Cytochalasin D-Induced Actin Gene Expression in Murine Erythroleukemia Cells

There is a dynamic equilibrium between monomeric G-actin and polymeric F-actin microfilaments (MFs) in eucaryotic cells. We have previously shown that disruption of MFs with cytochalasin D (CD) induced β-actin gene transcription, resulting in elevated levels of β-actin mRNA and protein synthesis. CD also inhibited cell growth by arresting progression through the S phase of the cell cycle. These CD-induced responses were reversible since recovering cells progressed through the G 2 phase and resumed normal growth while β-actin mRNA and protein synthesis rapidly returned to control levels. In the present study, we show that the response of βand γ-actin genes is due to the synthesis of a protein(s) acting at a 5′ regulatory element that may be independent of or require sequences in addition to the serum response element (SRE). CD induces β- and γ-actin mRNA in a dose-dependent manner, reaching a maximum of 20-fold over control mRNA levels at 30 μ M. βand γ-Actin gene expression was also induced 5-fold by serum stimulation of quiescent murine erythroleukemia (MEL) cells, while combined treatment with serum and CD had an additive effect. Two protein synthesis inhibitors, cycloheximide and puromycin, blocked the CD-induced increase in β-actin mRNA, in contrast to the serum-induced increase which is insensitive to inhibitors of protein synthesis. The rapid return of β-actin mRNA to basal levels following CD removal did not require protein synthesis nor did it require progression through the G 2 phase of the cell cycle. A vector containing the 5′ end of the β-actin gene linked to a CAT reporter responded to CD when transfected into MEL cells, localizing the responsive element to the 5′ portion of the β-actin gene. By contrast, a minimal 99-bp actin promoter-CAT construct containing a functional SRE did not respond to CD.