Search

We investigated the in vitro growth inhibitory and apoptotic effects of clinically achievable concentrations of As2O3 (0.5 to 2.0 μmol/L) against human myeloid leukemia cells known to be resistant to a number of apoptotic stimuli. These included chronic myelocytic leukemia (CML) blast crisis K562 and HL-60/Bcr-Abl cells, which contain p210 and p185 Bcr-Abl, respectively, and HL-60 cell types that overexpress Bcl-2 (HL-60/Bcl-2), Bcl-xL(HL-60/Bcl-xL), MDR (HL-60/VCR), or MRP (HL-60/AR) protein. The growth-inhibitory IC50 values for As2O3 treatment for 7 days against all these cell types ranged from 0.8 to 1.5 μmol/L. Exposure to 2 μmol/L As2O3 for 7 days induced apoptosis of all cell types, including HL-60/Bcr-Abl and K562 cells. This was associated with the cytosolic accumulation of cyt c and preapoptotic mitochondrial events, such as the loss of inner membrane potential (▵Ψm) and the increase in reactive oxygen species (ROS). Treatment with As2O3 (2 μmol/L) generated the activities of caspases, which produced the cleavage of the BH3 domain containing proapoptotic Bid protein and poly (ADP-ribose) polymerase. Significantly, As2O3-induced apoptosis of HL-60/Bcr-Abl and K562 cells was associated with a decline in Bcr-Abl protein levels, without any significant alterations in the levels of Bcl-xL, Bax, Apaf-1, Fas, and FasL. Although As2O3 treatment caused a marked increase in the expression of the myeloid differentiation marker CD11b, it did not affect Hb levels in HL-60/Bcr-Abl, K562, or HL-60/neo cells. However, in these cells, As2O3 potently induced hyper-acetylation of the histones H3 and H4. These findings characterize As2O3 as a growth inhibiting and apoptosis-inducing agent against a variety of myeloid leukemia cells resistant to multiple apoptotic stimuli.

Paclitaxel is a microtubule stabilizing drug that causes dividing cells to arrest and then undergo apoptosis. It also has antiangiogenic activity because it alters cytoskeletal structure, affecting migration and invasion. Paclitaxel is an effective treatment for AIDS-related Kaposi’s sarcoma (KS). KS is a tumor in which there is marked proliferation of endothelial cells in addition to the tumor cells, which themselves share many markers with activated (proliferating) endothelial cells.We sought to determine the mechanism by which paclitaxel exerts its anti-KS tumor effects.In vitro, KS cells are very sensitive to paclitaxel, with half-maximal growth inhibition observed at 0.8 nM. Inhibition of migration of KS cells was also observed at nanomolar concentrations of the drug. Paclitaxel induced cell cycle arrest with an accumulation of cells in sub-G1.This was accompaniedin vitroby various events typical of apoptosis: phosphorylation of two anti-apoptotic proteins Bcl-2 and Bcl-xL, release of cytochrome c into the cytoplasm, cleavage and activation of caspase-3.In vitroresults were borne out by studies of KS tumor xenografts in nude mice. Paclitaxel (10 mg/kg) inhibited tumor growth by 75% over 21 days. Histological examination of the tumors revealed a decrease in proliferative index, a decrease in the number of mitotic figures and an increase in apoptotic cells compared to tumors from untreated mice.