Expression of oncogenic K-ras from its endogenous promoter leads to a partial block of erythroid differentiation and hyperactivation of cytokine-dependent signaling pathways

When overexpressed in primary erythroid progenitors, oncogenic Ras leads to the constitutive activation of its downstream signaling pathways, severe block of terminal erythrold differentiation, and cytokine-independent growth of primary erythroid progenitors. However, whether high-level expression of oncogenic Ras is required for these phenotypes is unknown. To address this issue, we expressed oncogenic K-ras (K-raS(G12D)) from its endogenous promoter using a tetracyclineinducible system. We show that endogenous K-raS(G12D) leads to a partial block of terminal erythroid differentiation in vivo. In contrast to results obtained when oncogenic Ras was overexpressed from retroviral vectors, endogenous levels of K-raS(G12D) fail to constitutively activate but rather hyperactivate cytokine-dependent signaling pathways, including Stat5, Akt, and p44/42 MAPK, in primary erythroid progenitors. This explains previous observations that hematopoietic progenitors expressing endogenous K-ras(G12D) display hypersensitivity to cytokine stimulation in various colony assays. Our results support eff orts to modulate Ras signaling for treating hematopoietic malignancies.