ABSTRACT
Lysophosphatidic acid (LPA) protects chronic lymphocytic leukemia (CLL) cells from apoptosis. Vascular endothelial growth factor (VEGF) also protects CLL cells against apoptosis. The mechanism for LPA protection against apoptosis in CLL cells is unknown. Herein, we show CLL cells express LPA receptors LPA(1-5) but in normal B cells, LPA(1) was rarely expressed and LPA(3,) LPA(4,) and LPA(6) were undetectable whereas the other LPA receptors were expressed. LPA plasma levels are similar in patients with CLL compared to healthy controls. In contrast, plasma levels of VEGF are elevated in patients with CLL compared to healthy controls and LPA treatment induced VEGF secretion in CLL cells. CLL cells also express VEGF receptors and LPA protection against Flu induced apoptosis is blocked by inhibition of VEGF receptor activation. These results indicate that LPA protects CLL cells from apoptosis through higher expression of LPA receptors and autocrine production of VEGF.
Subject(s)
Leukemia, Lymphocytic, Chronic, B-Cell/genetics , Receptors, Lysophosphatidic Acid/genetics , Receptors, Purinergic P2/genetics , Receptors, Vascular Endothelial Growth Factor/genetics , Vascular Endothelial Growth Factor A/blood , Aged , Aged, 80 and over , Apoptosis/drug effects , Cell Line, Tumor , Cell Survival/drug effects , Cells, Cultured , Female , Gene Expression Profiling , Humans , Leukemia, Lymphocytic, Chronic, B-Cell/metabolism , Leukemia, Lymphocytic, Chronic, B-Cell/pathology , Lysophospholipids/blood , Lysophospholipids/pharmacology , Male , Middle Aged , Reverse Transcriptase Polymerase Chain Reaction , Signal Transduction/drug effects , Vascular Endothelial Growth Factor A/genetics , Vascular Endothelial Growth Factor A/metabolism , Vidarabine/analogs & derivatives , Vidarabine/pharmacologyABSTRACT
The standard treatments for chronic lymphocytic leukemia (CLL) include the alkylating agent chlorambucil (CLB) and the nucleoside analog fludarabine (F-ara-AMP, Flu). Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a death receptor ligand that induces apoptosis preferentially in tumors. However, CLL cells seem to be resistant to TRAIL-induced apoptosis. The TRAIL apoptotic signaling pathway has also been implicated in genotoxin-induced apoptosis through upregulation of TRAIL death receptors DR4 and DR5. In the present study, we demonstrate that the treatment of primary CLL cells with CLB or Flu increases the mRNA, protein and cell surface expression levels of DR4 and DR5 in a dose-dependent manner. In contrast to CLL cells, drug treatment fails to increase significantly the expression of DR4 or DR5 in normal lymphocytes. CLL cells are, however, resistant to TRAIL-induced apoptosis compared to B-cell lines. In contrast, combinational treatment using CLB or Flu with TRAIL (100 ng/ml) gave a synergistic apoptotic response. Furthermore, TRAIL is readily detectable on the cell surface of CLL cells, but TRAIL expression fails to increase following drug treatment. Preventing TRAIL from interacting with DR4 and DR5 decreases CLB-induced apoptosis in CLL cells. A similar, but less marked effect is observed with Flu. These findings indicate the involvement of the TRAIL apoptotic pathway in the mechanism of action of chemotherapy, and this mechanism could be utilized to sensitize CLL cells to TRAIL-induced apoptosis.
