High-level expression of human c-jun gene causes cellular transformation of chicken embryo fibroblasts

To analyze the transforming activity of c-jun, a Rous sarcoma virus (RSV) variant that carries human c-jun instead of v-src (JH1) was constructed. After infection onto chicken embryo fibroblasts (CEF), JH1 formed foci with a titer comparable to that of wild-type RSV, and the infected cells grew in soft agar, indicating that the human c-jun gene has transforming potential, like the v-jun gene. The expression of Fra-2, one of the recently isolated Fos-related antigens, but not Fos was detected in both JH1-infected CEF and CEF infected with the control retrovirus vector (DS3). Gel shift analysis using nuclear extracts from DS3-infected CEF revealed that the Fra-2/Jun complex contributes to the basal level of AP-1 DNA binding activity. A similar activity was detected in JH1-infected CEF, but these cells have an additional AP-1 binding activity derived from Jun homodimers that seems to play important roles in the cellular transformation.