs) are characterized by amplification of centrosomes through unknown mechanisms. We have discovered that the centrosomal kinase Nek2 is overexpressed in testicular seminomas and we have characterized its function in neoplastic germ cells. One unexpected finding of our study was the nuclear localization of Nek2 in germ cells of patients. The same nuclear localization was observed in the seminoma cell line Tcam-2. We found that Nek2 was localized in the nucleus also in undifferentiated embryonal male primordial germ cells (PGCs) and in spermatogonial stem cells from post-natal testis. These results suggest that nuclear Nek2 is a novel marker of the undifferentiated stage of male germ cells that is maintained in testicular seminomas, but not in other TGCTs. The nuclear localization of Nek2 is not a unique feature of testicular seminomas, we show that Nek2 is mainly distributed in the nucleus of cancer cells from other tissues, including breast, prostate and colon cancer cells. The subnuclear distribution of Nek2 in speckles closely resembled that of many regulators of pre-mRNA splicing. We found that Nek2 physically associates with several splicing factors, such as SR proteins (ASF/SF2), hnRNPs (A1, F and H), and the STAR protein Sam68. We focused our study on Sam68 because this splicing regulator is also up-regulated in breast and prostate carcinomas like Nek2. Our study shows that Sam68 is also overexpressed in testicular seminomas but not in other TGCTs, like Nek2. Moreover, Nek2 phosphorylates Sam68 and affects Sam68-dependent splicing of CD44v5 pre-mRNA, an alternatively spliced form of the receptor, frequently altered in cancer cells, that promotes cell proliferation and invasiveness. These results identify a novel nuclear function of Nek2 and suggest that modulation of alternative splicing events by this kinase can contribute to neoplastic transformation