p21 expression is induced by activation of nuclear nerve growth factor-induced Bα (Nur77) in pancreatic cancer cells

1,1-Bis(3′-indolyl)-1-(p-anisyl)methane (DIM-C-pPhOCH₃) activates the orphan receptor nerve growth factor-induced Bα (Nur77)in cancer cells, and in this study, DIM-C-pPhOCH₃ decreased Panc1 pancreatic cancer cell survival and arrested cells in G₀-G ₁. These responses were accompanied by induction of the cyclin-dependent kinase inhibitor p21 in pancreatic cancer cells. Mechanistic studies showed that induction of p21 mRNA and protein by DIM-C-pPhOCH ₃ was Nur77 dependent but did not depend on Krüppel-like factor 4, which was also induced by DIM-C-pPhOCH₃. Activation of p21 promoter constructs by DIM-C-pPhOCH₃ required the GC-rich proximal region of the promoter, and results of RNA interference studies showed that Nur77-dependent activation of the p21 promoter involved interactions with Sp1 and Sp4 but not Sp3. Interactions of Nur77 with the p21 promoter in Panc1 cells treated with DIM-C-pPhOCH₃ were also confirmed in chromatin immunoprecipitation assays. These data show that activation of nuclear Nur77 results in a novel pathway for induction of p21, which is independent of Nur77 response elements but dependent on Sp proteins bound to the GC-rich proximal region of the p21 promoter.