Muscle wasting and interleukin-6-induced atrogin-I expression in the cachectic Apc ( Min/+ ) mouse

Abstract

Interleukin-6 (IL-6) is necessary for cachexia in Apc ( Min/+ ) mice, but the mechanisms inducing this myofiber wasting have not been established. The purpose of this study was to examine gastrocnemius muscle wasting in the Apc ( Min/+ ) mouse and to determine IL-6 regulated mechanisms contributing to muscle loss. Gastrocnemius type IIB mean fiber cross-sectional area (CSA) from Apc ( Min/+ ) mice decreased 32% between 13 and 22 weeks of age. Apc ( Min/+ ) mice lacking IL-6 did not have type IIB fiber atrophy, while overexpression of circulating IL-6 exacerbated the loss of type IIB fiber CSA in Apc ( Min/+ ) mice. Muscle Atrogin-I mRNA expression was induced at least ninefold at 18 and 22 weeks of age compared to 13-week-old mice. Atrogin-I gene expression was also induced by overexpression of circulating IL-6. These data suggest that high circulating IL-6 levels induce type IIB fiber CSA loss in Apc ( Min/+ ) mice, and circulating IL-6 is sufficient to regulate Atrogin-I gene expression in cachectic mice.

Fig. 1

Time course of gastrocnemius muscle wasting in Apc^Min/+ mice. Gastrocnemius muscle mass (a), mean gastrocnemius fiber CSA of type IIA and type IIB fibers (b), frequency distribution for type IIB gastrocnemius fibers (c), and frequency distribution of type IIA fibers (d). Muscles were harvested from 13-wk-old (n=13), 18-wk-old (n=11), and 22-wk-old (n=7) Apc^Min/+ mice. Values are means ± SE. *Signifies different from 13-wk-old mice.

Fig. 2

Gene and protein expression related to protein degradation in cachectic Apc^Min/+ mouse gastrocnemius muscle. IL-6 mRNA (a) and STAT-3 phosphorylation (b) were indicators of IL-6 signaling. MuRF-I mRNA (c) and Atrogin-I mRNA (d) were measured in 13-, 18-, and 22-wk-old Apc^Min/+ mice. Atrogin-I protein levels were also elevated during cachexia (e). There was a significant correlation (r=-0.675; P=0.002) between Atrogin-I mRNA and gastrocnemius muscle mass (f). Values are means ± SE. *Signifies different from 13-wk-old mice.

Fig. 3

Gastrocnemius fiber CSA in 26-week-old Apc^Min/+ mice lacking IL-6. Mean gastrocnemius type IIA and type IIB fiber CSA (a) in 26-week old C57BL/6, Apc^Min/+, and Apc^Min/+ / IL-6^-/- mice. Type IIB (b) and type IIA (c) fiber frequency distributions in C57BL/6, Apc^Min/+, and Apc^Min/+ / IL-6^-/- mice. Values are means ± SE. *Signifies both C57BL/6 and Apc^Min/+ / IL-6^-/- different from Apc^Min/+ mice.

Fig. 4

Over-expression of circulating IL-6 in Apc^Min/+ mice. Mean gastrocnemius type IIA and type IIB fiber CSA (a) and fiber frequency distributions of type IIB (b) and type IIA fibers (c) following 4 weeks of increased IL-6. Values are means ± SE. *Signifies different from Apc^Min/+ Control.

Fig. 5

Atrogin-I and MuRF-I mRNA and protein expression in gastrocnemius muscle from mice over-expressing circulating IL-6. Atrogin-I and MuRF-I gene expression during increased circulating IL-6 in cachectic Apc^Min/+ mice (a). There was a positive correlation between Atrogin-I mRNA levels and plasma IL-6 in Apc^Min/+ mice (b). Atrogin-I protein levels during IL-6 over-expression in Apc^Min/+ mice (c). Atrogin-I and MuRF-I gene expression during increased circulating IL-6 in non-cachectic, wild-type mice (d). Atrogin-I protein levels during IL-6 over-expression in C57BL/6 mice (e). Values are means ± SE. *Signifies different from Control.

Fig. 6

Atrogin-I mRNA and protein expression in 6-mo-old Apc^Min/+ and Apc^Min/+ / IL-6^-/- mice. Atrogin-I mRNA (a) and Atrogin-I protein (b) in 6-mo-old cachectic (Apc^Min/+) and non-cachectic (Apc^Min/+ / IL-6^-/-) mice. Values are means ± SE.