Resistance to phorbol 12-myristate 13-acetate-induced cell growth arrest in an HL60 cell line chronically exposed to a glutathione S-transferase pi inhibitor.

Glutathione S-transferase pi (GSTpi; EC 2.5.1.18) has been shown recently to be a regulator of mitogen-activated protein kinases (MAPK). We have developed, by chronic exposure of HL60 cells to increasing concentrations of a peptidomimetic GSTpi inhibitor TLK199, a 10-fold resistant cell line (HL60/TLK199). Among the cellular adaptations observed in this cell line was an increase in extracellular signal-regulated kinase (ERK) activity without modification of basal expression levels. Phorbol 12-myristate 13-acetate (PMA) induced monocyte/macrophage cytodifferentiation in both HL60 wild-type (WT) and HL60/TLK199 cells. In contrast, PMA induced a pronounced cell growth inhibition and G(0)/G(1) cell cycle arrest in HL60 WT cells, while this differentiating agent had only a mild effect on cell growth without G(0)/G(1) cell cycle arrest in HL60/TLK199. This effect was associated with a rapid and sustained activation of ERK (up to 6hr) in HL60 WT cells but only a transient induction of these kinases (between 30 and 60min) in HL60/TLK199. Furthermore, treatment of both cell lines with PMA in combination with the protein tyrosine phosphatase inhibitors sodium orthovanadate (OV) or 3,4-dephostatin (DPN) circumvented the resistance to cell growth arrest and potentiated differentiation in HL60/TLK199 but had no effect on HL60 WT cells. The circumvention of the resistance to PMA was associated with a sustained activation of ERK. These data suggest that chronic exposure of HL60 cells to TLK199 alters cellular ERK activation by PMA, which may contribute to the differential response of the WT and resistant cells to PMA.