Different Response to Retinoic Acid of Two Teratocarcinoma Cell Lines

Retinoic acid (RA), a well-known inducer of differentiation, has been shown to regulate its own receptor gene expression in F9 teratocarcinoma cells. The homologous regulation of receptors by RA might be critical for RA-induced F9 cell differentiation. F9 cell lines from two different laboratories, named F9-1 and F9-2, were compared for retinoic acid receptor (RAR) and retinoid x receptor (RXR) gene expression in response to RA. The data show that both F9-1 and F9-2 cell lines are embryonal carcinoma cells, but of different phenotypes and different sensitivity to RA. In F9-1 cells, RA regulates all three RARs (α, β, and γ), two RXRs (α and γ), two activin receptors (ActR II and IIB), and tissue-specific plasminogen activator (t-PA) gene expression. In F9-2 cells RA regulates only the RARβ, RXRα, and t-PA genes. The induction of mRNA levels was much higher in F9-1 than in F9-2 cells. Different basal RARγ and RXRγ mRNA levels were also noted. In these two cell lines F9-2 cells expressed greater amounts of RARγ1, γ2, and γ3 mRNA isoforms, but lacked RXRγ mRNA compared with F9-1 cells. Since RARγ1 has been shown to exert an antagonistic effect on other types of RA receptors, the decreased sensitivity of F9-2 cells to RA might be due to its high level of RARγ1 and/or low level of RXRγ. This notion was in part supported by gel shift assay which demonstrated constitutive binding of RARγ to a RA responsive element (RARβE) in F9-2 cells. Further, the binding of nuclear protein to RARβE was increased upon RA treatment in F9-1 cells, but not in F9-2 cells. These differences in the regulation of RA receptors might determine the sensitivity of the two substrains of F9 cells to RA.