ANKHD1 Interacts with the Proapoptotic Protein SIVA and Plays a Role in the Proliferation and Stathmin Activation of Acute Leukemia Cells.

Acute leukemia and solid tumors result from alterations in the essential pathways of cell physiology including apoptosis, proliferation and genome instability. In solid tumors, the proapoptotic SIVA protein modulates apoptosis, proliferation, migration and promotes Stathmin inhibition through phosphorylation. Stathmin regulates microtubules dynamics and its hyperactivity confers chromosome instability in leukemia cells. Using two-hybrid system assay, we have identified SIVA as a binding partner of ANKHD1, an ankyrin-repeat-containing protein. ANKHD1 is overexpressed in acute myeloid leukemia (AML) and acute lymphoblast leukemia (ALL) and has the potential role of regulating multiple cellular functions via their repeat motifs. We thus hypothesized that ANKHD1 and SIVA could be involved in leukemogenesis. We aimed to evaluate SIVA expression in normal and leukemia hematopoietic cells, to confirm the endogenous ANKHD1/SIVA association, and to investigate the functional role of both proteins in apoptosis, proliferation, migration, and Stathmin activation.

Expression of SIVA was evaluated by qPCR in total bone marrow cells from 22 healthy donors, 42 AML and 21 ALL patients at diagnosis. All normal donors and patients provided informed written consent and the study was approved by the ethics committee of the Institution. Leukemia cell lines (Jurkat, Namalwa or U937 cells) were used for functional studies. Endogenous protein interaction was verified by immunopreciptation and cofocal microscopy. We stably knocked down the endogenous expression level of ANKHD1 or SIVA with specific shRNA-expressing lentiviral vector and in vitro apoptosis was examined by AnnexinV/PI, cell growth by MTT assay and colony formation, and migration by transwell assays. In addition, we investigated in vivo tumor growth; leukemia cells were implanted in the dorsal sub cutis of NOD/SCID mice and tumors were excised, measured and weighed after 15 days. Stathmin activation proteins (Stathmin, phospho-Stathmin, alpha tubulin and acetylated-alpha tubulin) and apoptotic proteins (BCL-XL, BAX, JNK and phospho-JNK) were evaluated by Western blot. Appropriated statistical analysis was performed.

SIVA expression was significantly decreased in AML and ALL cells compared with normal hematopoietic cells (P<0.05), a reverse pattern of ANKHD1 expression, when compared with published data. Immunopreciptation and confocal analyses confirmed that ANKHD1 and SIVA interact and co-localize in the cytoplasm of leukemia cells. Functional studies revealed that SIVA and ANKHD1 have antagonistic effects on migration, Stathmin activation, and in vivo tumor growth. SIVA silencing resulted in a significantly increased cell migration, Stathmin activation (decreased Stathmin phosphorylation), and augmented in vivo tumor growth (P<0.05). On the other hand, ANKHD1 silencing resulted in a significantly decreased cell migration, Stathmin inactivation (increased Stathmin phosphorylation and alpha tubulin acetylation), and reduced in vivo tumor growth (P<0.05). Regarding apoptosis and proliferation, SIVA knockdown resulted in a significant decrease in apoptosis response to UV and daunorubicin induction and a downregulation of proapoptotic proteins p-JNK and BAX, an upregulation of the antiapoptotic protein BCL-XL, but no modulation was observed in proliferation and clonal growth in vitro. In contrast, ANKHD1 knockdown resulted in a significant decrease of proliferation and clonogenicity (P<0.05), but no changes were observed in apoptosis in vitro.

Our data indicate SIVA as a tumor suppressor gene in leukemia cells, and SIVA downmodulation may contribute to the apoptosis resistance and chromosome instability. ANKHD1 may be an oncogene, and the upregulation of this protein in leukemia cells might lead to increased proliferation and generate chromosomal instability through increased Stathmin activation. The results suggest that ANKHD1 inhibits SIVA and restoration of SIVA expression or inhibition of ANKHD1 may be an attractive approach in leukemia.

No relevant conflicts of interest to declare.