The effects of cholera toxin (CT) on transforming growth factor β1-stimulated protooncogene expression, fr-^JGTP binding, GTPase activity and growth under anchorage-independent and -dependent conditions were studied in AKR-2B fibroblast cells. CT was shown to inhibit TGF01-stimulated c-sis and c-myc mRNA expression. Actinomycin D decay and nuclear runon experiments demonstrated that this inhibition was not due to an increased decay of protooncogene message, but to a decreased transcriptional activation. These inhibitory effects were not secondary to changes in the ability of TGF01 to bind to its receptors) since radioreceptor assays and affinity labeling studies demonstrated that CT had no effect on TGF01 binding. ADP ribosylation of AKR-2B plasma membranes with [x-32P]NAD+ revealed a Mt 45,000 protein as the major CT substrate. The labeling of this Mt 45,000 protein in membranes could be inhibited by prior pretreatment of the cells with increasing concentrations of CT. Treatment of membranes with nanogram concentrations of CT abolished the increase in fr-^jGTP binding following addition of TGF01 as well as decreased basal binding. Similarly, CT pretreatment of membranes inhibited TGF01-stimulated GTPase activity. Unexpectedly however, the stimulatory effects of TGF01 on anchorage-independent growth in soft agar were unaffected by CT. Only pertussis toxin was able to inhibit TGF01 -induced colony formation in soft agar in a dose-dependent manner. Furthermore, differential effects of both CT and pertussis toxin were observed on TGF01-stimulated monolayer growth; CT was inhibitory, whereas pertussis toxin was without effect. These results suggest that the diverse biological effects of TGF01 are mediated through multiple intracellular pathways distinguishable by their toxin sensitivities.
|Original language||English (US)|
|Number of pages||8|
|State||Published - Nov 1 1989|
ASJC Scopus subject areas
- Cancer Research