Cathepsin S (CTSS) is a cysteine protease that regulates many physiological processes and is increased in obesity and type 2 diabetes. While previous studies show that deletion of CTSS improves glycemic control through suppression of hepatic glucose output, little is known about the role of circulating CTSS in regulating glucose and energy metabolism. We assessed the effects of recombinant CTSS on metabolism in cultured hepatocytes, myotubes and adipocytes, and in mice following acute CTSS administration. CTSS improved glucose tolerance in lean mice and this coincided with increased plasma insulin. CTSS reduced G6pc and Pck1 mRNA expression and glucose output from hepatocytes but did not affect glucose metabolism in myotubes or adipocytes. CTSS did not affect insulin secretion from pancreatic beta-cells, rather CTSS stimulated glucagon-like peptide (GLP)-1 secretion from intestinal mucosal tissues. CTSS retained its positive effects on glycemic control in mice injected the GLP-1 receptor antagonist exendin (9-39) amide. The effects of CTSS on glycemic control were not retained in high-fat fed mice or db/db mice, despite the preservation of CTSS’ inhibitory actions on hepatic glucose output in isolated primary hepatocytes. In conclusion, we unveil a role for CTSS in the regulation of glycemic control via direct effects on hepatocytes, and that these effects on glycemic control are abrogated in insulin resistant states.
Hamzeh Karimkhanloo, Stacey N Keenan, Emily W Sun, David A Wattchow, Damien J Keating, Magdalene K Montgomery, and Matthew J. Watt
Anna G Holmes, Jose L Mesa, Bronwyn A Neill, Jason Chung, Andrew L Carey, Gregory R Steinberg, Bruce E Kemp, Robert J Southgate, Graeme I Lancaster, Clinton R Bruce, Matthew J Watt, and Mark A Febbraio
Chronic elevations in interleukin (IL)-6 have been associated with insulin resistance, but acute IL-6 administration can enhance insulin sensitivity. Our aim was to exogenously administer IL-6 to rats to elicit either chronic or repeated acute elevations in systemic IL-6. We hypothesized that a continuous elevation of IL-6 would inhibit glucose tolerance and insulin sensitivity while acute intermittent elevations would improve it. Male Wistar rats were treated for 14d with recombinant human IL-6 (2.4 μg/day) or saline administered either by miniosmotic pump (continuous IL-6) or via twice-daily injection (intermittent IL-6). Glucose and insulin tolerance tests were performed following 14-d treatment and 24 h later rats were administered a bolus of insulin (150 mU/g) or saline intraperitoneally. Approximately, 10 min after insulin injection soleus, gastrocnemius and liver were excised and rapidly frozen in liquid nitrogen for subsequent metabolic measures. Irrespective of the mode of delivery, IL-6 treatment increased basal insulin sensitivity, as measured by the homeostatic model assessment of insulin resistance, and enhanced glucose clearance during an i.p. glucose tolerance test. IL-6 increased circulating fatty acids, but did not increase triglyceride accumulation in either skeletal muscle or liver, while it increased the protein expression of both PPARα and UCP2 in skeletal muscle, suggesting that IL-6 can enhance fat oxidation via mitochondrial uncoupling. These data demonstrate that, irrespective of the mode of delivery, IL-6 administration over 2 weeks enhances glucose tolerance. Our results do not support the notion that prolonged chronically elevated IL-6 impairs insulin action in vivo.