Failure to induce inhibition of cyclin A and up-regulation of p21 expression in phorbol ester-resistant U937 cells by phorbol ester

Resistance to phorbol ester-induced differentiation in human myeloid leukemia cells: a hypothetic role for the mRNA stabilization process

UM384 cells, derived from the human myeloid leukemia U937 cell line, fail to differentiate in response to 12-O-tetradecanoylphorbol-13-acetate (TPA). Using cDNA microarray and real-time quantitative PCR (RT-QPCR) approaches, we observed a difference in the response to TPA treatment: all the genes from U937 cells were continuously modulated from 2 to 24h. In UM384 cells, 60% of the genes were transiently modulated at 2h, then returned to control levels at 24h. Moreover, HuR, an AU-rich element-binding protein (ARE-BP), was differentially located in the two cell lines. Therefore, a defect of mRNA stabilization could be responsible for the resistance of UM384 cells to TPA-induced differentiation, suggesting a possible role for the post-transcriptional regulation in the leukemogenesis.

Failure to activate caspase 3 in phorbol ester-resistant leukemia cells is associated with resistance to apoptotic cell death

The protein kinase C (PKC)-specific inhibitor, Ro-31-8220, has been shown to induce anti-proliferation and apoptosis of human cancer cell lines. In the present study, we determined the molecular pathways that lead to apoptosis after treatment of cells with the PKC-specific inhibitor RO-31-8220. For this, we used the U937 human leukemia cell line and a phorbolmyristate acetate (PMA)-resistant derivative cell line, R-U937. Ro-31-8220 treatment of U937 cells leads to apoptosis, which is accompanied by activation of caspase 3 (as measured by decreased levels of the 32kDa inactive form and increased proteolytic cleavage of phospholipase C (PLC)-gamma1). The broad-range caspase inhibitor z-VAD-fmk inhibits this induction of apoptosis, supporting a direct link between caspase activation and Ro-31-8220 induction of apoptosis. This activation of apoptosis is also accompanied by release of cytochrome c, but not by altered expression of Bcl-2 family protein or IAP family proteins. In R-U937 cells, Ro-31-8220 fails to cause release of cytochrome c, activation of caspase 3, or apoptosis. Activation of Akt occurs to a greater extent in the R-U937 cells than the U937 cells and thus might be related to protection from Ro-31-8220-induced apoptosis.

Activation of PKC but not of ERK is required for vitamin E-succinate-induced apoptosis of HL-60 cells

Vitamin E-succinate (VES) induced HL-60 human leukemia cells to undergo apoptosis. Treatment with VES induced membrane translocation of Fas; cleavages of caspase-3, PARP, and lamin B; hypophosphorylation of retinoblastoma protein; and increase of p21(WAF1) protein level. During the induction of apoptosis, activity of PKC was gradually increased with downregulation of VES-induced ERK activity and accompanied by activation of caspase-3. Inhibition of PKC by GF109203X blocked VES-mediated membrane translocation of PKC-alpha and cleavage of caspase-3 cascade, resulting in prevention of VES-induced apoptosis. On the contrary, PKC activation by cotreatment with LPC or thapsigargin and VES synergistically increased VES-mediated apoptosis. However, inhibition of ERK activity by PD98059 showed no significant effect on VES-induced PKC activity and apoptosis. Taken together, our data suggest that VES induces activation of PKC and PKC-dependent hypophosphorylation of retinoblastoma protein, which results in induction of apoptosis, and that VES-induced early activation of ERK and ERK-dependent induction of p21(WAF1) are not required for apoptosis.

Chemopreventive agent resveratrol, a natural product derived from grapes, reversibly inhibits progression through S and G2 phases of the cell cycle in U937 cells

Resveratrol, a natural product derived from grapes, has been shown to prevent carcinogenesis in murine models. We report here that resveratrol induces antiproliferation and arrests the S phase in human histiocytic lymphoma U937 cells. Resveratrol induces arrest in the S phase at low concentrations (30-60 microM), but high concentrations do not induce S phase accumulation in U937 cells. Removal of resveratrol from the culture medium stimulates U937 cells to reenter the cell cycle synchronously, as judged by the expression patterns of cyclin E, A and by fluorescent activated cell sorting analysis. These data demonstrate that resveratrol causes S phase arrest and reversible cell cycle arrest. Thus, resveratrol provides an important new cell cycle blocker as well as a cancer chemopreventive agent.