Rho GAPs inactivate Rho GTPases by the conservative GAP domain that promotes the GTP hydrolysis and accelerates the intrinsic GTPase activity, limiting the duration of the regulated reaction [29, 30]. The abnormal expression of Rho GAPs was observed in various tissues from patients with cancer and immunological diseases [31–34]. The expression of Rho GAPs was found to be heterogeneous among different tumors. ARHGAP30, for instance, was downregulated in lung cancer and colorectal cancer, whereas over-expressed in pancreatic cancer [35–37]. A low expression of ARHGAP9 was found in hepatocellular carcinoma and bladder cancer, whereas a high expression was observed in breast cancer [17, 19, 38]. In this study, we found that ARHGAP9 is over-expressed in both AML samples and cell lines compared with the normal counterparts . Furthermore, the low expression of ARHGAP9 was significantly correlated with t(15; 17) AML.
Some studies have been explored the role of Rho GAPs in cancer and found that most of Rho GAPs were associated with good outcomes in many kinds of solid tumors [33, 36, 37, 39]. ARHGAP9 also showed a good prognosis in bladder cancer and gastric cancer [18, 38]. However, in our survival analysis study, we found ARHGAP9 overexpression was associated with a poor prognosis in AML. These results indicated that ARHGAP9 may play different roles in various cancers. Meanwhile, DOCK2 belonging to the Rho GEFs family activates GTPase which has the opposite function compared with Rho GAPs. Also, it was an independent favorable prognostic factor for both EFS and OS in AML . Thus, in addition to being associated with GTPase, some regulators of Rho GTPase may have other functions in cancer. Despite the high expression of ARHGAP9 was related to CN-AML, we did not find any relationship between ARHGAP9 expression and prognosis in CN-AML. In the other study, some genes such as NCALD, IL2RA, and BCL2 are associated with prognosis in AML patients with auto/allo-HSCT and/or chemotherapy [41–43]. In the present study, ARHGAP9high groups had poor prognosis in post-chemotherapy AML patients, whereas no significant difference were found in OS and EFS between ARHGAP9high group and ARHGAP9low group in patients after auto/allo-HSCT, suggesting that the effects of ARHGAP9 over-expression could be eliminated by auto/allo-HSCT, instead of chemotherapy.
Previous studies showed that some of Rho GAPs expressed mainly in hematopoietic cells [8, 44–46]. Costa and his colleagues have shown that lacking ARHGAP15 leads to enhanced chemotactic responses, straighter directional migration, amplified reactive oxygen species production, increased phagocytosis as well as improved bacterial killing in neutrophils . ARHGAP25 negatively regulates leukocyte transendothelial migration in mice and phagocytosis acting in human neutrophilic granulocytes [45, 47]. Studies of both in vitro and in vivo indicated that ARHGAP21 knockdown could impair the function of T cells, reduced erythroid commitment and differentiation, and enhanced RhoC activity . In T lymphocytes, ARHGAP19 was shown to affect the stiffness and shape of lymphocytes by regulating cytokinesis and chromosome segregation . Taken together, Rho GAPs may play an important role in hematopoietic cells and regulate cell motility, cell cycle, adhesion, phagocytosis, NADPH oxidase, auto/allo-HSCT development, inflammatory responses, and neutrophil chemotaxis. This hypothesis was well supported by the functional analysis of KEGG and GO enriched by the overlapping genes in our study.
Upon immunization, Rho GAPs are critical for innate immunity and adaptive immunity [48–50]. While more than 11 members of Rho GAPs take part in various neutrophil functions that belong to the adaptive immunity . We speculated that ARHGAP9 probably also takes part in immune response in the AML because the overlapping genes were enriched in the immune system and the immune tissues. Our study showed the differently expressed genes associated with ARHGAP19 expression were enriched in APL. While 95% of APL is the abnormalities of t(15;17) which encoded the PML-RARA fusion protein . What is more, the expression of ARHGAP9 is the lowest in t(15;17) AML compared with the other chromosome abnormalities in AML, and all patients with t(15;17) were in the ARHGAP9low group in our study. Therefore, the expression of ARHGAP9 may be suppressed by the PML-RARA fusion protein. We should further investigate ARHGAP9 physiological role in APL.