Increase In C-fos mRNA Levels Research Articles

Transcriptional and posttranscriptional control of c-fos gene expression in human monocytes.

We examined the mechanisms that are responsible for the regulation of c-fos gene expression in human monocytes. Levels of c-fos mRNA were low or undetectable in resting monocytes. Results of run-on transcription assays, however, demonstrated that both the first two and last two exons of the c-fos gene were transcribed at similar rates, and that only the sense strand of this gene was transcribed. These findings suggest that the level of c-fos transcripts in resting human monocytes is controlled at a posttranscriptional level. Activation of resting monocytes with phorbol ester was associated with a rapid and transient increase in c-fos mRNA levels. This increase in c-fos transcripts was related to an enhanced rate of c-fos transcription. Moreover, exposure of resting monocytes to inhibitors of protein synthesis induced (i) a rapid and marked (300-fold) increase in c-fos mRNA levels, despite only a 9-fold increase in c-fos transcription, and (ii) a prolongation of the half-life of c-fos mRNA. Thus, while posttranscriptional control was responsible for the down-regulation of c-fos transcripts in both resting and activated human monocytes, transcriptional mechanisms were responsible for the transient increase in c-fos expression induced by phorbol ester. Furthermore, the marked increases in c-fos mRNA associated with inhibition of protein synthesis were regulated by both transcriptional and posttranscriptional mechanisms. These findings may be related to recent observations which indicate that both positive and negative factors transcriptionally regulate c-fos gene expression and that sequences found in the 3'-untranslated region of the c-fos mRNA are responsible for the stability of this transcript.

Arachidonic acid and cyclic adenosine monophosphate stimulation of c-fos expression by a pathway independent of phorbol ester-sensitive protein kinase C.

The stimulation of cell proliferation by platelet-derived and other growth factors is associated with a rapid increase in the expression of the c-fos protooncogene. We and others have shown that phosphosphoinositide turnover and protein kinase C play a role in the activation of this gene by growth factors, but that a second, kinase C-independent pathway(s) exist. Because cAMP potentiates the actions of a number of growth factors and is elevated in platelet-derived growth factor-stimulated Swiss 3T3 cells, we examined the ability of cAMP to stimulate c-fos expression in this cell type. Forskolin, a direct activator of adenylate cyclase, elicited marked increases in c-fos mRNA levels. Receptor-mediated activation of adenylate cyclase by prostaglandin E1 and stimulation with the cAMP analog 8-bromo-cAMP also enhanced c-fos expression. In cells made protein-kinase C deficient, c-fos induction by phorbol ester was abolished; by contrast, c-fos was still induced by cAMP-elevating agents in protein kinase C-depleted cells. Platelet-derived growth factor causes cAMP accumulation by stimulating arachidonic acid release and the formation of prostaglandins capable of activating adenylate cyclase. The addition of arachidonic acid and the arachidonate metabolite prostaglandin E2 to Swiss 3T3 cultures stimulated c-fos expression. These data suggest the existence of a pathway from growth factor receptor to gene induction that is mediated by cAMP and does not depend on a phorbol ester-sensitive protein kinase C.

Changes in gene expression associated with induced differentiation of erythroleukemia: protooncogenes, globin genes, and cell division.

Hexamethylenebisacetamide (HMBA)-induced differentiation of murine erythroleukemia cells (MELC) is a multistep process involving an early latent period during which a number of metabolic changes have been detected, but the cells are not yet committed irreversibly to differentiate. Commitment is defined as the capacity of MELC to go on to express the program of terminal cell division and gene expression (such as the accumulation of globin mRNA) upon removal of the HMBA from the culture. In the presence of HMBA, a small proportion of MELC are committed by 10-12 hr and greater than 90% by 48-60 hr. The present study shows that, during the initial 4 hr of culture, HMBA causes a marked decrease in c-myb and c-myc and an increase in c-fos mRNA levels. With continued culture, the decrease in c-myb and the increase in c-fos mRNA persists, while c-myc mRNA returns to control levels before the time that MELC begin to show irreversible differentiation. Dexamethasone, which blocks expression of HMBA-induced MELC differentiation, does not alter the early pattern of changes in protooncogene mRNA nor the sustained elevation of c-fos, but it does inhibit the continued suppression of c-myb allowing c-myb to return toward control levels. Hemin, which induces MELC to accumulate globins but does not initiate commitment to terminal cell division, does not alter these protooncogene mRNA levels. These studies suggest that, although the early decrease in c-myb and c-myc and increase in c-fos mRNAs may be involved in the multistep events leading to differentiation, the continued suppression of c-myb is critical for HMBA-induced MELC commitment to terminal cell division.