Platelet-Derived Growth Factor (PDGF) Secreted by Chronic Lymphocytic Leukemic B-Cells Is Capable of Regulating the Activation and Function of Mesenchymal Stem Cells: Implications for Leukemic Cell/Stromal Cell Crosstalk

Background: It is believed that malignant cells can “condition” microenvironment to facilitate tumor cell survival. Our previous finding demonstrated that the culture of chronic lymphocytic leukemic (CLL) B-cells and marrow-derived mesenchymal stem cells (MSC) impacts both cell types bi-directionally (ASH 2007, Blood, 110: 337). Thus, MSC are capable of promoting CLL B-cell activation and proliferation while soluble factors secreted from CLL B cells were found to induce MSC activation in terms of both Erk and Akt activation. However, the exact mechanism and extent of this leukemic cell-MSC crosstalk has not been explored yet in CLL.

Methods: To investigate if factors secreted from the CLL B-cells can modulate the migration and activation of marrow derived MSC, the conditioned medium (CM) of CLL B-cells was generated by collecting the supernatant of freshly isolated PBMC from CLL patients cultured for approximately 4 days at a concentration of 5 × 106 per ml in AIM-V medium. The migration and proliferation capacities of MSC were measured when they were cultured with or without the CM of CLL cells. Subsequently, the membrane receptors activated in MSC after 30 minutes of exposure to the CM of CLL cells were tested using a receptor tyrosine kinase array assay. In addition, we studied the downstream signal pathways of MSC by immunoblot approaches pre and post exposure to CLL CM and in some experiments we added specific, commercially available inhibitors for individual signaling pathways of interest. ELISA was used to measure angiogenic cytokines including PDGF, VEGF, BFGF and TSP-1 levels from CLL plasma as well as from CM of CLL B cells.

Results: To test if CLL B-cells were capable of secreting soluble factors that activate or signal MSC, we examined the migration and proliferation capacities of CLL-MSC when they were exposed to CM generated from cultured CLL B-cells compared to AIM-V medium. Bone marrow MSC of CLL patients exhibited an increased migration as measured by a modified Boyden chamber assay (n = 4, mean increase of migration: 15%, p = 0.01) and proliferation as measured by direct cell counting with trypan blue staining (n =4, mean increase of total cell levels: 3.2 fold, p = 0.02) when cultured with CM of CLL B-cells compared to AIM-V medium. Using a receptor tyrosine kinase array assay (R&D system), we found that the sole growth factor receptor activated on CLL –MSC was PDGFRα when MSC were exposed to CM of cultured CLL B-cells for 30 minutes. This finding was further confirmed by demonstrating that PDGFRα was phosphorylated by immunoprecipitating MSC lysates with anti-PDGFRα followed by immunoblot with anti-phosphotyrosine antibody. We subsequently found that both PDGFRα and Akt were activated within 10 minutes of exposure by CM of CLL B-cells. When CLL-MSC were pretreated with a PDGFR inhibitor (PDGFR tyrosine kinase inhibitor III, Calbiochem), an inhibitor known to block the ATP binding site of PDGFR, neither PDGFRα nor Akt activation was detectable when MSC were exposed to CM from CLL B-cells. These results imply that Akt is likely activated downstream of the PDGFR signal pathway in CLL-MSC. We next found that PDGF was secreted by CLL B-cells by detecting its presence using ELISA in both the CM (n = 12,113.7 ± 23.6 pg/ml, mean ± sem) and plasma of CLL patients (n = 21, 3296.7 ± 800.1 pg/ml). Since we have previously found that a switch in pro- vs. anti-angiogenic cytokines can occur when CLL B cells are added to MSC, we tested if PDGF can upregulate MSC angiogenic cytokine levels. When PDGF (5ng/ml, R&D system) was introduced to the CLL-MSC, we found that VEGF production, but not TSP-1 or BFGF significantly increased over control levels (n = 3, mean increase: 3.3 fold, p = 0.05) in the CM of the MSC harvested after a 72 h culture period.

Conclusions: These results indicate that CLL B-cells are capable of activating MSC function by increasing their migration and proliferation capacity likely via signaling through the PDGF receptor. The PDGF activated MSC results in downstream Akt activation and the increased secretion of VEGF, a cytokine known to enhance CLL B-cellsurvival and drug resistance. Further interrogation of the mechanism(s) that regulate the interaction between leukemic CLL B cells and stromal cells should yield important information that can be used for therapeutic strategies in CLL.