Caspase-1 deficiency in mice reduces intestinal triglyceride absorption and hepatic triglyceride secretion

Abstract

Caspase-1 is known to activate the proinflammatory cytokines IL-1β and IL-18. Additionally, it can cleave other substrates, including proteins involved in metabolism. Recently, we showed that caspase-1 deficiency in mice strongly reduces high-fat diet-induced weight gain, at least partly caused by an increased energy production. Increased feces secretion by caspase-1-deficient mice suggests that lipid malabsorption possibly further reduces adipose tissue mass. In this study we investigated whether caspase-1 plays a role in triglyceride-(TG)-rich lipoprotein metabolism using caspase-1-deficient and wild-type mice. Caspase-1 deficiency reduced the postprandial TG response to an oral lipid load, whereas TG-derived fatty acid (FA) uptake by peripheral tissues was not affected, demonstrated by unaltered kinetics of [(3)H]TG-labeled very low-density lipoprotein (VLDL)-like emulsion particles. An oral gavage of [(3)H]TG-containing olive oil revealed that caspase-1 deficiency reduced TG absorption and subsequent uptake of TG-derived FA in liver, muscle, and adipose tissue. Similarly, despite an elevated hepatic TG content, caspase-1 deficiency reduced hepatic VLDL-TG production. Intestinal and hepatic gene expression analysis revealed that caspase-1 deficiency did not affect FA oxidation or FA uptake but rather reduced intracellular FA transport, thereby limiting lipid availability for the assembly and secretion of TG-rich lipoproteins. The current study reveals a novel function for caspase-1, or caspase-1-cleaved substrates, in controlling intestinal TG absorption and hepatic TG secretion.

Fig. 1.

Caspase-1 deficiency in mice reduces the postprandial lipid response. Overnight-fasted WT and caspase-1^−/− mice received an intragastric olive oil gavage, and blood samples were drawn at the indicated time points. Plasma TG (A) and FFA (B) concentrations were determined in WT mice (open circles) and caspase-1^−/− mice (closed circles). Values are means ± SD (n = 8). *P < 0.05; **P < 0.01.

Fig. 2.

Caspase-1^−/− mice have normal clearance of VLDL-like TG-rich particles. WT and caspase-1^−/− mice were fasted for 4 h and received an intravenous bolus of [³H]TO and [¹⁴C]CO-double labeled TG-rich emulsion particles (1 mg TG per mouse) (n = 7–8). Blood was collected at the indicated time points, and ³H (A) and ¹⁴C (C) activities were determined in plasma of WT mice (open circles) and caspase-1^−/− mice (closed circles). At 15 min after injection, organs were isolated, and uptake of ³H (B) and ¹⁴C (D) activity by the organs was measured. Values are means ± SD. *P < 0.05.

Fig. 3.

Caspase-1 deficiency in mice reduces intestinal TG production. Overnight-fasted WT and caspase-1^−/− mice received an intragastric load of 200 µL olive oil containing [³H]triolein ([³H]TO) (n = 6–7). Blood samples were drawn at the indicated time points, and ³H activity was determined in plasma of WT mice (open circles) and caspase-1^−/− mice (closed circles) (A). At 2 h after the oral lipid load, organs were collected, and uptake of ³H activity was measured (B). Intestine was isolated and washed twice to remove intestinal luminal nonabsorbed olive oil. ³H activity in the intestinal tissue (C) and the nonabsorbed ³H activity in the lumen (D) were determined. Feces was collected from individual housed WT and caspase-1^−/− mice (n = 8), and fecal fatty acids were derivatized and extracted followed by fatty acid separation and quantification using gas chromatography. Individual fatty acids were determined (E), and total fatty acid content (µmol/g feces) was calculated from the sum of all individual fatty acids (F). Values are means ± SD. *P < 0.05; ***P < 0.001.

Fig. 4.

Caspase-1 deficiency in mice decreases VLDL-TG production without affecting VLDL-apoB production and increases hepatic steatosis. WT and caspase-1^−/− mice were fasted for 4 h and received injections of Tran³⁵S to label newly synthesized protein and Triton WR1339 to block lipolysis (n = 8). Blood samples were drawn at the indicated time points, and TG concentrations were determined in plasma of WT mice (open circles) and caspase-1^−/− mice (closed circles) and plotted as the increase in plasma TG relative to t = 0 (A). TG production rates were determined by linear regression analysis. At 120 min, mice were exsanguinated, and VLDL was isolated and assayed for ³⁵S-apoB (B). Livers were obtained from fed and 4 h-fasted WT and caspase-1^−/− mice, and lipids were extracted (n = 3). Triglycerides (TG) (C) and total cholesterol (TC) (D) were measured and expressed per mg protein. Plasma FFA levels (E) were measured in plasma from fed and 4 h-fasted WT and caspase-1^−/− mice. Values are means ± SD. *P < 0.05; ***P < 0.001.