Reduction of thymosin beta4 and actin in HL60 cells during apoptosis is preceded by a decrease of their mRNAs.

Thymosin beta4 (Tbeta4) is the most prominent representative of the beta-thymosins, a family of highly conserved polar 5 kDa peptides. This peptide is now regarded to be the main G-actin sequestering peptide in mammals and therefore plays an important role in organization of the microfilamental system. During apoptosis of cells this microfilamental system is disrupted. Therefore we studied changes in thymosin beta4 and actin content of HL60 cells after induction of apoptosis using cytosine arabinoside (araC). Thymosin beta4 content decreased to about 30% of the control value after incubation for 48 h in the presence of araC. Also the amount of total actin decreased to about half of the control, while total cellular protein remained constant. To further elucidate if the changes of thymosin beta4 and actin content correlate with the gene expression the relative mRNA content of thymosin beta4 and beta-actin was determined using the ribonuclease protection assay (RPA). Already after 24 h the relative amount of mRNA of thymosin beta4 and beta-actin was greatly reduced to 71 and 58%, respectively. Upon a 48 h araC treatment, the mRNA of these two proteins decreased to 15 and 10% compared to the control, whereas the content of total RNA and protein per cell was nearly unchanged. According to our data araC has a significant influence on the transcriptional level of thymosin beta4 and actin.