REGULATION OF RIBOSOMAL DNA-TRANSCRIPTION BY INSULIN

The experiments reported here used 3T6-Swiss albino mouse fibroblasts and H4-II-E-C3 rat hepatoma cells as model systems to examine the mech anism(s) through which insulin regulates rDNA transcription. Serum sta rvation of 3T6 cells for 72 h resulted in a marked reduction in rDNA t ranscription. Treatment of serum-deprived cells with insulin was suffi cient to restore rDNA transcription to control values. In addition, tr eatment of exponentially growing H4-II-E-C3 with insulin stimulated rD NA transcription. However, for both cell types, the stimulation of rDN A transcription in response to insulin was not associated with a chang e in the cellular content of RNA polymerase I. Thus we conclude that i nsulin must cause alterations in formation of the active RNA polymeras e I initiation complex and/or the activities of auxiliary rDNA transcr iption factors. In support of this conclusion, insulin treatment of bo th cell types was found to increase the nuclear content of upstream bi nding factor (UBF) and RNA polymerase I-associated factor 53. Both of these factors are thought to be involved in recruitment of RNA polymer ase I to the rDNA promoter. Nuclear run-on experiments demonstrated th at the increase in cellular content of UBF was due to elevated transcr iption of the UBF gene. In addition, overexpression of UBF was suffici ent to directly stimulate rDNA transcription from a reporter construct . The results demonstrate that insulin is capable of stimulating rDNA transcription in both 3T6 and H4-II-E-C3 cells, at least in part by in creasing the cellular content of components required for assembly of R NA polymerase I into an active complex.