ABSTRACT
Vitamin D deficiency leads to muscle wasting in both animals and humans. A vitamin D-deficient rat model was created using Sprague Dawley male rats. We studied the involvement of the ubiquitin proteasome and other proteolytic pathways in vitamin D deficiency-induced muscle atrophy. To delineate the effect of hypocalcemia that accompanies D deficiency, a group of deficient rats was supplemented with high calcium alone. Total protein degradation in muscle was assessed by release of tyrosine; proteasomal, lysosomal, and calpain enzyme activities were studied using specific substrates by fluorometry, and E2 enzyme expression was assessed by Western blot analysis. Muscle histology was done by myosin ATPase staining method, whereas 3-methylhistidine in the urine was estimated using HPLC. Muscle gene expression was measured by semiquantitative RT-PCR. Total protein degradation in muscle and the level of 3-methylhistidine in urine were increased in the deficient group compared with the control group. Proteasomal enzyme activities, expression of the E2 ubiquitin conjugating enzyme, and ubiquitin conjugates were increased in the deficient group compared with controls. On the other hand, lysosomal and calpain activities were not altered. Type II fiber area, a marker for muscle atrophy, was decreased in the deficient muscle compared with control muscle. Muscle atrophy marker genes and proteasomal subunit genes were up-regulated, whereas myogenic genes were down-regulated in D-deficient muscle. From the results it appears that the ubiquitin proteasome pathway is the major pathway involved in vitamin D deficiency-induced muscle protein degradation and that calcium supplementation alone in the absence of vitamin D partially corrects the changes.
