Increased expression of matrix metalloproteinase-1 in osteocytes precedes bone resorption as stimulated by disuse: evidence for autoregulation of the cell's mechanical environment?

An in vivo animal model of bone adaptation was used to examine a possible role for matrix metalloproteinase-1 in the local mediation of bone remodeling: to corrode the coupling of osteocytes to the matrix in an attempt to autoregulate the cell's perception of its mechanical environment. Twelve young (12-16 months old) skeletally mature turkeys were separated into groups to be studied for stimulus periods of either 3 or 30 days. In each animal, the left ulna was functionally isolated and subjected to either disuse or 3,000 microstrain at 1 Hz for 10 minutes per day. The right ulna remained intact and served as an intra-animal control. No significant differences in bone area were detected at 3 days; however, ulnae subjected to disuse lost 8 +/- 4% (+/-SD) of bone area by 30 days. Over the same period, ulnae subjected to the mechanical stimulus gained 21 +/- 9% of bone area. With use of in situ reverse transcription-polymerase chain reaction, less than 2% of the osteocytes examined from the intact control ulnae stained positively for matrix metalloproteinase-1 mRNA. An antibody raised against matrix metalloproteinase-1 revealed no positively labeled osteocytes in the intact ulnae. This low percentage of osteocytes expressing matrix metalloproteinase-1 mRNA was similar to that seen in ulnae subjected to the osteogenic mechanical stimuli. In contrast, ulnae subjected to either 3 or 30 days of disuse showed evidence of matrix metalloproteinase-1 mRNA activity in a high percentage of osteocytes (89 +/- 5 and 66 +/- 8%, respectively; each time point significantly different from intact ulnae, as well as from each other, p < 0.05). The percentage of osteocytes labeled with the anti-matrix metalloproteinase-1 antibody was also highly elevated following 3 days of disuse (74 +/- 17%). These data demonstrate that an early response of bone to disuse is the upregulation of matrix metalloproteinase-1 activity in osteocytes. It is proposed that this upregulation of collagenase activity is indicative of the cell's degradation of coupling to the matrix, and it thus reflects the osteocyte's regulation of its own mechanical environment. We believe that such autoregulation of the osteocyte's physical environment will accommodate subtle changes in the bone's functional environment without the need to add or resorb bone tissue.