Heat‐Induced bFGF gene expression in the absence of heat shock element correlates with enhanced AP‐1 binding activity

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Abstract
Basic fibroblast growth factor (bFGF) has been shown to be a potent mitogen and a promoter of angiogenesis. It has been hypothesized that the expression of the bFGF gene may be induced by stress of various types. To test that hypothesis, we investigated the expression of the bFGF gene during heat treatment in adriamycin‐resistant (MCF‐7/ADR) and‐sensitive (MCF‐7) human breast carcinoma cells. Under normal growth conditions, the bFGF mRNA was detected in MCF‐7/ADR cells, while it was not detectable in MCF‐7 cells by Northern blot analysis. During heating at 41°C, the level of bFGF mRNA increased in MCF‐7/ADR cells and the message became detectable in the MCF‐7 cell line. However, after continuous heating at 41°C for 24 h, the bFGF mRNA level decreased to control level in MCF‐7/ADR cells. Interestingly, simultaneous treatment with heat and 60 m̈g/ml H‐7 (1‐(isoquinolinylsulfonyl)‐2‐methylpiperzine, a potent PKC inhibitor) decreased the level of bFGF mRNA in MCF‐7/ADR cells. These results suggest that a protein kinase, likely PKC, is involved in the transcriptional regulation of the heat‐enhanced bFGF gene expression in human breast carcinoma cells. Although no heat shock element can be identified in the promoter of the bFGF gene, we observed that the AP‐1 binding activity to a TPA responsive element (TRE)‐like sequence in the promoter of bFGF gene was enhanced by heat, as tested by mobility shift assay. Antibody developed against the c‐Jun and c‐Fos proteins inhibited the AP‐1 binding activity to TRE. Therefore, the AP‐1 complex appears to be responsible for the heat‐enhanced binding to the TRE‐like motif of the bFGF gene. Furthermore, the increased AP‐1 binding activity does not require new protein synthesis but activation of the preexisting c‐Jun proteins.