Regulation of expression of the chicken ovalbumin gene: Interactions between steroid hormones and second messenger systems

The chicken ovalbumin gene is subject to multihormonal regulation. Maximal expression of it requires not only the synergistic effects of estrogen and corticosterone, but also the permissive effects of insulin. In addition to effects on transcription, the stability of its message is greatly enhanced by estrogen. Furthermore, two signal transduction pathways involving protein kinases have been implicated in the regulation of the ovalbumin gene. To better define the role of second messengers on expression of the ovalbumin gene, the effects of the protein kinase-C (PKC) and the cAMP-dependent protein kinase (PKA) pathways on the endogenous levels of ovalbumin mRNA and the transcription of an ovalbumin fusion gene were investigated. Primary cultures of oviduct cells were treated with phorbol 12-myristilate 13-acetate (an activator of PKC) or with forskolin and 3-isobutyl-1-methylxanthine (an activator of PKA) alone, activators plus estrogen and corticosterone, or activators plus both steroids and insulin. The results indicate that phorbol 12-myristilate 13-acetate causes a dramatic destabilization of ovalbumin message, resulting in a reduction in ovalbumin mRNA levels. In contrast, the activators of the PKA system can substitute for insulin and, thereby, increase expression of the ovalbumin gene synergistically with the steroids. The effect of the activators of the PKA system is at the level of transcription. Thus, in chicken oviduct cell cultures, the PKA and PKC signal transduction pathways act in opposing ways to modulate the steroid-induced expression of the ovalbumin gene.