HGAL is a recently identified germinal center (GC)–specific gene whose expression by tumor cells correlates with a favorable prognosis in patients with diffuse large B-cell and classical Hodgkin lymphomas. HGAL is involved in negative regulation of lymphocyte migration, thus potentially constraining lymphocytes to the GC and decreasing lymphoma cell migration and dissemination. The latter effect may contribute to the less aggressive clinical behavior of HGAL-expressing lymphomas. Actin filaments, usually in association with myosin, are responsible for many types of cell movements. The interaction between myosin and actin is responsible for muscle contraction, migration of nonmuscle cells and also plays a role in cell division. The movement of cells appears to be driven directly by actin polymerization and by actin-myosin interactions. In our previous work (Lu et al, Blood 2007), we demonstrated by co-immunoprecipitation and co-localization studies that HGAL interacts with myosin II and actin. However, the mechanistic consequences of this interaction are unknown. Because the interaction of actin with myosin is the key factor in cell motility and HGAL negatively regulates lymphocyte migration, we hypothesized that HGAL may decelerate cell migration by affecting the binding of myosin to actin. To confirm the direct and not interdependent interaction between myosin-HGAL and actin-HGAL, we performed in vitro co-sedimentation experiments with recombinant HGAL protein and purified actin and myosin, respectively. These studies demonstrated direct and independent binding of HGAL to both myosin and actin. We next demonstrated that the N terminal portion of the HGAL protein (aa 1–118) can bind the head region of myosin II (S1) that contains the actin and ATP binding sites, and to the rod part of the myosin molecule that is responsible for filament formation. While no effect of the HGAL on the actomyosin ATPase activity was observed, it reduced the rate of actin polymerization from G-actin to F-actin. Further binding studies using pyrene-labeled F-actin showed that HGAL increases the binding of myosin to F-actin. Taken together, our results indicate that HGAL is a cellular regulator of actin assembly that can also regulate the actin-myosin interaction. This in turn may explain the role of HGAL as a favorable prognosis marker for patients with diffuse large B-cell and classical Hodgkin lymphomas.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.