Possible involvement of IGF-1 signaling on compensatory growth of the infraspinatus muscle induced by the supraspinatus tendon detachment of rat shoulder

Abstract

A rotator cuff tear (RCT) is a common musculoskeletal disorder among elderly people. RCT is often treated conservatively for functional compensation by the remaining muscles. However, the mode of such compensation after RCT has not yet been fully understood. Here, we used the RCT rat model to investigate the compensatory process in the remaining muscles. The involvement of insulin-like growth factor 1 (IGF-1)/Akt signaling which potentially contributes to the muscle growth was also examined. The RCT made by transecting the supraspinatus (SSP) tendon resulted in atrophy of the SSP muscle. The remaining infraspinatus (ISP) muscle weight increased rapidly after a transient decrease (3 days), which could be induced by posttraumatic immobilization. The IGF-1 mRNA levels increased transiently at 7 days followed by a gradual increase thereafter in the ISP muscle, and those of IGF-1 receptor mRNA significantly increased after 3 days. IGF-1 protein levels biphasically increased (3 and 14 days), then gradually decreased thereafter. The IGF-1 protein levels tended to show a negative correlation with IGF-1 mRNA levels. These levels also showed a negative correlation with the ISP muscle weight, indicating that the increase in IGF-1 secretion may contribute to the ISP muscle growth. The pAkt/Akt protein ratio decreased transiently by 14 days, but recovered later. The IGF-1 protein levels were negatively correlated with the pAkt/Akt ratio. These results indicate that transection of the SSP tendon activates IGF-1/Akt signaling in the remaining ISP muscle for structural compensation. Thus, the remaining muscles after RCT can be a target for rehabilitation through the activation of IGF-1/Akt signaling.

Keywords: Animal study; functional compensation; rotator cuff tear.

Figure 1.

Schematic showing the surgical procedure. Surgical transection of the supraspinatus tendon was performed in the left shoulder. (A) A skin incision was made to expose the rotator cuff tendons. (B) The supraspinatus tendon was exposed by splitting the deltoid muscle (trapezius muscle is not shown in this figure for clarity). (C) The supraspinatus tendon was partly removed from the greater tuberosity of the humerus. For clarity, deltoid muscle, acromion, and clavicle are not shown in this figure.

Figure 2.

Ratio of the SSP (A) or ISP (B) muscle weight to whole body weight. Data are shown as mean ± SEM. (A) The ratio of the SSP muscle weight significantly decreased at 14 days compared with that at 0 day. This decrease was maintained until 84 days. (B) The weight ratio of the ISP muscle was significantly higher at 28, 56, and 84 days than at 3 days. *significant versus 0 day (P < 0.05). ^†significant versus 3 days (P < 0.05).

Figure 3.

Changes in IGF‐1 and IGF‐1 receptor mRNA levels in the ISP muscle. (A) Representative amplified PCR bands visualized by ethidium bromide staining. (B) Change in relative levels of IGF‐1 mRNA. Data are expressed as arbitrary units, which were normalized with the corresponding density of GAPDH mRNA. The levels were significantly higher at 84 days than at 0 day, and those were also significantly higher at 7, 56, and 84 days than at 3 days. (C) Change in relative levels of IGF‐1 receptor mRNA. IGF‐1 receptor mRNA levels were significantly higher at 3 days following surgery than at 0 day. (D) The IGF‐1 mRNA and the weight ratio of the ISP muscle tended to show a positive correlation, but it was not statistically significant (r = 0.331, P = 0.052). *significant versus 0 day (P < 0.05). ^†significant versus 3 days (P < 0.05).

Figure 4.

IGF‐1 protein levels measured by ELISA and correlation with IGF‐1 mRNA levels. (A) IGF‐1 protein levels were significantly higher at 3 days than at 0, 7, 28, 56, and 84 days. The levels were also significantly higher at 14 days than at 0 day. (B) The IGF‐1 mRNA and IGF‐1 protein levels tended to show a negative correlation, but it was not statistically significant (r = −0.313, P = 0.067). (C) The IGF‐1 protein levels and the weight ratio of the ISP muscle were negatively correlation (r = −0.572, P < 0.01). *significant versus 0 day (P < 0.05). ^†significant versus 3 days (P < 0.05).

Figure 5.

Ratio of pAkt/Akt levels and its correlation with IGF‐1 protein levels. (A) Representative Western blot results of Akt and pAkt. Western blot analysis was carried out using rabbit monoclonal antibodies against Akt or pAkt. (B) Expression ratio of pAkt/Akt in ISP muscle. The ratios at were significantly lower 14 days than at 0 day. (C) IGF‐1 protein levels and pAkt/Akt ratio were negatively correlated (r = −0.499, P = 0.002). *significant versus 0 day (P < 0.05).