High-fat/high-sucrose feeding increases systemic intermediate monocyte population in two rat strains, but major bone changes are observed only in sprague dawley rats

Abstract

Purpose: Monocytes are the circulating precursors of macrophages and osteoclasts and three main monocyte subsets are defined: classical, intermediate and non-classical. The proportion of intermediate monocytes in the blood is known to increase in diseases such as asthma and rheumatoid arthritis, but also during obesity; one of the known risk factor for osteoarthritis (OA). This monocyte subset also has the highest capacity for bone resorption, when differentiated to osteoclasts in vitro. In addition, we know that especially early development of OA is associated with an increased bone resorption and fast remodeling, whereas only at later stages a slow turnover accompanied by subchondral plate thickening or sclerosis can be observed. In this study, we compared the effects of high-fat (HF) and high-fat/high-sucrose (HFS) diets on circulating monocyte populations in rats. We also monitored the effects on bone via uCT scans. We included both Wistar(Han) and Sprague Dawley (SD) rats. Methods: 24 Wistar(Han) (Crl:WI(Han)) rats and 8 Sprague Dawley (Crl:CD(SD)) rats received different diets for 12 weeks. All 32 rats were male and 12 weeks old at the start of the study. The 24 Wistar(Han) rats were randomized in 3 groups: standard chow diet (ST group; 9 kcal% from fat, 67 kcal% from carbohydrates of which 9.8% sucrose), high fat diet (HF group; 60 kcal% from fat, 20 kcal% from carbohydrates of which 0% sucrose) and high fat-high sucrose diet (HFS group; 40 kcal% from fat, 40 kcal% from carbohydrates of which 83% sucrose). The 8 SD rats received the same high fat-high sucrose diet (HFS-SD group). MicroCT scans were performed at week 0 and week 12. Blood samples were taken at week 1 and 11 to perform flow cytometry analysis and determine the classical (CD172a+CD43-), intermediate (CD172a+CD43low) and non-classical (CD172a+CD43hi) monocytes. Results: The Sprague Dawley (SD) rats had a significantly different monocyte distribution in week one of the study compared to the Wistar rats, with a larger proportion of classical monocytes (21.7% ±5.8 vs. 14.0% ±4.5, p=0.001) and less non-classical monocytes (71.6% ±5.4 vs. 79.7% ±4.8, p=0.001) (Figure 1A). The proportion of intermediate monocytes was similar between the two rat strains at week one, 5.5% ±1.4 in the Wistar rats and 5.6% ±1.8 in the SD rats (p=0.8). Interestingly, the SD monocyte distribution was more comparable to that the older Wistar rats in week 11 (Figure 1A). After 12 weeks of diet intervention the intermediate monocyte proportion had increased significantly in all rats that received HFS diet, compared to baseline (p=0.03 and p=0.008, respectively) (Figure 1B). The HF group showed an intermediate monocyte increase to 9.2% ±1.2, but this was not statistically significant (p=0.08) compared to baseline. Looking at uCT scans, changes in bone structure were observed in the SD rats after 12 weeks of HFS diet (Figure 2). The changes were mainly characterized by subchondral bone remodeling, but osteophyte formation was also observed. Changes observed in the HF and HFS groups of Wistar rats were minor in comparison (Figure 2). Conclusions: In this study we found an increased intermediate monocyte population in rats that were fed a HFS diet for 12 weeks, when compared to standard chow diet. In addition, we saw subchondral bone remodeling and osteophytes in the HFS-SD group on uCT to a larger extend than in the Wistar HFS and HF group. The effects of obesity on bone metabolism are paradoxical; on the one hand it is thought to increase bone density due to more mechanical loading, but on the other hand obesity associated systemic inflammation is associated with negative effects on the bone microstructure. In this study we see larger effects on the bone structure in SD rats in response to HFS-feeding than in Wistar rats, this could possibly be explained by the differences in monocyte populations and uCT scans we observe at the start of the study, which both indicate that this strain of rats may have a more advanced biological age. [Formula presented] [Formula presented]