Expression and transcriptional activity of Ap‐1, CRE, and URE binding proteins in B16 mouse melanoma subclones

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Abstract
The expression and DNA binding activity of members of the activating protein‐1 (AP‐1) and activating transcription factor (ATF) families of transcription factors were analyzed in sham and ultraviolet (UV)‐irradiated subclones of the B16 mouse melanoma cell system. The four subclones we used represent sequential stages in the development and progression of malignant melanoma and exhibit differences in growth and metastatic potential. Western blot analysis revealed differential expression of some AP‐1 (c‐jun, jun‐B, and jun‐D) and ATF (43‐ and 47‐kDa cyclic AMP‐responsive element binding protein (CREB) family members) in the different subclones; while c‐jun expression was noted in the subclones with the greater malignant potential, jun‐D was expressed in those with the lesser malignant potential. Furthermore, a delicate balance between the two forms of CREB was noted; the 47‐kDa CREB appeared, when expressed exclusively, in subclones that exhibit a greater malignant potential. Electrophoretic mobility shift assays using AP‐1, CRE, and UV‐responsive element (URE) consensus sequences indicated that distinct complexes were formed with extracts from each of the four subclones. The complexes were competitively inhibited by each of the target sequences used, suggesting that “cross‐talk”occurs between some AP‐1 and ATF family members in this cell system. Moreover, a multimer of the URE sequence, cloned upstream of a chloramphenicol acetyltransferase reporter gene, was transcriptionally active and responsive to UV irradiation in two of the four subclones. UV‐related transcriptional activation was directly correlated with the expression of a 43‐kDa CREB. Together, these observations identify members of AP‐1 and CREB families whose expression and activities correlate with the malignant potential of subclones that represent different stages in melanoma development and progression.