Anja Troeger

RhoH is critical for cell-microenvironment interactions in chronic lymphocytic leukemia in mice and humans

Trafficking of B-cell chronic lymphocytic leukemia (CLL) cells to the bone marrow and interaction with supporting stromal cells mediates important survival and proliferation signals. Previous studies have demonstrated that deletion of Rhoh led to a delayed disease onset in a murine model of CLL. Here we assessed the impact of RhoH on homing, migration, and cell-contact dependent interactions of CLL cells. Rhoh(-/-) CLL cells exhibited reduced marrow homing and subsequent engraftment. In vitro migration toward the chemokines CXCL12 and CXCL13 and cell-cell interactions between Rhoh(-/-) CLL cells and the supporting microenvironment was reduced. In the absence of RhoH the distribution of phosphorylated focal adhesion kinase, a protein known to coordinate activation of the Rho GTPases RhoA and Rac, appeared less polarized in chemokine-stimulated Rhoh(-/-) CLL cells, and activation and localization of RhoA and Rac was dysregulated leading to defective integrin function. These findings in the Rhoh(-/-) CLL cells were subsequently demonstrated to closely resemble changes in GTPase activation observed in human CLL samples after in vitro and in vivo treatment with lenalidomide, an agent with known influence on microenvironment protection, and suggest that RhoH plays a critical role in prosurvival CLL cell-cell and cell-microenvironment interactions with this agent.

A Unique Carboxyl-terminal Insert Domain in the Hematopoietic-specific, GTPase-deficient Rho GTPase RhoH Regulates Post-translational Processing

Background: RhoH is required for T cell development and non-coding RhoH mutations are found in B-cell lymphoma. Results: The C-terminal insert domain regulates RhoH protein stability via chaperone-mediated autophagy. Conclusion: Although the insert domain is required for lysosomal uptake, it is dispensable for T cell development. Significance: Targeting the insert domain may permit alteration of RhoH protein levels without impairing vital protein functions. RhoH is a hematopoietic-specific, GTPase-deficient member of the Rho GTPase family that was first identified as a hypermutable gene in human B lineage lymphomas. RhoH remains in a constitutively active state and thus its effects are regulated by expression levels or post-translational modifications. Similar to other small GTPases, intracellular localization of RhoH is dependent upon the conserved “CAAX” box and surrounding sequences within the carboxyl (C) terminus. However, RhoH also contains a unique C-terminal “insert” domain of yet undetermined function. RhoH serves as adaptor molecule in T cell receptor signaling and RhoH expression correlates with the unfavorable prognostic marker ZAP70 in human chronic lymphocytic leukemia. Disease progression is attenuated in a Rhoh−/− mouse model of chronic lymphocytic leukemia and treatment of primary human chronic lymphocytic leukemia cells with Lenalidomide results in reduced RhoH protein levels. Thus, RhoH is a potential therapeutic target in B cell malignancies. In the current studies, we demonstrate that deletion of the insert domain (LFSINE) results in significant cytoplasmic protein accumulation. Using inhibitors of degradation pathways, we show that LFSINE regulates lysosomal RhoH uptake and degradation via chaperone-mediated autophagy. Whereas the C-terminal prenylation site is critical for ZAP70 interaction, subcellular localization and rescue of the Rhoh−/− T cell defect in vivo, the insert domain appears dispensable for these functions. Taken together, our findings suggest that the insert domain regulates protein stability and activity without otherwise affecting RhoH function.

RhoH participates in a multi-protein complex with the zinc finger protein kaiso that regulates both cytoskeletal structures and chemokine-induced T cells

ABSTRACT RhoH is a haematopoietic -specific, GTPase-deficient Rho GTPase that plays an essential role in T lymphocyte development and haematopoietic cell migration. RhoH is known to interact with ZAP70 in T cell receptor (TCR) signaling and antagonize Rac GTPase activity. To further elucidate the molecular mechanisms of RhoH in T cell function, we carried out in vivo biotinylation and mass spectrometry analysis to identify new RhoH-interacting proteins in Jurkat T cells. We indentified Kaiso by streptavidin capture and confirmed the interaction with RhoH by co-immunoprecipitation. Kaiso is a 95 kDa dual-specific Broad complex, Trantrak, Bric-a-brac/Pox virus, Zinc finger (POZ-ZF) transcription factor that has been shown to regulate both gene expression and p120 catenin-associated cell-cell adhesions. We further showed that RhoH, Kaiso and p120 catenin all co-localize at chemokine-induced actin-containing cell protrusion sites. Using RhoH knockdown we demonstrated that Kaiso localization depends on RhoH function. Similar to the effect of RhoH deficiency, Kaiso down-regulation led to altered cell migration and actin-polymerization in chemokine stimulated Jurkat cells. Interestingly, RhoH and Kaiso also co-localized to the nucleus in a time-dependent fashion after chemokine stimulation and with T cell receptor activation where RhoH is required for Kaiso localization. Based on these results and previous studies, we propose that extracellular microenvironment signals regulate RhoH and Kaiso to modulate actin-cytoskeleton structure and transcriptional activity during T cell migration.