The mechanism of immunosuppressive drug azathioprine is not clear, while azathioprine has been used for 60 years in clinical
practice in patients undergoing transplantation surgery or to combat autoimmune disease. Part of the function of azathioprine
became evident in specific immune cells, namely T cells, demonstrating that small GTPase Rac1 was inhibited by azathioprine
and thereby reduced their inflammatory response.
We show that 6-mercaptopurine and thiopurines 6-thio-GDP and 6-thio-GTP,
all metabolites of azathioprine, cause Rac1 inhibition in endothelial cells, macrophages and gut epithelial cells, thus this
effect is not exclusive to T cells. The beneficial effect of azathioprine on aortic aneurysm development (dilation due to
degradation of the aorta) involved Rac1 inhibition in endothelial cells and macrophages. Since endothelial cells (a key cell
type in blood vessels) play a role in almost all diseases, it implies that part of the immunosuppressive response of azathioprine
is mediated by reducing the activation status of these non-immune cells.
Inflammatory bowel disease (IBD) is the autoimmune
disease where azathioprine is most frequently used in the clinic. The fact that thiopurines reduce Rac1 activation in gut
epithelial cells explains that, also in IBD, azathioprine affects activation in non-immune cells, next to the inflammatory
cells. Finally, we reveal, by in silico molecular docking studies, that the thiopurines can bind directly to Rac1, making
them true Rac1 inhibitors. Conclusively, the vasculature and Rac1 are involved in many diseases, thus inhibition of Rac1 in
vascular cells is an important function of immunosuppressive drug azathioprine.