The Ras/Erk pathway induces primitive endoderm but prevents parietal endoderm differentiation of F9 embryonal carcinoma cells

The formation of parietal endoderm (PE) is one of the first differentiation processes during mouse development and can be studied in vitro using F9 em bryonal carcinoma (EC) cells. Treatment of F9 EC cells with retinoic acid ( RA) induces differentiation toward primitive endoderm (PrE), while differen tiation toward PE is induced by subsequent addition of parathyroid hormone (PTH) or PTH-related peptide (PTRrP). The signal transduction mechanisms in volved in this two-step process are largely unclear. We show that the RA-induced differentiation toward PrE is accompanied by a sustained increase in Ras activity and that ectopic expression of oncogenic Ha-Ras is sufficient to induce PrE differentiation Ras activity subsequent ly decreases upon PTH-induced differentiation toward PE. This is a necessar y event, since expression of oncogenic Ha-Ras in PrE like cells prevents PT H-induced PE differentiation. Expression of active PKA in PrE-like F9 cells mimics PTH-induced PE differentiation and is again prevented by oncogenic Ha-Ras. The effect of oncogenic Ras on both differentiation steps is abolis hed by the MEK inhibitor PD98059 and can be mimicked by constitutively acti ve forms of Raf and MEK. In conclusion, our data suggest that activation of the Ras/Erk is sufficien t to induce differentiation to PrE and to prevent subsequent differentiatio n toward PE. Activation of PISA down-regulates Ras activity, resulting in d isappearance of this blockade and transmission of signal(s) triggering PE d ifferentiation.