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ALTERED AUTOPHAGY AND ENDOPLASMIC RETICULUM STRESS RESPONSE TO MECHANICAL OVERLOAD IN CACHECTIC APCMIN MICE

AN Milton
S Sato

Abstract

Alexis N. Milton, Shuichi Sato. University of Louisiana at Lafayette, Lafayette, LA.

BACKGROUND: Cachexia is a pernicious multi-organ syndrome that affects individuals with progressive stages of cancer. Tumors induce a catabolic state, targeting skeletal muscle. The endoplasmic reticulum (ER) and autophagy assist in maintaining skeletal muscle homeostasis. Cancer cells elicit ER stress, triggering unfolded protein response (UPR), but inhibiting ER stress activates protein degradation and autophagy, exacerbating muscle-wasting conditions. UPR mediates mechanical stress-induced adaptation in skeletal muscle; however, the changes in UPR and autophagy in cachexia are unknown. This study examined whether ER stress and autophagy responses to mechanical overload would differ between healthy and cachectic mice. METHODS: ApcMin (Min, n=5) mice and age-matched wild-type (WT, n=5) mice were used. Synergist ablation (SA) was performed on the left leg, whereas the right leg served as an internal control. After seven-day mechanical overload, plantaris muscles were excised. The tissues were homogenized, and routine western blotting was performed using 80~120 µg of the total protein. Student's t-test (WT vs. Min) and two-way ANOVA (genotype x treatment) were used for statistical analysis. The coefficient of determination (r2) was used to examine a significant correlation. RESULTS: Min mice showed smaller body weight (BW, 34.2 ± 1.2 g vs. 25.7 ± 0.7 g, for WT and Min mice, respectively, p<0.05). Min mice lost 9.8% of BW compared to their peak BW. Likewise, control plantaris weight in Min mice was smaller than that of WT mice (15.1 ± 0.3 mg vs. 18.7 ± 0.6 mg, respectively). Mechanical overload increased the plantaris weights, but WT mice displayed more percent change than Min mice (43.3 ± 5.8% vs. 21.3 ± 4.6%, p<0.05). Western blot analysis showed that Min mice had reduced phosphorylated state of p70S6K after mechanical overload than WT mice (1.9-fold vs. 4.2-fold, p<0.05). The levels of phosphorylated eukaryotic translation initiation factor 2 (p-eIF2α) and Beclin, indicators of UPR and autophagy activation, were decreased in Min mice. There was a significant correlation between p70S6K and eIF2α (p<0.01) and between p70S6K and Beclin (p<0.05). The slopes of the regression curve in Min mice were smaller than WT mice (p<0.05). CONCLUSIONS: These results suggest that autophagy and ER stress responses following mechanical overload attribute to a reduction of anabolic response pathways in cachectic mice.

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