Role of Platelet-Derived Growth Factor On Vascular Endothelial Growth Factor Synthesis by Mesenchymal Stem Cells in Chronic Lymphocytic Leukemia: Implications for Clinical Progression.

We have demonstrated that crosstalk between CLL B-cells and marrow-derived mesenchymal stem cells (MSC) can modulate the activation of both. Soluble factors in the CLL-condition media (CM) regulate the migration and proliferation of MSC. We previously found that platelet-derived growth factor (PDGF) receptors were selectively activated in MSC by CLL-CM and are necessary for the downstream Akt activation in MSC. PDGF is capable of promoting vascular endothelial growth factor (VEGF) production in MSC; however, several questions remain unanswered. Can CLL B-cells produce PDGF; is PDGF the specific mediator in CLL-CM responsible for PDGFR activation in MSC; what is the mechanism of PDGF mediated VEGF production in MSC; and is there any clinical significance of PDGF mediated VEGF production by MSC for CLL patients?

MSC were generated from CLL patients and controls as previously described. To determine the specific ligand responsible for PDGFR activation mediated by CLL-CM blocking antibody to PDGF or VEGF or both was used to neutralize the binding activity of PDGF or VEGF in the CLL-CM. Subsequently, the pattern of membrane receptor activation was evaluated using a commercially available receptor tyrosine kinase (RTK) array assay. ELISA was performed to measure PDGF and VEGF levels from plasma and CM of CLL patients and normals. The protein expression of PDGF isoforms in CLL and normal B-cells was assessed using Western blot. To assess the mechanism of PDGF mediated VEGF production by MSC, we used either a PI3K inhibitor, LY294002 (20 mM), or a p38 MAPK inhibitor, SB202190 (100 nM), to block these pathways in MSC.

The known ligands for PDGF receptors, PDGF and VEGF, were detected in the CLL-CM (PDGF, 113.7 ± 23.6 pg/ml, VEGF, 186.94 ± 54.57 ng/ml; n = 12). We also found that PDGF is present in CLL cells by Western blot analysis and expression of PDGF in CLL cells was up regulated by hypoxia. By using PDGF- or VEGF-blocking antibody or both to neutralize the binding activity of PDGF or VEGF in CLL-CM, we found only PDGF-blocking antibody was able to neutralize the majority of PDGFRa activation, indicating that PDGF in the CM is the predominant ligand for PDGFR activation in MSC. CLL MSC was exposed to either pooled CLL (n = 5) or pooled normal plasma (n = 5). Interestingly, PDGFR was more potently activated by CLL plasma than the normal plasma, suggesting PDGF levels were elevated in the CLL plasma. Indeed, PDGF in the normal plasma appeared to be much lower compared to the CLL plasma [normal (n = 11): 554.43 ± 88.55 pg/ml; CLL (n=43): 3115.36 ± 428.79 pg/ml. p < 0.0001]. In addition, the plasma PDGF level was found to be weakly correlated with VEGF level in 43 CLL patients assessed using Spearman correlation analysis (r = 0.3, p = 0.001). However, when CLL patients were grouped by clinical prognostic factors, the plasma PDGF level was found to be strongly correlated with plasma VEGF level in CLL patients with high-risk features including more advanced Rai stage 2-4 (r = 0.87, p < 0.0001), ZAP-70 positive (r = 0.8, p =0.04) and CLL patients who had required treatment (r = 0.8, p = 0.0008). To examine the mechanism for PDGF augmentation of VEGF by MSC, we found that PI3K pathway interruption using LY294002 (n = 6, p = 0.04), but not p38 MAPK inhibition (n = 6, p = 0.5), was able to abrogate the PDGF mediated VEGF production in MSC. Importantly, we also found that the secreted VEGF level in the CM of unstimulated CLL MSC appeared to be significantly higher compared to the level from culture media of unstimulated normal MSC [CLL (n = 14) vs. normal (n = 5), 298.1± 61.2 vs. 109.9 ± 52.3, p = 0.03].

These results indicate that PDGF secreted by CLL B-cells is necessary for PDGFR activation in MSC and can lead to increased VEGF production by MSC via a PI3K-Akt dependent mechanism. These findings have clinical implications as plasma PDGF levels are strongly correlated with plasma VEGF levels in high risk CLL. The finding that CLL MSC are more prone to secrete VEGF than normal MSC suggests the stromal microenvironment may function to facilitate CLL disease progression. Finally, as VEGF is known to enhance CLL B-cell survival and drug resistance, interrogation of the PDGF-VEGF regulation in the microenvironment of CLL patients should yield important information that can be used for therapeutic strategies in CLL.

Kay:Biogenc-Idec, Celgene, Genentech, genmab: Membership on an entity's Board of Directors or advisory committees; Genentech, Celgene, Hospira, Polyphenon Pharma, Sanofi-Aventis: Research Funding.