Role of p53 in aziridinylbenzoquinone-induced p21waf1 expression

In Vitro Anti-Neuroblastoma Activity of Thymoquinone Against Neuro-2a Cells via Cell-cycle Arrest

We have recently shown that thymoquinone (TQ) has a potent cytotoxic effect and induces apoptosis via caspase-3 activation with down-regulation of XIAP in mouse neuroblastoma (Neuro-2a) cells. Interestingly, our results showed that TQ was significantly more cytotoxic towards Neuro-2a cells when compared with primary normal neuronal cells. In this study, the effects of TQ on cell-cycle regulation and the mechanisms that contribute to this effect were investigated using Neuro-2a cells. Cell-cycle analysis performed by flow cytometry revealed cell-cycle arrest at G2/M phase and a significant increase in the accumulation of TQ-treated cells at sub-G1 phase, indicating induction of apoptosis by the compound. Moreover, TQ increased the expression of p53, p21 mRNA and protein levels, whereas it decreased the protein expression of PCNA, cyclin B1 and Cdc2 in a dose- dependent manner. Our finding suggests that TQ could suppress cell growth and cell survival via arresting the cell-cycle in the G2/M phase and inducing apoptosis of neuroblastoma cells.

Epidermal growth factor receptor is a common mediator of quinone-induced signaling leading to phosphorylation of connexin-43: role of glutathione and tyrosine phosphatases

Rat liver epithelial cells were exposed to three quinones with different properties: menadione (2-methyl-1,4-naphthoquinone, vitamin K3), an alkylating as well as redox-cycling quinone, the strongly alkylating p-benzoquinone (BQ), and the non-arylating redox-cycler, 2,3-dimethoxy-1,4-naphthoquinone (DMNQ). All three quinones induced the activation of extracellular signal-regulated kinase (ERK) 1 and ERK 2 via the activation of epidermal growth factor receptor (EGFR) and MAPK/ERK kinases (MEK) 1/2. ERK activation resulted in phosphorylation at Ser-279 and Ser-282 of the gap junctional protein, connexin-43, known to result in the loss of gap junctional intercellular communication. Another EGFR-dependent pathway was stimulated, leading to the activation of the antiapoptotic kinase Akt via phosphoinositide 3-kinase. The activation of EGFR-dependent signaling by these quinones was by different mechanisms: (i) menadione, but not BQ or DMNQ, inhibited a protein-tyrosine phosphatase regulating the EGFR, as concluded from an EGFR dephosphorylation assay; (ii) although menadione-induced activation of ERK was unimpaired by pretreatment of cells with N-acetyl cysteine, activation by BQ and DMNQ was prevented; (iii) cellular glutathione (GSH) levels were strongly depleted by BQ. The mere depletion of GSH by application of diethyl maleate EGFR-dependently activated ERK and Akt, thus mimicking BQ effects. GSH levels were only moderately decreased by menadione and not affected by DMNQ. In summary, EGFR-dependent signaling was mediated by protein-tyrosine phosphatase inactivation (menadione), GSH depletion (BQ), and redox-cycling (DMNQ), funneling into the same signaling pathway.

Transcriptional activation of p21WAF1 by PTEN/MMAC1 tumor suppressor

The recently discovered tumor suppressor gene PTEN has been found mutated in many types of advanced tumors. When introduced into tumor cells that lack the wild-type allele of the gene, PTEN was able to suppress the growth of these cells. Here, we have analyzed how PTEN might alter cell cycle-regulatory controls to achieve this growth-inhibitory effect. We found that overexpression of PTEN stimulates the synthesis of three inhibitors of cyclin-dependent kinases, p21WAF1, p27KIP1, and p57KIP2. This effect is very specific, as the expression of other components of the cell cycle engine, various cyclins and cyclin-dependent kinases, is not affected. For p21WAF1 we show that this induction is due to the p53-independent transcriptional activation of its promoter. In addition, increased expression of PTEN rendered the cells more sensitive to apoptotic cell death. Therefore, our data suggest a two-fold mechanism of growth inhibition by PTEN: one that acts via the increased expression of CKIs such as p21WAF1, and another that augments the cellular propensity for apoptotic cell death.