The Atypical Gtpase Rhou Lies Downstream IL6/STAT3 and Regulates Myeloma Plasma Cells Adhesion/Motility

RhoU is an atypical G protein of the Rho GTPase family endowed with a very high intrinsic guanine nucleotide exchange activity ensuring a prevalent GTP-loaded conformation. It is present at very low levels in most cells, however some reports have shown that the Wnt-1 and STAT3 pathways might regulate its expression which resulted highly increased in malignant tumors. Since the IL6R/STAT3 signaling is of critical in Multiple Myeloma (MM) growth, homing and adhesion to marrow stroma and RhoU GTPase may mediate the induction of filopodia formation and stress fiber dissolution, we aimed at studying the role of this protein in MM malignancy.

mRNA expression studies unraveled that MM patients have heterogeneous levels of RhoU ranging from -0.7 fold change to 18.5 fold change when normalized over controls. By analyzing available gene expression data sets, we found that RhoU expression changes were associated with distinctive expression of 557 genes. We also found a correlation between RhoU levels and genetic abnormalities that associate with a poor prognosis in MM patients, namely the t(4;14), del(13) and 1q gain. RhoU was positively modulated by IL-6 and by soluble factors from bone marrow stromal cells' culture. Its expression was down-modulated after STAT3 inhibition with the chemical inhibitor Stattic, even in the presence of IL-6. The treatment with Stattic also decreased MM cells' migration capability. Remarkably, RhoU knock down by siRNA caused changes in MM cells' phenotype and deranged adhesion/migration functional properties. Lastly we have looked into changes in RhoU expression after the treatment of MM cells with Lenalidomide, known to act on MM microenvironment and alter MM cells' adhesion to protective bone marrow niches. Surprisingly, treatment of MM cell lines or from patients with Lenalidomide led to an upregulation of RhoU expression in the cells before onset of apoptosis after 72 hours.

Our data point to a functional role in cell adhesion, migration and shape for RhoU GTPase in a subset of MM, in particular in cases with genetic abnormalities correlating with a poor prognosis. RhoU is downstream from STAT3/IL-6 and likely from growth factor delivered by bone marrow stromal cells. The upregulation of RhoU expression after treatment with Lenalidomide was associated to a stall of the cellular cytoskeleton and subsequent apoptosis. Thus RhoU might be important in MM pathogenesis and could become a suitable target to disrupt the MM plasma cell interaction with protective bone marrow niches.