Specific stimulation of actin gene transcription by epidermal growth factor and cycloheximide.

Abstract

Stimulation of quiescent AKR-2B mouse embryo cells with epidermal growth factor (EGF) results in a rapid and specific induction of actin mRNA sequences. These mRNAs include those coding for both P-and y-cytoskeletal, but not a-skeletal muscle, actin isotypes. Elongation of nascent RNA chains in isolated nuclei (run-off transcription) demonstrates that the mRNA accumulation is preceded by an increase in actin gene transcription. This increase is transient, however, and is followed by a rapid attenuation of transcriptional activity. An inhibitor of protein synthesis, cycloheximide, was also found to induce 18-and r-actin mRNA accumulation. Furthermore, the simultaneous addition of EGF and cycloheximide produced a synergistic effect on actin sequences in both steady-state nuclear and polysomal RNA. Run-off transcription experiments demonstrate that this synergistic effect results from an increase in the magnitude and duration of actin gene transcription. It is also specific in that a-tubulin gene transcription is not similarly affected. These data suggest the existence of a specific labile repressor of actin gene transcription.The binding of epidermal growth factor (EGF) to specific receptors in the membranes of quiescent cells initiates a variety of biochemical events that can culminate in the initiation of DNA synthesis and cell division (reviewed in ref. 1). Inhibitors of RNA synthesis can block this process, implying that the transcription of certain genes is required for a quiescent cell to reenter the cell cycle (2). The molecular mechanisms that regulate these genes are poorly understood but do not appear to involve the nuclear translocation of the hormone-receptor complex. Indeed, the complex itself is rapidly internalized and degraded several hours prior to the initiation of DNA synthesis (3,4). This observation has led to speculation that a second messenger(s) of hormone action must be responsible for initiating the expression of specific genes required for cell proliferation (1).A major constraint on any proposed mechanism of EGF or second messenger action is the requirement for a high degree of specificity. This consideration arises from several studies that have shown that peptide growth factors regulate a very limited domain of specific genes (5-8). This specificity could be achieved by a specific DNA binding protein acting either as a positive or negative regulator of gene transcription. Although theoretical constraints have been imposed on specific protein-DNA interactions in the context of a mammalian nucleus, these constraints are not absolute and a variety of compensating strategies are available to a eukaryotic cell (9). Therefore, it may be noteworthy that recent studies have shown that inhibitors of protein synthesis can potentiate the induction of specific mRNA sequences by platelet-derived growth factor (7, 10). Although this observation is consistent with a protein repressor of growth-factor-regulated genes, a direct effect on gene transcription rates was not shown.Prompted by a rep...

Cross-binding of factors to functionally different promoter elements in c-fos and skeletal actin genes.

“…Furthermore, this induction is prevented by a mutation in the MRE. It has been reported that the endogenous muscle actin gene is not induced by a serum growth factor (11). The discrepancy between this previous report and these data may result from differences in expression that result from the episomal rather than chromosomal location of the muscle gene, from species differences, or because a sensitive RNase protection assay was employed in these experiments to measure transcript levels.…”

Expression of a set of growth-related immediate early genes in BALB/c 3T3 cells: coordinate regulation with c-fos or c-myc.

“…By 3-4 hr the entire set ofgenes was nearly inactive again. This response is similar to that of the c-fos and actin genes (9,31). The response of gene 3CH482 (like the kinetics of RNA accumulation) resembles that of the c-myc gene (see Fig.…”

Gene expression in regenerating liver in relation to cell proliferation and stress

“…Accumulation of actin mRNA in regenerating liver occurs as soon as 1 h after partial hepatectomy and could be related to the G0/G1 transition, as described in AKR-2B mouse embryonic cells [21]. The late phase (included in the 3 -24-h post-hepatectomy period) of actin, c-myc and topoisomerase-I mRNA accumulation is on the contrary clearly related to the hepatocyte proliferation, since it is not observed after a sham operation or after Freund's adjuvant injection.…”