HSP47: Function

Hsp47 exhibits distinct functions at divers cellular compartments. Hsp47 is an ER-resident unique molecular chaperone crucially involved in the correct folding of procollagen ^1-3. More detailed information on the proposed functions of Hsp47 during procollagen processing will be provided under the section Mechanisms and Interaction. Beside its role in collagen biosynthesis, Hsp47 has been reported to represent an essential factor in the mammalian Golgi stress response ⁴. The Golgi stress response represents an autoregulatory functional part of the Golgi apparatus, which up-regulates Golgi function when it acts insufficiently (i.e. under stressful conditions = Golgi stress). Although the molecular mechanisms underlying the Golgi stress response are less well understood and analyzed in comparison to the ER stress response, current investigations identified at least three main regulatory pathways involved: the TFE3, CREB3, and the Hsp47 pathway (Figure 4). A summarized by Taniguchi et al. (2017), secretory and membrane proteins are subjected to post-translational modifications in the Golgi, such as glycosylation, phosphorylation and sulfation ¹⁵⁸. When the synthesis rate of  these proteins overcomes the Golgi’s capacity, post-translational modifications occur improperly. In order to compensate for this handicap, cells will enhance the expression of modifying enzymes. Certain detectors subsequently induce down-stream transcription factors, culminating in the transcriptional activation of Golgi-related genes ¹⁵⁸.

The Hsp47 pathway represents an essential regulatory part of the mammalian Golgi stress response as it up-regulates the expression of Hsp47, aiming to block Golgi stress-mediated apoptosis. There is a wealth of evidence illustrating that the Golgi apparatus is expanded in mucous cells in the Brunner’s glands that release a large quantity of mucins ¹⁵⁹. As mucins are O-glycosylated in the Golgi, the capacity of the Golgi gets inadequately during the differentiation of mucous cells. Consequently, these cells expand the Golgi apparatus by using the Golgi stress response. Glycosylation is one of the central post-translational events, and the recognition of O-glycans on proteins represents a prerequisite for glycoprotein trafficking ¹⁶⁰. Blockage of glycosylation thus interrups glycoprotein trafficking and acts as an efficient trigger for the Golgi stress response. Benzyl 2-acetamido-2-deoxy-α-d-galactopyranoside (GalNAc-bn), a potent O-glycosylation inhibitor ¹⁶¹, has recently been identified as an inductor of Golgi stress capable of modulating ER function and inducing Hsp47 expression in the ER while down-regulation of Hsp47 led to malfunctioning of the Golgi and, consecutively, apoptosis induction ⁴. Likewise, Hsp47 expression was found as being up-regulated not only by GalNAc-bn, but also by the selective Golgi inhibitor monensin implying a possible role of Hsp47 in regulating Golgi functions, albeit not transferred to the Golgi in the presence of GalNAc-bn ¹⁶². Data raised by the group of Masaya Tohyama led to the following proposal of how Golgi stress induces apoptosis (Figure 5). Briefly, Golgi stress up-regulates Hsp47 expression in the ER and blocks caspase-2 cleavage in the Golgi thus preventing apoptosis-induced cell death. Notably, down-regulation of Hsp47 together with Golgi stress leads to activation of caspase-2 in the Golgi and ER stress sensors related to the unfolded protein response (UPR), e.g. activating transcription factor 6 (ATF-6) and C/EBP homologous protein (CHOP), accompanied by activation of mitochondrial caspase-9 and efflux of cytochrome c from the mitochondria to the cytosol. From these findings it may be concluded that Golgi stress induces Hsp47 expression in the ER, whereas down-regulation of Hsp47 causes Golgi dysfunction which culminates in cell death.  Thus, Golgi stress not only affects Golgi, but also ER and mitochondrial functions and induces cell death through blockage of Hsp47. As collagen dynamics remained unaffected by GalNAc-bn, it might be hypothesized that collagen does not contribute to the progression of apoptosis mediated by Golgi stress.

Beside its function as an ER-resident chaperone involved in collagen biosynthesis in collagen-producing cells ⁵, Hsp47 can be found on the surface of several cell types including activated human platelets and mouse embryo parietal endoderm cells ^7-10. In the latter, membrane-associated Hsp47 was proposed to act as a receptor for extracellular collagens due to its ability to bind to native collagen type IV and gelatin in vitro. Chondrocytic cell lines as well as human oral squamous carcinoma cells were also reported to express Hsp47 on their cell membrane ^{7, 8}. In chondrocytic cells, down-regulation of rheumatoid arthritis-related Hsp47 was reported to induce apoptosis and cell-surface expression of Hsp47 in association with CD9 implying a distinct mechanism of apoptosis induction independent of TNF. Thus, surface-expressed Hsp47 has been proposed to mediate immunoregulatory effects (e.g., induction of autoantibody production and inflammatory reactions) in autoimmune diseases such as rheumatoid arthritis ⁷. Studies by Hebert et al. identified Hsp47 as being anchored to the cell membrane in a complex with CD9 on squamous carcinoma cells where it regulates tumor cell invasion and motility most likely by operating as a serpin inhibitor or collagen receptor ⁸. It is of note that all of the three cell types mentioned synthesize collagen, so it remains an open question whether the role of Hsp47 on the plasma membrane was secondary to its chaperone activity. However, Hsp47 could also be identfied on the surface of activated human platelets where Hsp47 might affect the interaction of platelets with collagen following vessel injury ⁹. Platelets are known anucleate, non-collagen producing cells lacking the classical ER membrane system. They have been found to associate with exposed collagen at sites of vessel wall injury to form thrombi and plug the wound, thus blocking haemorrhaging. In a most recent study, Hsp47 was described to intensify platelet-collagen interactions in the formation of thrombi and hemostasis, highlighting a vital novel mechanism for chaperone protein actions in the extracellular regulation of cell functions ¹⁶³.

The expression of Hsp47 varies with tissue type, developmental stage and stress conditions. While Hsp47 is constitutively expressed under normal conditions, its expression is up-regulated by cellular stress including heat shock. Hsp47 expression levels tightly correlate with that of collagens in numerous tissues and cell types. Moreover, cancer cells often show up-regulated levels of Hsp47 (reviewed by Ibrahim et al., 2018) ¹⁶⁴. The correlation between Hsp47 and cancer might be due to the fact that Hsp47 is a product of the SERPINH1 gene located to chromosome 11q13, a region usually amplified in human cancers ^{165, 166}. Various extracellular matrix (ECM) proteins including collagen have also been demontrated as being expressed in association with Hsp47 to provide necessary biophysical and biochemical signals to drive growth, migration and invasion of cancer cells ¹⁶⁷. TGF-β signaling obviously seems to play a crucial role in the regulation of the Hsp47 expression, since the TGF-β signaling pathway conquers Hsp47-induced carcinogenesis and stemness of glioblastoma ¹⁶⁸.