Differential effects of dibutyryl cyclic adenosine monophosphate and retinoic acid on the growth, differentiation, and cyclic adenosine monophosphate-binding protein of murine neuroblastoma cells.

Dibutyryl cyclic adenosine 3':5'-monophosphate (Bt2cAMP) and beta-all-trans retinoic acid (RA) have been shown separately, and in some cases in combination, to modulate the growth, differentiation, and cAMP-dependent protein kinase (PK-A) activity of various tumor cells. The effects of Bt2cAMP and RA on a cholinergic clone (S20) of C1300 mouse neuroblastoma cells were explored in the present study. Treatment of these cells with 1 mM Bt2cAMP for 3 or more days resulted in 93% inhibition of cell proliferation in monolayer cultures and in 98% inhibition of colony formation in semisolid medium (0.5% agarose). In contrast, treatment of the cells with 1 or 10 microM RA had no inhibitory effects on cell proliferation in monolayer cultures but enhanced colony formation in agarose by up to 130%. The growth of cells treated with a combination of Bt2cAMP and RA was inhibited, although less so than with Bt2cAMP alone. Cells treated with Bt2cAMP alone or Bt2cAMP and RA extended long, neurite-like, cellular processes indicative of differentiation, whereas only a few untreated or RA-treated cells produced such extensions. The amount of [3H]cAMP-binding protein increased gradually up to 2-fold during a 3-day treatment with Bt2cAMP; in contrast it decreased by nearly 2-fold during RA treatment. These changes occurred in the level of the type I regulatory subunit (RI) of PK-A as determined by photoaffinity labeling with 8-azidoadenosine cyclic 3':5'-[32P]monophosphate. The increase in RI following Bt2cAMP treatment was corroborated by DEAE-cellulose chromatography. This analysis also demonstrated that type I PK-A is the predominant kinase in the untreated S20 cells and that RI exists as a free subunit in Bt2cAMP-treated cells. The activity of PK-A decreased by about 20% following treatment with either Bt2cAMP or RA and by 45% following treatment with a combination of both agents. These results suggest that the distinct effects of Bt2cAMP and RA on the anchorage-independent growth of S20 cells may be related to their opposite effects on the level of RI.