Inhibition of c-src Transcription by Mithramycin:  Structure−Activity Relationships of Biosynthetically Produced Mithramycin Analogues Using the c-src Promoter as Target

The aureolic acid antitumor antibiotic mithramycin (MTM) inhibits both cancer growth and bone resorption by cross-linking GC-rich DNA, thus blocking binding of Sp-family transcription factors to gene regulatory elements. Transcription of c-src, a gene implicated in many human cancers and required for osteoclast-dependent bone resorption, is regulated by the binding of Sp factors to specific elements in its promoter. Therefore, this gene represents an important anticancer target and a potential lead target through which MTM displays its so far uncharacterized action against osteoclastic bone resorption. Here we demonstrate, using DNA binding studies, promoter reporter assays, and RT-PCR, that MTM inhibits Sp binding to the c-src promoter region, thereby decreasing its expression in human cancer cells. Furthermore, selected mithramycin analogues, namely, premithramycin B, mithramycin SK, 7-demethylmithramycin, 4E-ketomithramycin, and 4C-ketodemycarosylmithramycin, generated through combinatorial biosynthesis, were compared with MTM for their ability to block Sp binding to the c-src promoter. Although most of the tested compounds lost their ability to bind to the DNA, alteration of the MTM 3-pentyl side chain led to a compound (mithramycin SK) with the same DNA binding specificity but with lower binding affinity than MTM. While this compound was comparable to MTM in promoter reporter, gene expression, and anticancer assays, given its weaker interaction with the DNA, it may be much less toxic than MTM. The results presented here supplement recent findings and, moreover, allow new conclusions to be made regarding both the structure−activity relationships, particularly with respect to the alkyl side chains, and the mechanism of action of aureolic acid drugs.