IN-VITRO CHARACTERIZATION OF AN ESTROGEN-REGULATED MESSENGER-RNA STABILIZING ACTIVITY IN THE AVIAN LIVER

Estrogen-mediated accumulation of the avian apolipoprotein (apo)II mRN A is in part due to its stabilization. To identify the biochemical act ivity responsible for this effect, radiolabeled, capped, and polyadeny lated apoII mRNA was incubated in vitro in liver cytosolic extracts fr om roosters who received either estrogen (estrogen-treated extract) or the vehicle (control extract) parenterally. The mRNA was very stable in estrogen-treated extract but was rapidly degraded in control extrac t. The RNA was degraded predominantly by endonuclease rather than exon uclease activity. The addition of the estrogen-treated extract to the control extract prevented the degradation of the mRNA in trans. This b iochemical activity was heat labile and was also destroyed by proteina se K but not by micrococcal nuclease, indicating that estrogen treatme nt resulted in the expression of a protein in the liver that stabilize d the apoII mRNA by inhibiting its nucleolytic degradation. This mRNA stabilization factor was labile around 60 degrees C, whereas the RNase remained stable up to 80 degrees C. Studies on mRNA protein interacti on showed that both control and estrogen-treated extracts contain mRNA -binding (mRNP) proteins that bind apoII mRNA. An increased binding to apoII mRNA by a subset of these proteins was observed with estrogen-t reated extract as compared with the control extract. This activity, al though it afforded complete protection from nucleolytic degradation to apoII and apo A1 mRNAs, appeared to provide less protection to mRNAs encoding chicken serum albumin and vitellogenin, suggesting differenti al stabilization of mRNAs. These studies indicate that a cytosolic mRN A-stabilization factor, providing apoII mRNA complete protection from nucleolytic degradation, is expressed in the avian liver upon estrogen treatment. This appears to be the first time that a biochemical activ ity responsible for hormone-mediated stabilization of mRNAs and estrog en induction of mRNA binding by specific mRNPs have been identified an d partially characterized in vitro.