A small molecule regulator of tissue transglutaminase conformation inhibits the malignant phenotype of cancer cells

The cover for issue 76 of Oncotarget features Figure 2, "The identification of TTGM 5826 as a potential modulator of tTG conformation," by Katt, et al.

This raises the possibility that strategies directed toward causing tTG to maintain an open state could potentially provide a therapeutic benefit for cancers in which tTG is highly expressed. Thus, TTGM 5826 represents the lead compound for a new class of small molecules that promote the toxicity of cancer cells by stabilizing the open state of tTG.

Dr. Richard A. Cerione from the Department of Molecular Medicine and the Department of Chemistry and Chemical Biology, at Cornell University, Ithaca, NY, USA said "Protein-glutamine γ-glutamyltransferase 2, more commonly referred to as tissue transglutaminase or type-2 transglutaminase, is a member of the transglutaminase family of proteins"

Specifically, epidermal growth factor treatment of He La cervical carcinoma cells caused tTG to localize to leading edges where it catalyzed protein crosslinking events necessary for EGF-stimulated cell motility.

Figure 2: The identification of TTGM 5826 as a potential modulator of tTG conformation. (A) For screening purposes, molecules were docked to the substrate binding site (boxed) of the open state of tTG (green, pdb ID: 2Q3Z). Shown is TTGM 5826 (space filling, white) docked to tTG. (B) Enlarged view of TTGM 5826 (white) docked to the crosslinking active site of tTG (green, pdb ID: 2Q3Z). The phenyl ring of TTGM 5826 projects into a deep pocket formed around the catalytic Cys 277, while the barbiturate ring is predicted to engage in hydrogen bonds with Trp 241, Gln 276, and Asn 333. The flexible linker allows the molecule to wrap around a 'hump' in the binding site, while the phthalamide projects into a second deep hydrophobic pocket. (C) The chemical structure of TTGM 5826 is composed of four moieties: a phenyl ring (lower left), a barbiturate (upper left), a tolyl ring (upper right), and a phthalamide (lower right). (D) tTG was incubated on ice with or without 20 mM CaCl₂ for 5 minutes, and then each sample was incubated without (lane 1) or with trypsin (lanes 2 & 3) for 3 hours. The samples were resolved by SDS-PAGE and stained with Coomassie blue. Lighter bands indicate that a greater amount of tTG was digested. Lanes of samples where lower concentrations of CaCl₂ were used were spliced out for clarity, and are indicated by the red line. (E) tTG was incubated on ice with the indicated small molecule (TTGM #), or DMSO, for 5 minutes, at which point trypsin was added, as indicated, and the reactions were processed as described in (C). TTGM molecules are labeled by the last four digits of their ChemBridge catalog number. Densiometric quantitation was performed with ImageJ. Band densities are reported as fractional density of the trypsin-free control band.

Recent evidence has also revealed an interesting connection between tTG and the maintenance of cellular pH, as it was shown that the inhibition of tTG crosslinking activity in highly aggressive cancer cells caused a decrease in extracellular pH and resulted in growth inhibition and increased apoptosis.

In contrast, wildtype tTG, or tTG mutants that are defective in their enzymatic crosslinking activity but retain their ability to bind guanine nucleotides, primarily adopt a closed state conformation and promote cell survival.

These findings, combined with the fact that tTG knockout mice are predominantly healthy, make tTG a potentially promising therapeutic target both for differentiated cancer cells and CSCs.

Treating cancer cell lines, as well as GSCs, that express tTG with TTGM 5826 induced cell death at concentrations that were not harmful to non-transformed cells.

The Richard A. Cerione research team concluded, "We have identified a novel small molecule tTG inhibitor, TTGM 5826, that targets and stabilizes the open state conformation of tTG."

Full text - https://doi.org/10.18632/oncotarget.26193

Correspondence to - Richard A. Cerione - rac1@cornell.edu

Keywords - transglutaminase, glioblastoma, signaling, glioma stem cells, cancer