Transglutaminase 2 is involved in homocysteine-induced activation of human THP-1 monocytes

Abstract

Aberrant transglutaminase 2 (TG2) expression and protein cross-linking activity have been associated with several chronic neurodegenerative disorders in which inflammatory processes triggered by activated microglia and monocytes play a key role, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis. Interestingly, mild-to-moderate hyperhomocysteinemia (HHcy), corresponding to increased plasma homocysteine (Hcy) concentrations in the range 16–60 μM, have recently been associated with the above-cited diseases. Using THP-1 monocytes, here we investigated the role of TG2 in cell response to mildly elevated Hcy concentrations.

A five-day incubation with Hcy (∼25 μM) increased reactive oxygen species, peroxide lipids, as well as 8-hydroxyguanosine levels by twofold, and decreased the endogenous cell antioxidant defenses, that is reduced glutathione, by 50% in Hcy-exposed cultures compared with controls (p < 0.01). Hcy-induced oxidative stress was associated with increases in TG2 expression and activity, as well as nuclear factor kappa B activation. Notably, the latter was reduced in the presence of the TG-specific inhibitor R283. Hcy exposure also significantly increased the mRNA levels of tumor necrosis factor alpha, interleukin (IL)-6, and IL-1β, as well as the level of Hcy-inducible endoplasmic reticulum (ER) stress protein, a marker of ER stress, in Hcy-exposed cultures compared with controls. Notably, these effects were dramatically reduced by R283.

These preliminary findings indicate that TG2 plays a key role in Hcy-induced activation of THP-1 monocytes, involving oxidative as well as ER stress and inflammation. This underlines the potential of TG2 inhibition in the therapeutic management of inflammatory processes contributing to neurodegenerative disorders associated with mild HHcy.

Keywords::

• transglutaminase 2
• homocysteine
• oxidative stress
• inflammation
• ER stress

Acknowledgements

We thank Prof. Martin Griffin of the School of Life & Health Sciences at Aston University in Birmingham (UK) for generously providing us R283. We thank Prof A. Di Pietro and Dr G. Visalli from University of Messina for generously providing us fluorescent probes and helping with flow cytometric analysis of oxidative stress markers. We also thank Prof M.T. Sciortino and Dr G. Siracusano from University of Messina for generously providing us mouse monoclonal antibody against LC3 and helping with Western blot analysis.

Declaration of interest

The authors declare that there is no conflict of interest. The authors alone are responsible for the content and writing of the paper.