N-Cadherin Expressing MM Cells Form Calcium-Dependent, Adherent Junctions That Likely Contribute to Focal Lesions and Nodular Growth of Multiple Myeloma Cells

Abstract 979

E-cadherin-mediated adhesion regulates homeostasis in tissues of epithelial origin and homotypic N-cadherin interactions are central to the interaction of hematopoietic stem cells and the endosteal niche. Loss of E-cadherin in carcinomas is characteristic of the epithelial-to-mesenchymal transition and metastasis. The loss of E-cadherin, causes the release of b-catenin from the adherent complex, increased nuclear translocation, and increased transcriptional activity of b-catenin/TCF. We and others have demonstrated that alterations in the Wnt/b-catenin pathway exists in multiple myeloma, a malignancy of terminally differentiated antibody secreting plasma cells. The first evidence of this deregulation came from studies showing that MM cells secrete the potent Wnt/b-catenin signaling inhibitor DKK1. While it is now clear that DKK1 mediated suppression of Wnt/b-catenin in the bone marrow contributes to the decoupling of bone formation, the role of Wnt/b-catenin in normal plasma cell development and myelomagenesis is less clear and often controversial. Myeloma cells grow exclusively in the bone marrow. This growth is characterized as being interstitial or nodular, with most disease exhibiting a mixed pattern. Nodular growth, recognized as focal lesions (FL) on MRI, characterizes the conversion of MGUS to symptomatic MM. Consistently, DKK1 levels are highest in CD138 cells isolated from FL where nodular tumor growth and bone destruction may be linked by Wnt/b-catenin suppression in MM cells and local microenvironment. Given the central role of classical cadherins in promoting cell-cell adhesion and regulating b-catenin, we hypothesized that abnormal expression of cadherins might play a direct role in myelomagenesis.

Cell lysates were prepared from 24 MM cell lines and from CD138⁺ cells from the BM of eight patients with MM. Immunoblotting was performed with antibodies specific to human N-cadherin protein. N-cadherin protein was observed in more than 82% of MM cell lines, with high protein levels in 55% of the cell lines. Similar levels of N-cadherin protein were seen in primary myeloma cells from the eight MM patients: two patients showed the highest levels of N-cadherin protein, two showed intermediate levels, and three weak levels; N-cadherin protein was absent in one patient. Similar results were obtained with three color-flow cytometry analysis of primary MM BM. To determine whether N-cadherin mediated MM cell interactions, a cell aggregation assay using GFP-expressing, N-cadherin+ MM cells observed under fluorescence microscopy was employed. When N-cadherin+ JJN3 cells were cultured in normal growth medium, they aggregated to form clusters. Similar results were observed in other MM cell lines, including OPM-2 and KMS-28-BM. Addition of a neutralizing N-cadherin antibody to these cultures significantly attenuated aggregation of JJN3 cells compared to control cells in normal medium or cells treated with control IgG. Homotypic N-cadherin interaction forms adherent junctions in a calcium dependent manner. To see if aggregation of MM cells results in the formation of adherent junctions, immunochemical staining was preformed to visualize the N-cadherin protein in OPM-2 cells. N-cadherin protein was clearly observed between myeloma cells in aggregation clusters that were significantly diminished in calcium-free medium. Taken together, these data suggest that N-cadherin induces homotypic adhesion of myeloma cells in a calcium-dependent manner and suggests that calcium release during bone resorption may enhance adherent junctions in MM that may in turn enhance plasma membrane localization of b-catenin. Studies are currently underway to determine whether DKK1 and N-cadherin adherent junctions cooperate to suppress b-catenin nuclear activity in MM cells and all converge to induce the nodular growth pattern and bone destruction often seen in N-cadherin-positive MM.