Expression of the intracisternal A-particle is elevated during differentiation of embryonal carcinoma cells

Three cDNA clones coding for the 3' region of the intracisternal A-particle (IAP), a mouse endogenous retrovirus, were isolated during screening of a library for genes whose expression was modulated during the retinoic acid-induced differentiation of the embryonal carcinoma cell line F9 into parietal endoderm-like (PE-like) cells. In contrast to previously reported results, no IAP transcripts were detected in either F9 cells or two pluripotent cell lines tested. Instead, IAP transcripts as well as IAPs were abundant in the PE-like cells PYS-2 and F9AcCl 9 and in retinoic acid-induced F9 cells but not in the other differentiated cell types of teratocarcinoma origin which were examined. A comparison of the nucleotide sequences of the three IAP cDNA clones with a genomically integrated proviral sequence (MIA14) demonstrated heterogeneity in both length and sequence among the clones. The position of the poly(A) addition site was determined to be 15 nucleotides from the proposed poly(A) addition signal and to occur after the sequence CAGA, not CA, as previously proposed. Length heterogeneity was greatest in a region of TC repeats 80 base pairs 5' to the poly(A) addition site. Additionally, the putative TATAA box found in MIA14 was deleted in the cDNA clones and in the long terminal repeat regions from two other genomic clones examined. The heterogeneity evident among the cDNA clones further demonstrated that at least two distinct IAP genes are activated during differentiation. An analysis of the rate of transcription in isolated nuclei indicated that the activation of expression of IAP genes in PE-like cells is the result of transcriptional regulation. Together, these observations suggest that the modulation of IAP transcription is regulated autonomously rather than by the fortuitous integration of an IAP sequence adjacent to a developmentally regulated cellular gene.