SP1 and cancer: Compounds studied as cancer chemopreventive agents, such as the antidiabetic drug metformin, curcumin, and also triterpenoid natural products, such as celastrol and its analogs [21,22,23] exhibit, besides exclusive activities, a common effect: they down regulate specific protein (Sp) transcription factors (TFs) Sp1, Sp3 and Sp4 and, consequently, pro-oncogenic Sp-regulated genes [24,25,26,27].