The inducible cyclooxygenase, COX-2, has been associated with vascular inflammation and cellular proliferation. We have discovered that hypoxia increases expression of the COX-2 gene in human vascular endothelial cells in culture independent of other stimuli. Western analysis of human umbilical vein endothelial cells (HUVEC) revealed a greater than 4-fold induction of protein by hypoxia (1% O2). The steady-state level of COX-2 mRNA was correspondingly elevated by both Northern blot and reverse transcriptase-polymerase chain reaction analysis. Using electrophoretic mobility shift assays with antibody supershifting, we also found that hypoxia causes increased binding of NF-kappaB p65 (Rel A) to the one out of the two NF-kappaB consensus elements in the COX-2 promoter which is closest to the transcription start site of the COX-2 gene. Transfection of an immortalized human microvascular endothelial cell line (HMEC-1) with mutation reporter gene constructs and HUVEC with both mutation and deletion reporter gene constructs suggested that transcription of the COX-2 gene was enhanced by hypoxia. In transcription factor decoy experiments, hypoxic HUVEC were exposed in culture to 20 microM of the same NF-kappaB element found to bind NF-kappaB protein. The wild type transcription factor decoy prevented hypoxic induction of COX-2, presumably by binding with cytoplasmic p65; however, mutated or scrambled oligonucleotides did not prevent the increase in COX-2 protein expression by hypoxia. Thus, the intracellular signaling mechanism that leads to induction of COX-2 by hypoxia includes binding of p65 to the relatively 3' NF-kappaB consensus element in the COX-2 upstream promoter region in human vascular endothelial cells.

p53 is a central regulator that turns on vast gene networks to maintain cellular integrity in the presence of various stimuli. p53 activates transcription initiation in part by aiding recruitment of TFIID to the promoter. However, the precise means by which p53 dynamically interacts with TFIID to facilitate assembly on target gene promoters remains elusive. To address this key issue, we have undertaken an integrated approach involving single-molecule fluorescence microscopy, single-particle cryo-electron microscopy, and biochemistry. Our real-time single-molecule imaging data demonstrate that TFIID alone binds poorly to native p53 target promoters. p53 unlocks TFIID's ability to bind DNA by stabilizing TFIID contacts with both the core promoter and a region within p53's response element. Analysis of single-molecule dissociation kinetics reveals that TFIID interacts with promoters via transient and prolonged DNA binding modes that are each regulated by p53. Importantly, our structural work reveals that TFIID's conversion to a rearranged DNA binding conformation is enhanced in the presence of DNA and p53. Notably, TFIID's interaction with DNA induces p53 to rapidly dissociate, which likely leads to additional rounds of p53-mediated recruitment of other basal factors. Collectively, these findings indicate that p53 dynamically escorts and loads TFIID onto its target promoters. KEYWORDS p53, TFIID, transcription, structure, single-molecule microscopy M ore than 50% of cancer patients harbor p53 mutations, highlighting the essential role of this protein in tumor suppression (1). To properly maintain genomic stability, p53 induces vast gene networks involved in diverse cellular pathways, including the cell cycle arrest, apoptosis, and DNA repair pathways (2). In response to various stress stimuli, p53 acts as a transcriptional activator that specifically binds consensus response elements (REs; two 10-base-pair half-sites [RRRCWWGYYY]) within its target genes to directly stimulate gene expression (2). p53 utilizes its ability to nonspecifically bind and slide along the DNA to expedite the search for target REs (3). Upon recognition of its response genes, p53 facilitates transcription at least in part by targeting the TFIID-mediated transcription machinery to the promoter (4-7). TFIID is composed of TATA-binding protein (TBP) and 14 TBP-associated factors (TAFs). TFIID recognizes and binds multiple core promoter elements (e.g., the TATA box, the initiator, and downstream core promoter elements [DPE]) surrounding the transcription start site (TSS) (8). Once bound, TFIID serves as a central scaffold for six basal factors, including RNA polymerase II, to form a preinitiation complex (PIC) directing transcription initiation. Importantly, without the assistance of activators such as p53, TFIID's core promoter recognition is weak and rate limiting for transcription initiation due to inefficient PIC assembly (9-16). As TFIID is responsible for transcription of at least ϳ90% of proteincoding genes (17), unraveling how p...