Thrombin-Induced PARI Signaling Pathway Modulates ß-Catenin in the Transformation of Symptomatic Myeloma Cells Into the Quiescent Phenotype,

Abstract 3932

Studies on the longevity of plasma cells (PCs) have provided evidence that PCs can survive for several months in the absence of repopulation by memory B cells, and PCs in the bone marrow appear to have a much longer lifespan than those found in the extramedullary environments. The concept of myeloma stem cells is based on the theory that a subset of B cells (plasmablasts) with the CD138⁻/CD20⁺/CD27⁺ phenotype coexisting with the majority of terminally differentiated myeloma cells give rise to tumor cells via constant clonogenic reproduction and differentiation. In previous studies, by depleting a large portion of CD138⁺/CD19⁻ myeloma PCs, a small number of CD138⁻/CD19⁺ cells remained. These cells proliferated with high efficiency, giving rise to progeny with strong CD138 expression (CD138⁺⁺) both in vitro and in vivo. By contrast, the counterpart of terminally differentiated CD138⁺⁺ myeloma cells had limited or no clonal potential to recapitulate growth. The researchers also found that identical cytogenetic aberrations were maintained indefinitely throughout the replications. Nevertheless, the developmental path of the myeloma stem cells to its fully differentiated progeny is still unknown; more intriguingly, the mechanism that modulates myeloma cell maturation in vitro has yet to be elucidated.

Using a combination of array-comparative genomic hybridization (aCGH) and gene expression profiles (GEP) we discovered that PAR1 (coagulation factor II [thrombin] receptor, or F2R; 5q13.3) expression is strongly correlated with gains of chromosome 5, a consistent feature of the hyperdiploid (HY) molecular subtype of multiple myeloma. At the diagnosis, PAR1 gene expression is elevated in 50% of primary myeloma cases. In myeloma bone marrow and human myeloma cell lines (HMCLs), we have been able to identify a subset of cells that are strongly positive for PAR1 and have weak CD138 and CD38 surface markers. The PAR1-positive phenotype defines a distinct subpopulation in heterogeneous bone marrow cells, and, to a greater degree, in a homogenous HMCL. Significantly, we have been able to demonstrate proliferation inhibition when HMCLs that express high levels of PAR1 are exposed to thrombin. Studies of PAR1 functions indicate it is not only a thrombin receptor, but also modulates the phosphorylation of ß-catenin, either parallel to or competing with the well-known Wnt pathway. The destination of stabilized ß-catenin is crucial to the formation of adherens junctions (AJs) at the cell membrane and for gene transcription regulation in the nucleus. In this study, we also discovered that most HMCLs and primary myeloma PCs retain high levels of ß-catenin, which is primarily accumulated in the nucleus, and thrombin-catalyzed stimulation can redistribute ß-catenin into cytoplasm to strongly enhance the AJs between myeloma cells and stromal cells. Based on this evidence, we believe that myeloma cells are in fact capable of transforming into a quiescent stem-like phenotype; and thrombin-induced PAR1 signaling modulates the intracellular distribution of ß-catenin that plays a major role in the reversible transformation of primary myeloma cells to indolent and drug resistant myelomablasts.