Expression of apoptosis regulatory proteins in the skeletal muscle of tumor-bearing rabbits.

We reported finding that apoptosis occurred in skeletal muscle in the early stage after implantation. In the present study, we investigated expression of the apoptosis-related proteins Bax and Bcl-2 to determine the mechanism of the apoptosis. In the early stage of tumor bearing, 20 days after implantation, lean body mass (LBM) was reduced by 5.06 +/- 1.10% in the tumor-bearing group, compared with an increase of 4.96 +/- 1.26% in the control group. The apoptotic index (AI) of the skeletal muscle in the tumor-bearing group increased to 40.5 +/- 3.20% but was 0% in the control group, and Bax expression was strongly positive in 5 of the 10 rabbits in the tumor-bearing group, and significantly stronger than in the control group (P = 0.0002). In the late stage of tumor bearing, 40 days after implantation, the AI had declined to 0.93 +/- 0.96% in the tumor-bearing group, but was still 0% in the control group. Bax expression was rarely detected in either the tumor-bearing group or the control group, and there was no significant difference between the two groups (P = 0.706). No significant changes in Bcl-2 were observed in either group. The above results showed that apoptosis via Bax played a role in muscle wasting associated with progression of the malignant tumor. However, the apoptosis and expression of Bax were seen only in the early stage, within 20 days after implantation, not in the late stage. This suggested that the muscle wasting in the early stage might be caused by a different mechanism from that in the late stage.