Differential expression of protein kinase C isozymes and small GTP-binding proteins during HL60 cell differentiation by retinoic acid and cyclic AMP: relation with phospholipase D (PLD) activation.

The differential expression of protein kinase C (PKC) isozymes and small GTP-binding proteins, and their relation to O2 generation and phospholipase D (PLD) activation were analyzed during the differentiation of human promyelocytic HL60 cells to neutrophil-like cells induced by either retinoic acid (RA) or dibutyryl cyclic AMP (dbcAMP). In response to either one of the inducers, nitroblue tetrazolium (NBT) reduction activity time-dependently increased. Although PLD activity was upregulated by dbcAMP-treatment, only a slight increase was observed in RA-treated cells. Small GTP-binding proteins Rac1, Rap1, and RhoA, which are reported to be implicated in O2- generation or PLD activation, were already expressed in undifferentiated HL60 cells and their significant changes were not detected during differentiation. The mRNAs of the cytosolic components of NADPH oxidase system, p47phox and p67phox, were present in trace amounts in undifferentiated cells. However, they rapidly increased in response to RA or dbcAMP. In response to either RA or dbcAMP, the increases were observed in cPKC isozymes (alpha, beta I, beta II) but not in other subtypes (delta, epsilon, theta, zeta) by both RT-PCR and Western blot analyses. In dbcAMP-treated cells PKC alpha increased remarkably, whereas PKC beta I and beta II mainly elevated in RA-treated cells. These results suggest the possibility that cPKCs are closely related to cell differentiation and that PKC alpha is involved in PLD activation.