Approximately one-third of people with diabetes develop diabetic retinopathy, which can impair vision and is a leading cause of blindness among people of working age. The condition is characterized by the formation of acellular capillaries and pericyte loss in the retina. A hallmark of diabetic retinopathy is retinal vascular basement membrane thickening, which has been demonstrated to induce apoptosis and contribute to the loss of vascular cells in the retina. Lysyl oxidase (LOX) is an extracellular enzyme that cross-links collagen and elastin molecules to form a stable extracellular matrix. LOX has previously been implicated in promoting high glucose (HG)-induced apoptosis. Notably, studies have also suggested that the LOX-PP precursor enzyme, also promotes apoptosis in a number of diseased tissues. Studies of pancreatic and breast cancer cells have also suggested that overexpression of LOX-PP may trigger cell death.

Researchers at Boston University School of Medicine have identified an enzymatic precursor that appears to be toxic to retinal cells in rodents with diabetes, a finding that could lead to the development of novel treatments for diabetic retinopathy. On studying retinal endothelial cell cultures and rat models of diabetes, the researchers found clear evidence that a high blood glucose concentration increases levels of the precursor lysyl oxidase propeptide (LOX-PP), which is already known to play a role in triggering apoptosis in various diseased tissues. LOX-PP may induce cell death by compromising a cell survival pathway, and in retinas of diabetic rats, increased LOX-PP contributed to retinal vascular cell death associated with diabetic retinopathy. The finding could pave the way for new drugs that could treat diabetic retinopathy by targeting LOX-PP or its metabolites.

However, the role LOX-PP may play in the development and progression of diabetic retinopathy is not known. Scientists have not yet determined whether LOX-PP is involved in high glucose-induced apoptosis in the retina and therefore promotes retinal vascular cell loss. As reported in The American Journal of Pathology, scientists have now investigated the role LOX-PP may play in the retinal tissue of diabetic rats. For the study, the team administered artificially synthesized LOX-pp (recombinant LOX-PP, rLOX-PP) directly into retinal blood vessels of rats with and without diabetes. The researchers then analyzed any vasculature changes associated with diabetic retinopathy such as swelling, leaking blood vessels, thickening or blockage of vascular walls and histological hallmarks of the condition, such as acellular capillaries and pericyte loss. The team reports that more acellular capillaries formed and more pericytes were lost in the retinas of the rats with diabetes, compared with the control animals.

When the team injected rLOX-PP directly into the eyes of the non-diabetic rats, more acellular capillaries formed and more pericytes were lost, compared with non-diabetic rats that received a control injection. Next, the team investigated the effects of high-glucose conditions when retinal endothelial cells were grown in culture. Compared with cells grown in a normal medium, adding glucose to the cell cultures increased the expression of LOX-PP and disrupted the PI3K-Akt cell signaling pathway involved in cell growth and survival. When endothelial cells from rat retinas were exposed to rLOX-PP, the number of apoptotic cells significantly increased and c-Akt phosphorylation decreased. This suggests that high-glucose conditions increase LOX-PP levels, which, in turn, induces cell death. Furthermore, LOX-PP seemed to promote cell death by compromising the aforesaid pathway. Recombinant LOX-PP alone was sufficient to induce cell death.

Therefore high blood glucose is, after all, the real culprit of every biological or pathological response in the diabetic eye, as high calorie diet is a recognized trigger of either diabetes and senile macular degeneration. Commenting on diabetic retinopathy as a leading cause of blindness in the working-age population, Dr. Roy says it is unfortunate that there is no cure for this devastating ocular complication: “This report shows novel functionality of LOX-PP in mediating cell death under high glucose condition in retinal endothelial cells as well as in diabetic animals. Our findings suggest a novel mechanism for high glucose-induced cell death involving LOX-PP, which may be a therapeutic target in preventing retinal vascular cell loss associated with diabetic retinopathy”.

• Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry

Scientific references

Kim D, Lee D et al. Roy S. Am J Pathol. 2019 Oct;189(10):1945-1952.

Zadravec P, Braunger BM et al. Exp Eye Res. 2019 Feb; 179:115-124.

Song B et al., Roy S. Invest Ophthalmol Vis Sci. 2018; 59(15):5965-72.