All Trans-Retinoic Acid (ATRA) in Combination with PPAR-g Ligand Synergistically Enhances Inhibition of Cell Growth and Induction of Tumor Suppressor PTEN in Leukemic Cells.

Peroxisome proliferator-activated receptor-γ (PPAR-γ), a transcriptional factor that plays an essential role in mediating the pharmacologic actions of PPAR-γ ligands, is highly expressed in normal monocytes, various leukemias, and epithelial malignancies. PPAR-γ ligands have been developed to induce differentiation, growth arrest, and apoptosis. PPAR-γ must form a heterodimer with the retinoic acid receptor (RAR) to bind DNA, and its transcriptional activity is thought to be maximal in the presence of both PPAR-γ ligand and RAR ligand. Recently, it has been shown that activated PPAR-γ and PPAR-γ ligand may influence tumor growth, directly and indirectly, through regulation of the tumor suppressor gene PTEN, a lipid phosphatase that plays a significant role in several cellular functions, including survival and proliferation, by antagonizing phosphatidylinositol 3-kinase (PI 3-kinase)-mediated signaling pathways. The human leukemia cell line HL60 displays controlled differentiation along the granulocyte or monocyte lineages in response to various inductive signals. Retinoic acid receptor ligands are inducers of HL60 granulocytic differentiation. PPAR-γ ligands and RAR ligands induced growth arrest and oxidative burst capacity in HL60 cells. Previously, we reported the induction of PTEN gene expression by the PPAR-γ ligand ciglitazone in HL60 cells in the presence of retinoic acid using northern and western blotting. Here, using RT-PCR and an AdenoVector system to examine the contribution of transcriptional control in PTEN regulation and for PTEN overexpression, we show that PPAR-g ligand and RAR ligand can synergistically upregulate PTEN in human promyeloid leukemia cells, resulting in inhibition of cell growth and cell cycle progression of acute leukemic cells. Adenovirus-delivered PTEN overexpression resulted in arrests of both cell growth and the G₁ phase of the cell cycle in HL-60 cells, suggesting that the growth-suppressive effect of PTEN is associated with its ability to induce cell cycle arrest. The PPAR-γ ligand, ciglitazone, and the RAR ligand, retinoic acid, increased PTEN expression of HL-60 cells in a time- and dose-dependent manner, respectively. Upregulation of PTEN was significantly enhanced by a combination of both ciglitazone and retinoic acid. Moreover, these compounds synergistically induced arrests of both cell growth and the G₁ phase of the cell cycle. These findings suggest that the activation of the PPAR-γ: RAR heterodimer represents a novel regulatory pathway for HL60 cells and there may be a possible role for PPAR-γ and RAR ligand in prophylactic and therapeutic approaches for controlling leukemia through the upregulation of PTEN.