Racts. (A) Normal flexor tendons (n = 19) showed a significant negative correlation between PGE2 levels and increasing age (P = 0.01, r2 = 0.31). (B) Injured tendons (n = 15) comprised of both sub-acute and chronic injuries showed a significant positive correlation (dotted line) between PGE2 levels and increasing age (P = 0.03, r2 = 0.3) although when separated for injury stage, neither sub-acute nor chronic injuries were significant in isolation (solid lines). SAI = sub-acute injury (3? weeks post injury, n = 6), CI = chronic injury (.3 months post injury, n = 9). doi:10.1371/journal.pone.0048978.ginduced 1531364 higher levels of LXA4 release compared to 0.01 mM PGE2 and IL-1b (Fig. 8, P = 0.32).DiscussionProstaglandins such as PGE2 are produced by tenocytes and other fibroblasts in response to injury and after stimulation with pro-inflammatory cytokines [21,22], initiating MMP mediated catabolism of tendon ECM [40]. Although seemingly destructive to the local tissue architecture, this process facilitates clearance of cellular debris and debridement of the affected ECM as described for wound healing in other connective tissues. Prostaglandins may also exert beneficial regulatory actions in healthy tissues maintaining normal physiologic processes such as local bone remodelling [41] and modification of renal blood flow [42]. Furthermore, their presence following injury signals the onset of lipoxin mediated resolution processes, such that the duration and magnitude of the inflammatory response can be regulated to restrict the degree of tissue damage [33]. Thus prostaglandins can be said to possess `double-edged sword’ properties in terms of their dichotomous roles in wound healing processes. The extent to which these properties play a role in tendinopathies with injury and repair stage remains unclear.In the current study, PGF2a levels were unchanged with injury and were substantially lower than PGE2 levels in normal tendons. This may imply differential regulation of these prostaglandins in tendon, with PGF2a less susceptible to changes with injury suggesting PGE2 is the main prostaglandin operative in tendon injury. PGE2 levels were found to decrease with aging in normal tendons. This could be a 298690-60-5 consequence of the reduction in tendon cellularity with increasing age [43,44] leading to a decreasing tendon prostaglandin synthetic capacity. Alternatively, it may be related to a lack of PGE2 synthesising pro-inflammatory macrophages as we have described previously for uninjured tendons [16]. The MedChemExpress AN-3199 relationship between age and the pattern of PGE2 levels was difficult to determine in injured tendons because of the confounding issue that sub-acute injury predominated in younger horses compared to chronic injury, which occurred with greater frequency in older individuals. However, the positive correlation between increasing tendon PGE2 levels with age in injured horses could be attributable 1317923 to a greater PGE2 synthetic capacity both by increased tendon fibroblast cellularity and infiltration of proinflammatory macrophages into injured regions of tendon [16]. This was supported by the increase in mPGES-1:PGDH ratio in sub-acute injury which suggests an interplay between PGE2 synthesis and degradation could lead to an increased syntheticFigure 4. mPGES-1 and PGDH expression in normal and injured tendons. mPGES-1 and PGDH mRNA expression were normalised to GAPDH or 18S ribosomal RNA and are shown expressed as mPGES-1: PDGH ratio in each case. (A) Median values.Racts. (A) Normal flexor tendons (n = 19) showed a significant negative correlation between PGE2 levels and increasing age (P = 0.01, r2 = 0.31). (B) Injured tendons (n = 15) comprised of both sub-acute and chronic injuries showed a significant positive correlation (dotted line) between PGE2 levels and increasing age (P = 0.03, r2 = 0.3) although when separated for injury stage, neither sub-acute nor chronic injuries were significant in isolation (solid lines). SAI = sub-acute injury (3? weeks post injury, n = 6), CI = chronic injury (.3 months post injury, n = 9). doi:10.1371/journal.pone.0048978.ginduced 1531364 higher levels of LXA4 release compared to 0.01 mM PGE2 and IL-1b (Fig. 8, P = 0.32).DiscussionProstaglandins such as PGE2 are produced by tenocytes and other fibroblasts in response to injury and after stimulation with pro-inflammatory cytokines [21,22], initiating MMP mediated catabolism of tendon ECM [40]. Although seemingly destructive to the local tissue architecture, this process facilitates clearance of cellular debris and debridement of the affected ECM as described for wound healing in other connective tissues. Prostaglandins may also exert beneficial regulatory actions in healthy tissues maintaining normal physiologic processes such as local bone remodelling [41] and modification of renal blood flow [42]. Furthermore, their presence following injury signals the onset of lipoxin mediated resolution processes, such that the duration and magnitude of the inflammatory response can be regulated to restrict the degree of tissue damage [33]. Thus prostaglandins can be said to possess `double-edged sword’ properties in terms of their dichotomous roles in wound healing processes. The extent to which these properties play a role in tendinopathies with injury and repair stage remains unclear.In the current study, PGF2a levels were unchanged with injury and were substantially lower than PGE2 levels in normal tendons. This may imply differential regulation of these prostaglandins in tendon, with PGF2a less susceptible to changes with injury suggesting PGE2 is the main prostaglandin operative in tendon injury. PGE2 levels were found to decrease with aging in normal tendons. This could be a consequence of the reduction in tendon cellularity with increasing age [43,44] leading to a decreasing tendon prostaglandin synthetic capacity. Alternatively, it may be related to a lack of PGE2 synthesising pro-inflammatory macrophages as we have described previously for uninjured tendons [16]. The relationship between age and the pattern of PGE2 levels was difficult to determine in injured tendons because of the confounding issue that sub-acute injury predominated in younger horses compared to chronic injury, which occurred with greater frequency in older individuals. However, the positive correlation between increasing tendon PGE2 levels with age in injured horses could be attributable 1317923 to a greater PGE2 synthetic capacity both by increased tendon fibroblast cellularity and infiltration of proinflammatory macrophages into injured regions of tendon [16]. This was supported by the increase in mPGES-1:PGDH ratio in sub-acute injury which suggests an interplay between PGE2 synthesis and degradation could lead to an increased syntheticFigure 4. mPGES-1 and PGDH expression in normal and injured tendons. mPGES-1 and PGDH mRNA expression were normalised to GAPDH or 18S ribosomal RNA and are shown expressed as mPGES-1: PDGH ratio in each case. (A) Median values.
