Retinoic acid-mediated decrease of G (alpha S) protein expression: involvement of G (alpha S) in the differentiation...

The amount of the heterotrimeric G protein subunit G (alpha S) decreases after the induction of human myeloblastic leukemia HL-60 cells to become granulocyte-like cells in the presence of retinoic acid (RA). Compared to untreated control cells, HL-60 cells expressed decreased levels of G (alpha S) protein and mRNA levels after addition of RA to the cultures as shown by immunoblot and Northern blot analysis. The reduction of the G (alpha S) protein in HL-60 cells by antisense RNA expression was associated with (i) decreased cell doubling time; (ii) induction of a granulocyte-like phenotype; (iii) and expression of a surface marker characteristic of myeloid differentiation. Expression of a constitutively active mutant G (alpha S) (Q227L) in HL-60 cells blocked RA-induced differentiation. In contrast, treatment with forskolin, prostaglandin E2, or 8-bromo-cyclic AMP, which increase intracellular cyclic AMP (cAMP) levels, did not inhibit the RA-mediated differentiation process. No changes in cAMP levels occurred in response to RA. The present study provides insights into the involvement of G (alpha S) protein in growth regulation during differentiation of the human myeloid cell line HL-60. These data suggest that in HL-60 human myeloid cells RA-mediated decrease of G (alpha S) plays a critical role in the regulation of differentiation which is independent of intracellular cAMP.