Synergistic action of WDR5 and HDM2 inhibitors in SMARCB1-deficient cancer cells

Rhabdoid tumors (RT) are rare and deadly pediatric cancers driven by loss of SMARCB1, which encodes the SNF5 component of the SWI/SNF chromatin remodeler. Loss of SMARCB1 is associated with a complex set of phenotypic changes including vulnerability to inhibitors of protein synthesis and of the p53 ubiquitin-ligase HDM2. Recently, we discovered small molecule inhibitors of the 'WIN' site of WDR5, which in MLL-rearranged leukemia cells decrease the expression of a set of genes linked to protein synthesis, inducing a translational choke and causing p53-dependent inhibition of proliferation. Here, we characterize how WIN site inhibitors act in RT cells. As in leukemia cells, WIN site inhibition in RT cells causes the comprehensive displacement of WDR5 from chromatin, resulting in a decrease in protein synthesis gene expression. Unlike leukemia cells, however, the growth response of RT cells to WIN site blockade is independent of p53. Exploiting this observation, we demonstrate that WIN site inhibitor synergizes with an HDM2 antagonist to induce p53 and block RT cell proliferation in vitro. These data reveal a p53-independent action of WIN site inhibitors and forecast that future strategies to treat RT could be based on dual WDR5/HDM2 inhibition.

Interaction of MYC with host cell factor-1 is mediated by the evolutionarily conserved Myc box IV motif

The MYC family of oncogenes encodes a set of three related transcription factors that are overexpressed in many human tumors and contribute to the cancer-related deaths of more than 70,000 Americans every year. MYC proteins drive tumorigenesis by interacting with co-factors that enable them to regulate the expression of thousands of genes linked to cell growth, proliferation, metabolism, and genome stability. One effective way to identify critical cofactors required for MYC function has been to focus on sequence motifs within MYC that are conserved throughout evolution, on the assumption that their conservation is driven by protein-protein interactions that are vital for MYC activity. In addition to their DNA-binding domains, MYC proteins carry five regions of high sequence conservation known as Myc boxes (Mb). To date, four of the Myc box motifs (MbI, MbII, MbIIIa, and MbIIIb) have had a molecular function assigned to them, but the precise role of the remaining Myc box, MbIV, and the reason for its preservation in vertebrate Myc proteins, is unknown. Here, we show that MbIV is required for the association of MYC with the abundant transcriptional coregulator host cell factor 1 (HCF-1). We show that the invariant core of MbIV resembles the tetrapeptide HCF-binding motif (HBM) found in many HCF-interaction partners, and demonstrate that MYC interacts with HCF in a manner indistinguishable from the prototypical HBM-containing protein VP16. Finally, we show that rationalized point mutations in MYC that disrupt interaction with HCF-1 attenuate the ability of MYC to drive tumorigenesis in mice. Together, these data expose a molecular function for MbIV and indicate that HCF-1 is an important co-factor for MYC.

A new type of fluorogenic substrates for determination of cellular dipeptidyl peptidase IV (DP IV/CD26) activity

The stability of cell associated fluorescence is an essential requirement for measurements of cellular enzymatic activity via enzyme catalyzed liberation of fluorophores. Rhodamine 110 (R110), a highly fluorescent xanthene dye, was used to synthesize nonfluorescent dipeptidyl peptidase IV (DP IV) substrates Xaa-Pro-R110-Y allowing the stable covalent binding of the enzymatically released fluorescent R110-Y on cells. All compounds have been characterized as substrates of isolated DP IV with kcat/Km values of about 10(6) M-1.s-1. The hydrophobicity of the residue Y affects the affinity of the substrate to the catalytic site of DP IV. The compounds are characterized as sensitive substrates of cell surface associated DP IV of DP IV rich U-937 cells. The binding of the enzymatically released R110-Y on cells results in a stable cellular fluorescence. This way, the quantitative determination of cell surface associated DP IV activity is possible.

Down-regulation of T cell activation following inhibition of dipeptidyl peptidase IV/CD26 by the N-terminal part of the thromboxane A2 receptor

Using synthetic inhibitors, it has been shown that the ectopeptidase dipeptidyl peptidase IV (DP IV) (CD26) plays an important role in the activation and proliferation of T lymphocytes. The human immunodeficiency virus-1 Tat protein, as well as the N-terminal nonapeptide Tat(1-9) and other peptides containing the N-terminal sequence XXP, also inhibit DP IV and therefore T cell activation. Studying the effect of amino acid exchanges in the N-terminal three positions of the Tat(1-9) sequence, we found that tryptophan in position 2 strongly improves DP IV inhibition. NMR spectroscopy and molecular modeling show that the effect of Trp(2)-Tat(1-9) could not be explained by significant alterations in the backbone structure and suggest that tryptophan enters favorable interactions with DP IV. Data base searches revealed the thromboxane A2 receptor (TXA2-R) as a membrane protein extracellularly exposing N-terminal MWP. TXA2-R is expressed within the immune system on antigen-presenting cells, namely monocytes. The N-terminal nonapeptide of TXA2-R, TXA2-R(1-9), inhibits DP IV and DNA synthesis and IL-2 production of tetanus toxoid-stimulated peripheral blood mononuclear cells. Moreover, TXA2-R(1-9) induces the production of the immunosuppressive cytokine transforming growth factor-beta1. These data suggest that the N-terminal part of TXA2-R is an endogenous inhibitory ligand of DP IV and may modulate T cell activation via DP IV/CD26 inhibition.

Steroid alkaloid glycosides from Solanum coccineum

In addition to solamargine, isoanguivine and solasonine, two new steroid alkaloid glycosides, xylosylsolamargine and xylosyl-beta-solamarine, have been isolated from the aerial parts of Solanum coccineum, the structures of which have been elucidated as (25R)-3 beta-[O-beta-D-xylopyranosyl-(1-->2)-O-alpha-L-rhamnopyranosyl- (1-->4)-O-[alpha-L-rhamnopyranosyl-(1-->2)]-beta-D-glucopyranosyloxy+ ++]-22 alpha N-spirosol-5-ene and (25S)-3 beta-[O-beta-D-xylopyranosyl-(1-->2)-O-alpha-L-rhamnopyranosyl- (1-->4)-O-[alpha-L-rhamnopyranosyl-(1-->2)]-beta-D-glucopyranosyloxy+ ++]-22 beta N-spirosol-5-ene.