TPA activates p21WAF-1 promoter in human T-cells through its second most upstream Sp1 site

Phosphorylated extracellular signal-regulated protein kinases 1 and 2 phosphorylate Sp1 on serine 59 and regulate cellular senescence via transcription of p21Sdi1/Cip1/Waf1

Expression of p21(Sdi1) downstream of p53 is essential for induction of cellular senescence, although cancer cell senescence can also occur in the p53 null condition. We report herein that senescence-associated phosphorylated extracellular signal-regulated protein kinases 1 and 2 (SA-pErk1/2) enhanced p21(Sdi1) transcription by phosphorylating Sp1 on Ser(59) downstream of protein kinase C (PKC) alpha. Reactive oxygen species (ROS), which was increased in cellular senescence, significantly activated both PKCalpha and PKCbetaI. However, PKCalpha, but not PKCbetaI, regulated ROS generation and cell proliferation in senescent cells along with activation of cdk2, proven by siRNAs. PKCalpha-siRNA also reduced SA-pErk1/2 expression in old human diploid fibroblast cells, accompanied with changes of senescence phenotypes to young cell-like. Regulation of SA-pErk1/2 was also confirmed by using catalytically active PKCalpha and its DN-mutant construct. These findings strongly suggest a new pathway to regulate senescence phenotypes by ROS via Sp1 phosphorylation between PKCalpha and SA-pErk1/2: employing GST-Sp1 mutants and MEK inhibitor analyses, we found that SA-pErk1/2 regulated Sp1 phosphorylation on the Ser(59) residue in vivo, but not threonine, in cellular senescence, which regulated transcription of p21(Sdi1) expression. In summary, PKCalpha, which was activated in senescent cells by ROS strongly activated Erk1/2, and the SA-pErk1/2 in turn phosphorylated Sp1 on Ser(59). Sp1-enhanced transcription of p21(Sdi1) resulted in regulation of cellular senescence in primary human diploid fibroblast cells.

PI 3-K signaling pathway suppresses PMA-induced expression of p21WAF1/Cip1 in human leukemia cells

Despite the understanding of the importance of phosphoinositide 3-kinase (PI 3-K) signaling pathway in the regulation of cellular proliferation, little is known about its role during phorbol 12-myristate 13-acetate (PMA)-induced differentiation in human leukemia cells. Here, we report a novel finding that PI 3-K inhibition by LY294002 significantly increases p21WAF1/Cip1 expression in PMA-stimulated human leukemia cells NB4 and THP1. LY294002 potentiated expression of p21WAF1/Cip1 via a p53-independent mechanism and did not affect mitogen activated protein kinase (MAPK) activation. Electrophoretic mobility shift (EMSA) experiments revealed that blocking of PI 3-K was associated with increased binding of transcription factor Sp1 to the PMA-responsive sites on the p21WAF1/Cip1 promoter. Pretreatment with rapamycin, an inhibitor of mTOR kinase, decreased the expression of p21WAF1/Cip1 protein in PMA-stimulated NB4 cells. The level of PMA-induced p21WAF1/Cip1 protein expression was lower in NB4 cells overexpressing wild type protein kinase C zeta (PKC zeta) compared to those transfected with empty vector or with kinase inactive PKC zeta. Sp1 binding to the p21WAF1/Cip1 promoter was completely lost in a wild type PKC zeta overexpressing and PMA-stimulated NB4 cells. We demonstrate that PI 3-K signaling pathway suppresses PMA-induced expression of p21WAF1/Cip1 in human leukemia cells, and that this effect is partly mediated by PKC zeta.

Inhibitory effects of 12-O-tetradecanoylphorbol-13-acetate alone or in combination with all-trans retinoic acid on the growth of cultured human pancreas cancer cells and pancreas tumor xenografts in immunodeficient mice

Treatment of cultured PANC-1, MIA PaCa-2, and BxPC-3 human pancreatic adenocarcinoma cells with 0.1 to 1.6 nM 12-O-tetradecanoylphorbol-13-acetate (TPA) for 96 h inhibited the proliferation of these cells in a dose-dependent manner, and PANC-1 and MIA PaCa-2 cells were more sensitive to TPA than BxPC-3 cells. Inhibition of proliferation by TPA in PANC-1 cells was associated with an increase in the level of p21, but this was not observed in MIA PaCa-2 or BxPC-3 cells. The TPA-induced increase of p21 in PANC-1 cells was blocked by bisindolylmaleimide or rottlerin (inhibitors of protein kinase C). Studies in NCr-immunodeficient mice with well established PANC-1 tumor xenografts indicated that daily i.p. injections of TPA strongly inhibited tumor growth, increased the percentage of caspase-3-positive cells, and decreased the ratio of mitotic cells to caspase-3-positive cells in the tumors. Studies with BxPC-3 tumors in NCr mice receiving daily i.p. injections of vehicle, TPA, all-trans retinoic acid (ATRA), or a TPA/ATRA combination showed that TPA had an inhibitory effect on tumor growth, but treatment of the animals with the TPA/ATRA combination had a greater inhibitory effect on tumor growth than TPA alone. Treatment with the TPA/ATRA combination resulted in a substantially decreased ratio of the percentage of mitotic cells to the percentage of caspase-3-positive cells in the tumors compared with tumors from the vehicle-treated control animals. The inhibitory effects of TPA on tumor growth occurred at clinically achievable blood levels.

Sp1 and NF-kappaB transcription factor activity in the regulation of the p21 and FasL promoters during promyelocytic leukemia cell monocytic differentiation and its associated apoptosis

Treatment of human acute promyelocytic leukemia cells with phorbol 12-myristate 13-acetate (PMA) results in growth arrest and differentiation toward monocytes, which subsequently die by apoptosis. However, the relationship between terminal differentiation and apoptosis remains unclear. Here we have studied Sp1 and nuclear factor kappaB (NF-kappaB) transcription factor activity in controlling promoters of cell cycle-regulating (p21/WAF1/CIP1) and cell death (FasL) genes during monocytic differentiation and apoptosis of the human acute promyelocytic leukemia cell lines NB4 and HL-60. Using the electrophoretic mobility shift assay, we observed that PMA treatment of NB4 cells caused an early response in Sp1 binding to the p21 and FasL promoters at 8 h. The firmly adherent cell phenotype, characteristic of differentiated cells, retained Sp1-binding activity to either promoter, but it was often lost completely in detached, apoptotic cells. The association of Sp1 with the p21 promoter during monocytic differentiation correlated with the levels of expressed p21 in the cytoplasmic fraction, as detected by immunoblotting. In HL-60 cells, very weak or no Sp1 binding to either promoter was observed. Low NF-kappaB affinity for its consensus sites and to the FasL promoter was characteristic of apoptotic cells. The results of this study suggest a positive role of Sp1 and NF-kappaB, as regulators of p21 and FasL genes, in leukemic cell survival and monocytic differentiation and a negative role in apoptotic cell death.

Integrin beta1 subunit overexpressed in the SMMC-7721 cells regulates the promoter activity of p21(CIP1) and enhances its transcription

Evidence has been emerging to suggest that integrin could induce growth inhibition in some cell types. Some of the molecular mechanisms underlying growth arrest have been elucidated. We reported here that overexpression of integrin beta1 imposed a growth inhibitory effect on the hepatocellular carcinoma cell line SMMC-7721, and this phenomenon was mainly attributed to the cyclin-dependent kinase inhibitor p21(CIP1). Furthermore, our findings suggested that transcription activity of the p21(CIP1) gene could be upregulated in the integrin beta1-overexpressing cells, and possibly controlled by the cis-elements in the core region of the p21(CIP1) promoter.