The A and J series of prostaglandins (PGs) accumulate in the nuclei to suppress the proliferation of cancer cells. Here we report that Δ7-PGA1 methyl ester, a synthetic anti-cancer PG, increased the level of mRNA for the cyclin-dependent kinase inhibitor p21 in human leukaemia HL-60 cells. The induction of p21 was associated with the accumulation of hypophosphorylated retinoblastoma protein (pRB) and the suppression of c-myc gene expression. Since the p53 gene is deleted in HL-60 cells, the anti-cancer PG is suggested to inhibit cancer cell growth by inducing p21 via a p53-independent pathway. Unlike HL-60 cells, cisplatin-resistant HL-60/R-CP cells were insensitive to Δ7-PGA1 methyl ester. While c-myc expression was transiently suppressed, neither G1 arrest nor hypophosphorylation of pRB was observed with the anti-cancer PG. Plasma membrane vesicles from HL-60/R-CP cells showed an enhanced level of GS-X pump (ATP-dependent glutathione S-conjugate export pump) activity towards the glutathione S-conjugate of Δ7-PGA1 methyl ester (Km 110 nM). GIF-0019 {N-carbomethoxy-S-[5-(4-benzoylphenyl)pentyl]glutathione dimethyl ester}, a specific inhibitor of the GS-X pump, dose-dependently enhanced the cellular sensitivity of HL-60/R-CP cells to Δ7-PGA1 methyl ester and induced G1 arrest. The GS-X pump is suggested to play a pivotal role in modulating the biological action of the anti-cancer PG.

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