We have reported previously that 12-O-tetradecanoylphorbol-13-acetate (TPA) disrupts the morphology and functional development of the rat embryonic visceral yolk sac (VYS) maintained in a whole-embryo culture system. The TPA-mediated disruption of the VYS is characterized by the abnormal progressive separation of the cellular layers that comprise the VYS and appears to be related to late-stage promotion. The present study further characterizes this effect of TPA on the VYS of rat conceptuses in vitro.

VYS ornithine decarboxylase levels were not induced but rather were initially depressed by TPA treatment. There was no major effect of TPA treatment on VYS hemoglobin content, as measured by absorbance at 414 nm and polyacrylamide gel electrophoresis. Changes in VYS hemoglobin synthesis during the culture period, measured by [14C]leucine incorporation with subsequent autoradiography, was likewise not a major effect of TPA treatment. VYS DNA synthesis and VYS RNA synthesis, measured by [3H]thymidine and [3H]uridine incorporation, respectively, were unchanged by TPA treatment. VYS protein synthesis, measured by [3H]leucine incorporation, was initially increased by TPA treatment but returned to control values by the end of the culture period. This increase in [3H]leucine incorporation was not due to a TPA-mediated change in the secretory function of the VYS.

The data suggest that the tumor promoter-induced disruption of the VYS is not associated with cellular proliferation, ornithine decarboxylase induction, or alterations in differentiation. Effects on the cell surface, altering cell-cell interactions and/or communication might best explain these actions of TPA.

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Supported by NIH Grants CA 32306-01 and RR 5359-20 Project 3-82. Presented in part at the Joint Meeting of the American Society for Pharmacology and Experimental Therapeutics and the Society of Toxicology, 1982 (22). This paper is the second in a series.

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