Differences in tumor necrosis factor (TNF)α and TNF receptor-1-mediated intracellular signaling factors in normal, inflamed and scar-formed horse tendons

Tumor necrosis factor (TNF) receptors (TNF-R)-mediated cell survival or apoptosis has been demonstrated in many cells, but little is known about survival or apoptotic signals via TNF-R I in tendinocytes. In this study, we focused on four signaling factors, TNFU, TNF-R1, TNFR-associated factor2 (TRAF2) and caspase-3, in order to elucidate the signaling events in tendinocytes. Samples were obtained from normal, inflamed and scar-formed equine superficial digital flexor tendons. To detect these signaling factors, samples were subjected to immunohistochemistry and Western blot analysis, and some samples were also subjected to reverse transcription-polymerase chain reaction (RT-PCR), PCR-Southern blot analysis and in situ hybridization to detect the expression of TNF alpha mRNA. Distribution of the four factors differed depending on the tendon condition, normal, inflamed or scar-formed. In the normal tendon, large amounts of TRAF2 were found in tendinocytes, but the amounts of TNF-R I were small. TNFa mRNA was expressed most highly in the inflamed tendon. TNF-R1, which was only faintly detected in the normal tendon, was detected at a high level in the inflamed tendon, and the amounts of TRAF2 and caspase-3 also increased. Activated caspase-3 was only detected in the inflamed tendon. TNFa mRNA was also expressed in the scar-formed tendon, though it showed weak signals, and the expression levels of TNF-R I, TRAF2 and caspase-3 proteins were very low. Two distinct intracellular signaling pathways of TNF alpha, which lead to cell survival and apoptosis, might be present in tendinocytes mediated through TNF-R1. These results, which reflect the dynamism of TNF alpha, provide important clues for means to prevent tendinopathy.