Background
Type 2 diabetes (T2D) is a chronic metabolic disease characterized by insulin resistance and increased levels of blood glucose, accounting for approximately 90% of all diabetes mellitus cases. The prevalence of T2D has been expanding over the past decades, resulting in an increasing global health problem. Previous studies in patients showed that plasma cathepsin D (CTSD) levels correlate with insulin resistance in T2D. Moreover, our team showed that plasma CTSD activity correlates with metabolic parameters of T2D in males. Other studies from our group show that extracellular CTSD leads to disturbances in several metabolic diseases and that it is a sensitive marker for disease progression. However, very little is known about factors that induce increased exocytosis of CTSD in plasma under pathological conditions. Therefore, this study aimed to determine factors that trigger alterations in plasma CTSD activity in T2D.

Methods
THP-1 cells and wild type bone-marrow derived macrophages (BMDMs) were incubated with glucose and fatty acids (palmitate), respectively, and CTSD activity was measured. WT and LDLR^-/- BMDMs were incubated with cortisol and CTSD activity was determined. Plasma CTSD activity was measured in 14 T2D, 14 metabolic syndrome and 14 healthy age-matched males, following a standardized breakfast (t = 0h), lunch (t = 4h) and diner (t = 8h). Blood samples were collected before and at t = 2, 4, 6, 8, 12, 16, 20 and 24h following breakfast.

Results
The results indicate that glucose and palmitate enhanced CTSD activity in vitro. Cortisol increased extracellular CTSD activity of LDLR^-/- BMDMs rather than wild type BMDMs. Moreover, analysis of plasma from patients who received three identical standard meals throughout the day demonstrated rhythmic activity of plasma CTSD.

Conclusion
Altogether, the results indicate that plasma CTSD activity is regulated by glucose, palmitate and cortisol, and that its activity fluctuates upon meal consumption