Characterization of the estrogen-responsive domain of avian liver and cloning of double-stranded cDNA derived from estrogen-inducible RNA species

Estrogen-dependent activation of the avian very low density apolipoprotein II and vitellogenin genes. Transient alterations in mRNA polyadenylation and stability early during induction

Administration of estrogen to egg-laying vertebrates activates unscheduled, hepatic expression of major, egg-yolk protein genes in immature animals and mature males. Two avian yolk protein genes, encoding very low density apolipoprotein II (apoVLDLII) and vitellogenin II, are dormant prior to stimulation with estrogen, but within three days their cognate mRNAs accumulate to become two of the most abundant species in the liver. Accumulation of these mRNAs has been attributed to both induction of transcription and selective, estrogen-dependent mRNA stabilization. We have detected alterations in the size of apoVLDLII mRNA that occur during the first 24 hours that are attributable to a shift in the extent of polyadenylation as steady-state is approached. In vitro transcription assays indicate that primary activation of both genes takes place relatively slowly and that maximal rates of mRNA accumulation occur when the apoVLDLII and vitellogenin II genes are expressed at only 30% and 10% of their fully induced levels, respectively. Transcription data combined with the structural alteration of apoVLDLII mRNA suggest that stability of the two mRNAs may change as steady-state is approached. We have assessed the compatibility of this suggestion with earlier estimates of the kinetics of accumulation of both mRNAs by developing a generally useful algorithm that predicts approach to steady-state kinetics under conditions where both the rate of synthesis and mRNA stability change throughout the accumulation phase of the response. The results predict that the stability of both mRNAs decreases by at least two- to threefold during the approach to steady-state and that, although an additional destabilization of apoVLDLII mRNA may occur following withdrawal of estrogen, the steady-state stability of vitellogenin mRNA is not significantly decreased upon removal of hormone.

Sequential comparative hybridizations analyzed by computerized image processing can identify and quantitate regulated RNAs

A method to analyze shifts in the relative abundance of many specific RNAs following any stimulus is presented. Hybridizations of two complex "total" cDNA probes (from the pre- and poststimulus states) to each member of a cDNA library are quantitatively analyzed and mathematically compared by using computer-assisted image processing and statistical analysis of sequential filter hybridizations. Experiments indicate that shifts in abundance between two states can be identified and reproducibly quantitated without purified probes. Direct isolation of recombinant cDNA colonies containing inserts corresponding to regulated RNAs is thus possible. The use of this system is demonstrated for partial analysis of the response in vivo of rat liver to glucocorticoids. Application to other biological systems in which a shift between two states occurs is discussed.

Estrogen receptor and the development of estrogenic responses in embryonic chick liver

We have examined the development of responsiveness to estrogen by embryonic chick liver with a view to determining common and unique factors involved in the establishment of different genomic responses to th e hormone. The major apoproteins of chick VLDL, apo VLDL-B and apo VLDL-II, both appear to be estrogen inducible at an earlier stage of of embryonic development than is vitellogenin. Apo VLDL-B, but not vitellogenin, exhibits a significant level of hepatic synthesis in the absence of estrogen treatment. This basal synthesis in the absence of estrogen treatment. This basal synthesis is tamoxifen-resistant and is detectable at very early stages of hepatic development, well before estrogen responsiveness is seen. Immunological cross-reactivity, electrophoretic behavior and the results of limited proteolysis mapping suggest that the apo VLDL-B synthesized under basal and estrogen-stimulated conditions is the same (or a very similar) protein. Inducibility of the VLDL apoproteins appears to parallel the appearance of the hepatic estrogen receptor system at days 10-12 while vitellogenin induction is delayed by several days. Cytosol receptor concentration undergoes a gradual increase up to the 19th day of development and thereafter declines. The properties of the 19-day receptor are very similar to those of cytosol receptor in hatched chickens, but the fall in concentration does not appear to be proportionately related to inducibility of estrogenic responses, as measured by the relative rates of synthesis in vitro.