Expression of interleukin-15 in human skeletal muscle effect of exercise and muscle fibre type composition

Abstract

The cytokine interleukin-15 (IL-15) has been demonstrated to have anabolic effects in cell culture systems. We tested the hypothesis that IL-15 is predominantly expressed by type 2 skeletal muscle fibres, and that resistance exercise regulates IL-15 expression in muscle. Triceps brachii, vastus lateralis quadriceps and soleus muscle biopsies were obtained from normally physically active, healthy, young male volunteers (n = 14), because these muscles are characterized by having different fibre-type compositions. In addition, healthy, normally physically active male subjects (n = 8) not involved in any kind of resistance exercise underwent a heavy resistance exercise protocol that stimulated the vastus lateralis muscle and biopsies were obtained from this muscle pre-exercise as well as 6, 24 and 48 h post-exercise. IL-15 mRNA levels were twofold higher in the triceps (type 2 fibre dominance) compared with the soleus muscle (type 1 fibre dominance), but Western blotting and immunohistochemistry revealed that muscle IL-15 protein content did not differ between triceps brachii, quadriceps and soleus muscles. Following resistance exercise, IL-15 mRNA levels were up-regulated twofold at 24 h of recovery without any changes in muscle IL-15 protein content or plasma IL-15 at any of the investigated time points. In conclusion, IL-15 mRNA level is enhanced in skeletal muscles dominated by type 2 fibres and resistance exercise induces increased muscular IL-15 mRNA levels. IL-15 mRNA levels in skeletal muscle were not paralleled by similar changes in muscular IL-15 protein expression suggesting that muscle IL-15 may exist in a translationally inactive pool.

Figure 1. IL-15 in triceps, vastus lateralis and soleus

IL-15 mRNA levels (RT-PCR) (A) and IL-15 protein levels (Western blotting) (B) are shown for triceps (T), vastus lateralis (V) and soleus (S) muscles (n= 14). Means ±s.e.m. are shown; * shows significant difference between triceps and soleus (P < 0.001).

Figure 2. IL-15 expression

A, IL-15 expression is shown for triceps (a), vastus lateralis (c) and soleus (e) muscles. Myofibrillar ATPase staining is shown in neighbouring tissue sections of the triceps (b), vastus lateralis (d) and soleus (f) muscles (b, d and f are consecutive sections to those shown in a, c and e). Fibre types are distinguished by ATPase staining and fibres appearing black are type 2 fibres. The asterisks depict matching fibres for each muscle group (n= 7). As displayed, IL-15 protein expression is comparable in muscle fibre types and in the different muscles of triceps, vastus lateralis and soleus. Scale bar: 89 μm. B, IL-15 expression as defined by our standard IL-15 IHC is shown in a, while negative control sections are shown in b and c. Bb, negative control section as seen after the primary antibody was pre-absorbed with its corresponding antigenic protein, thereby preventing the following IHC binding of primary IL-15 antibodies to endogenous IL-15 proteins of the muscle. Bc, negative control section after incubation in the absence of primary IL-15 antibodies, whereby the IHC results in negative immunostaining. Scale bar: 47 μm

Figure 3. IL-15 in resistance exercise

IL-15 mRNA levels (A) and IL-15 protein levels (Western blotting) (B) are shown for vastus lateralis muscle biopsies obtained before, as well as 6, 24 and 48 h post one bout of resistance exercise. Means ±s.e.m. are shown; * shows significant difference from pre-exercise value (P < 0.05).