Protein Secretion by Mesometrial and Antimesometrial Rat Decidual Tissue: Evidence for Differential Gene Expression*

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Abstract
Our recent finding that decidual luteotropin, a PRL-like horomone secreted by the rat decidua, is found primarily in the antimesometrial cells suggests strongly that the synthesis of this hormone may well be an important function of the antimesometrial tissue. The objective of this investigation was, therefore, 1) to determine whether antimesometrial tissue expresses mRNA for and actively secretes a protein (s) with PRL-like activity, and 2) to examine the pattern of protein production by the antimesometrial and mesometrial zones throughout decidual development. RNA obtained from decidual tissue of day 8 pseudopregnant rats was translated in a cell-free system. The translated products were subjected to PRL receptor affinity chromatography in the presence or absence of ovine PRL to assess binding specificity. The eluted 35S-labeled proteins were analyzed by both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting. A major 28K protein bound specifically to and was eluted from PRL receptor-enriched luteal membranes. This protein also cross-reacted with antibodies to human PRL. To determine where the mRNA for this 28K protein is expressed and whether this protein represents a prohormone, RNA isolated from both antimesometrial and mesometrial tissue was translated in the presence or absence of microsomal membranes. The 28K protein was synthesized specifically by RNA isolated from the antimesometrial zone. No apparent change in the relative mol wt of the 28K protein was observed when translation was performed in the presence of microsomal membranes. To determine whether this protein is a secreted product and to investigate the pattern of protein secretion by the mesometrial and antimesometrial decidua, tissue explants obtained from both zones between days 9-13 of pseudopregnancy were cultured in the presence of [35S]methionine. Antimesometrial tissue secreted one major 28K protein which was capable of binding to PRL receptors on luteal membranes and was immunoprecipitated by antibodies to human PRL, whereas the mesometrial tissue primarily secreted an approximately 180K protein. The overall pattern of protein synthesis and release not only differed between the mesometrial and antimesometrial tissue but also differed with each day of pseudopregnancy. The secretion of several proteins decreased with advancing gestational age, while the secretion of other proteins began abruptly after day 11, coincident with regression of the antimesometrial tissue. In summary, results of this investigation have established that rat decidual tissue synthesizes and selectively secretes proteins, and the specificity and rate of production of these distinct proteins depend on the spatial differentation and gestational age of the decidual tissue. One of the important findings of this study is that mesometrial and antimesometrial tissue each secrete a major protein which may serve as a marker for each tissue type. Whereas the role for the 180K mesometrial protein remains to be defined, the 28K protein, whose mRNA is localized in the antimesometrial tissue possesses both the immunological and biological activity of PRL.