Increased expression and nuclear localization of the centrosomal kinase Nek2 in human testicular seminomas

Germ cell tumors (GCTs) are a heterogeneous group of neoplastic diseases that occur both in the gonads and in extra-gonadal sites, such as the retroperitoneal district, the mediastinal region and brain. Testicular GCTs (TGCTs) can be distinguished in three epidemiologically, clinically and histologically diverse groups of tumors. The first group includes pre-puberal teratomas and yolk sac tumors and originate from immature germ cells, such as the migrating primordial germ cells (PGCs). The post-puberal testicular germ cell tumors (PTGCTs) include seminomas and non-seminomas (embryonal cell carcinoma, choriocarcinoma and post-puberal yolk sac tumors and teratomas). They originate from PGCs or gonocytes that have already reached the gonads. PTGCTs are the most common form of TGCT . Testicular germ cell tumors (TGCTs) are characterized by amplification of centrosomes and aneuploidy through unknown mechanisms. Protein kinases that regulate the centrosome cycle, like Plk1 and Aurora A, are often aberrantly regulated in neoplastic cells. Changes in their expression or activity can lead to perturbations in centrosome duplication. Nek2 is a centrosomal kinase, highly enriched in male germ cells which promotes centrosome separation at the onset of mitosis through phosphorylation and displacement of proteins involved in centrosome cohesion. Experimentally, upregulation of this kinase in human cells causes premature splitting of the centrosome, whereas overexpression of kinase-dead Nek2 induces centrosome abnormalities resulting in monopolar spindles and aneuploidy. Hence, a tight regulation of Nek2 abundance and activity is essential to ensure the correct centrosome cycle. Herein we investigated the expression of Nek2 in TGCTs. We found that this kinase is upregulated in human testicular seminomas as compared to control testes or other types of testicular germ cell tumours. In addition, immunokinase assays demonstrate that Nek2 activity is also increased in human seminomas. By immunohistochemistry we observed that Nek2 is prevalently localized in the nucleus of neoplastic cells of primary human seminomas. We confirm upregulation of Nek2 and its nuclear localization in the Tcam-2 , the only seminoma cell line available to date. Using several approaches we also demonstrate that nuclear localization of Nek2 is a feature of the more undifferentiated germ cells of mouse testis, (PGCs and gonocytes), and correlates with expression of the stemness markers OCT4 and PLZF. These studies suggest that upregulation of Nek2 is a frequent event in human seminomas and that it may participate in the onset or progression of neoplastic transformation through deregulation of centrosome duplication and/or nuclear events in germ cells.