Molecular and Pathological Analysis of DOCK Family Proteins in Hematopoietic Malignancy

Abstract 5230

Crk, originally identified as an oncogene product v-Crk encoding in a chicken retrovirus CT10 in 1988, is a signaling adaptor protein mostly composed of SH2 and SH3 domains, and has been shown to play a pivotal role in cell proliferation, differentiation, and migration in various human cancers. Crk SH3 domain binds to DOCK180 which has been reported to activate Rac regulating cytoskeletal reorganization leading to the enhancement of cell motility and engulfment. In 1999, we identified DOCK2, a hematopoietic cell-specific homolog of the DOCK family protein, and revealed that DOCK2 regulated cell motility and cytokine production through the activation of Rac in human hematopoietic cells (1,–3). Recently, we demonstrated the prominent expression of DOCK2 in human B cell lymphoma, and DOCK2 regulated cell proliferation through Rac and ERK activation in B cell lymphoma cell lines (4). Immunostaining for DOCK2 in 20 cases of human B cell lymphoma tissue specimens including diffuse large B cell lymphoma and follicular lymphoma revealed the prominent expression of DOCK2 in all of the lymphoma cells. DOCK2-knockdown (KD) of the B cell lymphoma cell lines, Ramos and Raji, using the lentiviral shRNA system presented decreased cell proliferation compared to the control cells. Furthermore, the tumor formation of DOCK2-KD Ramos cell in nude mice was significantly abrogated. Western blotting analysis and pull-down assay using GST-PAK-RBD chimeric protein suggested the presence of DOCK2-Rac-ERK pathway regulating the cell proliferation of these lymphoma cells. In addition, we confirmed the elevated expression levels of DOCK2 in various types of hematopoietic tumor cell lines as well as in the primary cells from the patients with leukemia and lymphoma. Interestingly, we also found the expression of DOCK180, not DOCK2, in Hodgkin/Reed-Sternberg cells of Hodgkin lymphoma tissue specimens as well as in three cell lines derived from Hodgkin lymphoma. We are currently analyzing the tumorigenic roles of DOCK2 and DOCK180 using the suitable cell lines derived from various hematopoietic neoplasms.

Here we summarize the results of our molecular and pathological analysis for the DOCK family proteins in various human hematopoietic malignancies such as lymphomas and leukemia. Our results suggest the possible target molecules of DOCK family proteins including DOCK2 and DOCK180, and also Rac for lymphoma and leukemia therapy.