Screening of substrate peptide sequences for tissue-type transglutaminase (TGase 2) using T7 phage cDNA library

18F-labeling and initial in vivo evaluation of a Hitomi peptide for imaging tissue transglutaminase 2

INTRODUCTION

Tissue transglutaminase 2 (TG2) is a calcium-dependent enzyme which cross-links proteins. It is overexpressed in many diseases and plays a key role in tissue remodeling, including cell adhesion and migration. Overexpression of TG2 in breast cancer is a marker for patients at risk of recurrence. Non-invasive imaging of TG2 can therefore play an important role in patient management. TG2 probes labeled with the positron emitters ¹¹C and ¹⁸F have thus far not found widespread application due to purity and metabolism issues. Our approach was to radiolabel a TG2 selective, 13-mer amino acid peptide, which was modified with a 5-azidopentanoic acid group at the N-terminus via a copper free click chemistry approach.

METHODS

Radiochemistry was performed and fully automated using an iPhase FlexLab module. We produced the radiolabeling synthon [¹⁸F]FBz-DBCO from [¹⁸F]SFB and DBCO-amine. After HPLC purification, [¹⁸F]FBz-DBCO was reacted with the modified peptide and the putative radiotracer purified by HPLC. In vivo imaging using the radiolabeled amine was performed in mice bearing either TG2 expressing MDA-MB-231 or non-TG2 expressing MCF-7 xenografts as negative control. Expression of the target was confirmed using immunohistochemistry and western blot techniques.

RESULTS

We obtained 9 ± 2 GBq of the radiolabeled peptide from 55 ± 5 GBq of fluorine-18 in an overall synthesis time of 160 min from end of bombardment (EOB), including HPLC purification and reformulation. Small animal PET/MR imaging showed that visualization of MDA-MB-231 tumors using the radiolabeled peptide could only be achieved due to differences in clearance between tumor and surrounding tissue. In the MCF-7 xenograft model, radiotracer clearance from tumor and surrounding tissue occurred at a similar rate, thus making it impossible to visualize MCF-7 tumors. The presence of TG2 in MDA-MB-231 tumors and absence in MCF-7 tumors was confirmed by immunohistochemistry staining and western blot analysis.

CONCLUSION

A fully automated synthesis of a TG2 selective, 13-amino-acid peptide modified with 5-azido pentynoic acid at the N-terminal was established using [¹⁸F]FBzDBCO as a prosthetic group. Although our results show that radiolabeled peptides have potential as imaging agents for TG2, more research needs to be performed to improve radiotracer kinetics.

Transglutaminse 2 and EGGL, the protein cross-link formed by transglutaminse 2, as therapeutic targets for disabilities of old age

Aging of the extracellular matrix (ECM), the protein matrix that surrounds and penetrates the tissues and binds the body together, contributes significantly to functional aging of tissues. ECM proteins become increasingly cross-linked with age, and this cross-linking is probably important in the decline of the ECM's function. This article reviews the role of ε-(γ-glutamyl)-lysine (EGGL), a cross-link formed by transglutaminase enzymes, and particularly the widely expressed isozyme transglutaminase 2 (TG2), in the aging ECM. There is little direct data on EGGL accumulation with age, and no direct evidence of a role of EGGL in the aging of the ECM with pathology. However, several lines of circumstantial evidence suggest that EGGL accumulates with age, and its association with pathology suggests that this might reflect degradation of ECM function. TG activity increases with age in many circumstances. ECM protein turnover is such that some EGGL made by TG is likely to remain in place for years, if not decades, in healthy tissue, and both EGGL and TG levels are enhanced by age-related diseases. If further research shows EGGL does accumulate with age, removing it could be of therapeutic benefit. Also reviewed is the blockade of TG and active removal of EGGL as therapeutic strategies, with the conclusion that both have promise. EGGL removal may have benefit for acute fibrotic diseases, such as tendinopathy, and for treating generalized decline in ECM function with old age. Extracellular TG2 and EGGL are therefore therapeutic targets both for specific and more generalized diseases of aging.

Recent advances in the development of tissue transglutaminase (TG2) inhibitors

Tissue transglutaminase (TG2) is a Ca(2+)-dependent enzyme and probably the most ubiquitously expressed member of the mammalian transglutaminase family. TG2 plays a number of important roles in a variety of biological processes. Via its transamidating function, it is responsible for the cross-linking of proteins by forming isopeptide bonds between glutamine and lysine residues. Intracellularly, Ca(2+) activation of the enzyme is normally tightly regulated by the binding of GTP. However, upregulated levels of TG2 are associated with many disease states like celiac sprue, certain types of cancer, fibrosis, cystic fibrosis, multiple sclerosis, Alzheimer's, Huntington's and Parkinson's disease. Selective inhibitors for TG2 both cell penetrating and non-cell penetrating would therefore serve as novel therapeutic tools for the treatment of these disease states. Moreover, they would provide useful tools to fully elucidate the cellular mechanisms TG2 is involved in and help comprehend how the enzyme is regulated at the cellular level. The current paper is intended to give an update on the recently discovered classes of TG2 inhibitors along with their structure-activity relationships. The biological properties of these derivatives, in terms of both activity and selectivity, will also be reported in order to translate their potential for future therapeutic developments.