Surfactant protein d deficiency in mice is associated with hyperphagia, altered fat deposition, insulin resistance, and increased basal endotoxemia

Abstract

Pulmonary surfactant protein D (SP-D) is a host defence lectin of the innate immune system that enhances clearance of pathogens and modulates inflammatory responses. Recently it has been found that systemic SP-D is associated with metabolic disturbances and that SP-D deficient mice are mildly obese. However, the mechanism behind SP-D's role in energy metabolism is not known.Here we report that SP-D deficient mice had significantly higher ad libitum energy intake compared to wild-type mice and unchanged energy expenditure. This resulted in accumulation but also redistribution of fat tissue. Blood pressure was unchanged. The change in energy intake was unrelated to the basal levels of hypothalamic Pro-opiomelanocortin (POMC) and Agouti-related peptide (AgRP) gene expression. Neither short time systemic, nor intracereberoventricular SP-D treatment altered the hypothalamic signalling or body weight accumulation.In ad libitum fed animals, serum leptin, insulin, and glucose were significantly increased in mice deficient in SP-D, and indicative of insulin resistance. However, restricted diets eliminated all metabolic differences except the distribution of body fat. SP-D deficiency was further associated with elevated levels of systemic bacterial lipopolysaccharide.In conclusion, our findings suggest that lack of SP-D mediates modulation of food intake not directly involving hypothalamic regulatory pathways. The resulting accumulation of adipose tissue was associated with insulin resistance. The data suggest SP-D as a regulator of energy intake and body composition and an inhibitor of metabolic endotoxemia. SP-D may play a causal role at the crossroads of inflammation, obesity, and insulin resistance.

Figure 1. Mean body masses of male mice fed ad libitum for 18 weeks.

A) Spd−/− mice had a higher body mass than their wild-type counterparts. ANOVA of AUC showed a genotype effect (p = 0.005) and a diet effect (p = 0.02). Diet did not influence the body mass difference of genotypes. Data are means ± SE, n = 6. B) Carcass mass of protein, fat and water in 28 week old mice. Protein and fat mass was increased in Spd−/− mice. Genotype effect, * p<0.001 and ** p = 0.038. Data are least square means ± SE, n = 11–12. C) Mean masses of three fat pads in 28 week old mice. The gonadal fat pad mass was similar in the two genotypes. Retroperitoneal fat, subcutaneous fat and the total of the three pads were increased in Spd−/− mice. Genotype effect, * p = 0.001, ** p = 0.03, *** p = 0.047. Data are least square means +/− SE, n = 11–12.

Figure 2. A) Mean body mass gain in male Spd−/− and wild-type mice fed on a 80% energy restricted diet for 18 weeks.

ANOVA of body mass gain after 18 weeks did not show any genotype difference. B) Mean body masses of Spd−/− and wild-type mice fed a 95% restricted low fat diet. The body masses in Spd−/− and wild-type mice were equal after 25 weeks of diet treatment. C) The masses of three fat pads from 32 week old mice fed a 95% restricted low fat diet. The total mean mass of the three pads and the retroperitoneal pads was equal in the two genotypes. The gonadal fat pad mass was decreased and the subcutaneous pad was increased in Spd−/− mice. * p = 0.003, ** p = 0.008. Data are means + or − SE, n = 6.

Figure 3. Average energy intake in Spd−/− and wild-type mice.

At age 7–17 weeks energy intake was significantly higher in Spd−/− mice, but from age 18 to 25 the energy intake was identical. Two-way anova, genotype effect, * p = 0.02. Data are least square means + SE, n = 12.

Figure 4. Heat exchange in 28 week old Spd−/− and wild-type mice.

A) Heat exchange per animal was not significantly different between genotypes. B) Nutrient oxidation in ad libitum fed mice. Protein oxidation showed a borderline genotype effect, * p = 0.049. C) Nutrient oxidation in restricted fed mice. Protein oxidation showed a genotype effect, * p = 0.035. Data are least square means + SE, n = 11–12.

Figure 5. Linear association between ln(leptin) levels and mouse body weights.

Mice from different feeding groups show linear relation between systemic leptin levels and body weights (Spd−/−, wild type, high fat, low fat) when fed with 80% energy restriction or ad libitum.

Figure 6. Serum endotoxin levels.

Serum endotoxin levels were significantly increased in 16 SP-D deficient mice compared to 15 wild-type mice during the light phase period and fed a standard chow ad libitum. * p<0.05. Data were logaritmically transformed to obtain normality before testing. Data are means + standard deviations.