Nuclear translocation of protein kinase C-α and -ζ isoforms in HL-60 cells induced to differentiate along the granulocytic lineage by all-trans retinoic acid

We investigated whether members of the protein kinase C (PKC) family of enzymes were involved in the nuclear events underlying granulocytic differentiation induced by 10^-6 M all-trans retinoic acid (ATRA) in HL-60 cells. PKC activity was analysed by using a serine substituted specific peptide which enabled the evaluation of the whole catalytic activity of both Ca⁺⁺-dependent and Ca⁺⁺-independent PKC isoforms. In parallel, the subcellular distribution of various PKC isoforms was evaluated by Western blot, immunoprecipitation and in situ immunocytochemistry analyses. The level of PKC catalytic activity in the nuclei of HL-60 cells significantly (P <0.01) and progressively increased from 1 h of ATRA treatment onwards. Consistently, PKC-α and -ζ showed a striking and selective accumulation inside the nucleus upon treatment with ATRA. On the other hand, PKC-βI and -βII, the only two other isoforms present at nuclear level, did not show any significant modification upon ATRA treatment. The remaining PKC isoforms were not detectable inside the nucleus and showed only modest and non-significant variations, also in whole cell homogenates, upon ATRA treatment, except PKC-δ which showed a progressive down-regulation. Our data suggest that a selective nuclear translocalion of PKC-α and -ζ might be involved in the process of granulocytic differentiation induced by ATRA in HL-60 cells.