Keratinocyte-releasable stratifin functions as a potent collagenase-stimulating factor in fibroblasts

Termination of wound healing requires a fine balance between collagen deposition and its hydrolysis. To dissect the underlying control mechanisms for this process, we established a keratinocyte/fibroblast co-culture system and subsequently demonstrated more than a 10-fold increase in collagenase expression in fibroblasts co-cultured with keratinocytes relative to that of control cells. This finding was further confirmed in fibroblasts grown in a keratinocyte/fibroblast collagen-GAG gel. The efficacy of keratinocyte-derived collagenase stimulatory factors on collagenase activity was evaluated, and the results showed that only conditioned medium derived from fibroblasts co-cultured with keratinocytes was able to break down markedly type I collagen to its one-quarter and three-quarter fragments of both alpha (alpha1 and alpha2) and beta (beta1.1 and beta1.2) chains. The results of a dose-response experiment showed that keratinocyte-conditioned medium (KCM) stimulates the expression of collagenase mRNA by dermal fibroblasts in a concentration-dependent fashion. In a similar experiment, the results of a time-response experiment revealed that KCM treatment increases the expression of collagenase mRNA in dermal fibroblasts as early as 6 h and reaches its maximum level within 24-48 h. Considering that this keratinocyte-releasable factor has a potent collagenase stimulatory effect on fibroblasts, which favors the resolution of accumulated type I and type III collagen found in fibrotic tissue, we referred to this protein as a keratinocyte-derived anti-fibrogenic factor (KDAF). In a series of chromatography experiments and a direct trypsin digestion of the proteins and subsequent peptide mapping, a keratinocyte-derived collagenase-stimulating factor turned out to be a releasable form of stratifin, also known as 14-3-3 sigma protein. To validate this finding, stratifin cDNA was cloned into a pGEX-6P-1 expressing vector and more than 50 mg of recombinant stratifin was generated and used to treat fibroblasts with various concentrations for 24 h. The results of northern analysis showed a remarkable dose-response increase in the expression of collagenase mRNA in stratifin-treated fibroblasts relative to that of the control. This finding was consistent with that obtained from collagenase activity assay. In conclusion, we identified a keratinocyte-releasable form of stratifin in KCM that mimics the collagenase stimulatory effect of KCM for dermal fibroblasts. This finding suggests that stratifin is likely to be, at least, one of the KDAFs found in KCM.