Coordinated expression of five tropomyosin isoforms and β‐actin in astrocytes treated with dibutyryl cAMP and cytochalasin D

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Abstract
Cytochalasin D and dBcAMP cause cultured astrocytes to change from flat cells to retrated process‐bearing cells. F‐actin was present throughout cells stimulated with dBcAMP for 16 h, whereas cytochalasin D caused F‐actin to form massive aggregates at the tips of the cell processes. The two drugs differently regulated the expression of both β‐actin and tropomyosin genes in astrocytes cultured in the presence or absence of serum: dBcAMP caused down‐regulation and cytochalasin D caused up‐regulation. Northern blot analyses indicated that: (1) serum deprivation halved the concentration of all tropomyosin transcripts (TM‐1, TM‐2, TM‐4, TMBr‐1, TMBr‐2). Serum induced TM‐4 via transcriptional activation, independent of protein synthesis, (2) dBcAMP induced down‐regulation of β‐actin (–50%) and tropomyosin transcripts (–35 to 52%) even in the presence of serum. The concentration of profilin mRNA decreased in dBcAMP‐reactive astrocytes (‐46%). The decrease in β‐actin mRNA concentration was not blocked by cycloheximide, whereas down‐regulation of tropomyosin transcripts was completely reversed when protein synthesis was inhibited, and (3) cytochalasin D induced an increase in the concentration of tropomyosin transcripts (+ 69 to 185%) which was cumulative with serum stimulation. Cytochalasin D induction of both β‐actin and TM‐4 operated through transcriptional activation, independent of protein synthesis.The production of all tropomyosin transcripts examined here were strictly coordinated with β‐actin expression in serum‐, dBcAMP‐ and cytochalasin D‐treated astrocytes. This indicates that the differential expression of tropomyosin isoforms occurring during astrocyte maturation is due to more complex regulation than that involved in serum‐ or cAMP‐stimulated astrocytes.

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