In most living tissues, fibrosis is an essential wound-healing response to noxious stimuli. If excessive, however, it can disrupt the normal function of affected organs. For subconjunctival fibrosis, TGF-β plays a crucial role and causes the transdifferentiation of Tenon’s fibroblasts to myofibroblasts and the subsequent production of extracellular matrix including collagen from the transdifferentiated myofibroblasts [2–5].
Hsp47 is a 47-kDa glycoprotein and specialized molecular chaperone in collagen biosynthesis. In the ER of any collagen-producing cells, Hsp47 binds to the triple helical procollagen and stabilizes its higher-order structure. It prevents the premature secretion of procollagens from the ER into the Golgi apparatus and concentrates them within the ER before finishing their maturation process [6–11]. The importance of Hsp47 in normal development was documented by Nagai et al. . Using Hsp47 knockout mice, they showed that disruption of Hsp47 caused severe deficiencies in collagen maturation and organogenesis. The Hsp47 null embryos did not develop beyond 11.5 days postcoitus. In some animal models, siRNAs targeting Hsp47 effectively reduced experimental fibrosis [25, 26].
The majority of previous studies have focused on the role of Hsp47 in collagen synthesis. However, in the present study using TGF-β-stimulated human Tenon’s fibroblasts in vitro, we noticed another putative role of Hsp47 in the fibrotic process. The silencing of Hsp47 with specific siRNA molecules significantly attenuated the expression of αSMA that was induced by TGF-β1 treatment. Our data imply that Hsp47 may be involved in the initial stage of fibrosis, the transdifferentiation of stressed fibroblasts to myofibroblasts, as well as the later stage of fibrosis, the collagen synthesis in already transdifferentiated myofibroblasts. Since the transdifferentiation of fibroblasts to myofibroblasts is thought to be a vital initial step for the whole fibrotic process, if Hsp47 directly or indirectly participates in this step, controlling its action might be a novel therapeutic strategy for patients suffering from excessive fibrosis. Actually, the Hsp47-associated transdifferentiation signaling may be not the major pathway in pathologic fibrotic processes. However, due to the fibrosis is the essential wound healing processes, its partial modification using Hsp47 pathway rather than complete suppression might be a better option to control the postoperative inflammation. To better understand the precise working mechanisms of Hsp47 in transdifferentiation of fibroblasts to myofibroblasts, the further investigations are needed.
Regarding expression of Hsp47 itself, our findings differ from previous reports that documented increased expression of Hsp47 in various extraocular fibrotic diseases [12–21]. Increased expression of Hsp47 was even reported in ocular cicatrical pemphigoid . However, in this in vitro study using human Tenon’s fibroblasts, TGF-β1 increased the mRNA expression of Hsp47 but did not influence its protein expression. Data of immunofluorescence staining ( Additional file 2) support the results of western immunoblots. Though fibroblasts have been considered universal cells, they have different characteristics depending on their origin . Their response to TGF- β1 and expression pattern of Hsp47 are able to be tissue-specific and/or disease-specific. In addition, a gap between mRNA and protein levels of Hsp47 is assumed to be caused by the post-transcriptional modulation.