NUCLEAR TRANSLOCATION OF PROTEIN-KINASE-C-ALPHA AND PROTEIN-KINASE-C-ZETA 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 o f 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 s pecific peptide which enabled the evaluation of the whole catalytic ac tivity of both Ca++-dependent and Ca++-independent PKC isoforms. In pa rallel, the subcellular distributions of various PKC isoforms was eval uated by Western blot, immunoprecipitation and in situ immunocytochemi stry 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-alpha and -zeta showed a striking and selective accumulation inside the nucleus upon treatmen t with ATRA. On the other hand, PKC-beta I and -beta II, 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-significa nt variations, also in whole cell homogeneates, upon ATRA treatment, e xcept PKC-delta which showed a progressive down-regulation. Our data s uggest that a selective nuclear translocation of PKC-alpha and -zeta m ight be involved in the process of granulocytic differentiation induce d by ATRA in HL-60 cells.