Hexamethylenebisacetamide-induced erythroleukemia cell differentiation involves modulation of events required for cell cycle progression through G₁

Hexamethylenebisacetamide (HMBA), a potent inducer of differentiation of transformed cells such as murine erythroleukemia cells, causes a prolongation of the G₁ phase of the cell cycle during which commitment to terminal differentiation is first detected. Removal of HMBA prior to the G₁ phase aborts commitment. To further define the relationship between the G₁ phase and commitment to differentiation, we used two inhibitors of cell cycle progression: aphidicolin, which blocks cells at the G₁/S interphase, and deferoxamine, which blocks cells at an earlier stage during G₁. HMBA-induced prolongation of G₁ is associated with the accumulation of underphosphorylated retinoblastoma protein, decrease in cyclin A protein levels, and commitment to differentiation. G₁ arrest of murine erythroleukemia cells induced by aphidicolin or deferoxamine is not associated with accumulation of underphosphorylated retinoblastoma protein, suppression of cyclin A protein, or commitment of cells to terminal differentiation. Neither of the cell cycle inhibitors alters the effect of HMBA in inducing the G₁-associated changes or commitment to differentiation. Taken together, the present findings indicate that the site of action of HMBA which leads to commitment is in a stage of the G₁ phase prior to the point of cell cycle block caused by deferoxamine or aphidicolin. HMBA appears to cause cell differentiation with suppression of cell cycle progression by an action that affects events required for cell progression through G₁, including accumulation of underphosphorylated retinoblastoma protein and changes in regulation of cyclin levels.