The UPR has varied functions depending upon cell type. It is most active in what have been termed, “professional secretory cells.” These are cells that have a highly active protein translation machinery and include hepatocytes, adipocytes, oligodendrocytes, and pancreatic β-cells.10, 15, 22
With the increase in protein translation comes increased amounts of improperly folded proteins that can have a negative impact on cellular function. The UPR is up-regulated in these cells as a protective mechanism and its dysregulation has been implicated in a number of diseases including neurodegenerative disease, obesity, diabetes, and cancer.6, 10, 15
The role of the UPR contributing to the pathogenesis of various forms of cancer led to our interest in its possible role in keloid pathogenesis. Malignant tumors have a tremendous capacity to aggressively divide despite residing in an environment depleted of nutrients and oxygen. It has been noted that despite tumor cells’ physiologic barriers to cell survival, there is a paradoxical drive in malignant progression. Interestingly, it appears as if they are better adapted to carryout cell proliferation despite significant stress.6
There has been a great deal of investigation into the role of the UPR in tumorgenesis.6, 23, 24
These data suggest that there is up-regulation of the UPR as a adaptive response to allow tumors to survive in this relatively nutrient- and oxygen-poor environment.6, 25, 26
Although they lack malignant potential, keloids share many characteristics including persistent growth and the tendency to recur after excision, and are recognized as benign dermal fibroproliferative tumors.1, 4, 17–19
We describe here a differential response of the UPR to hypoxia in KFs compared to NFs. Our PCR data shows that there is activation of the UPR in both KFs and NFs, but this was not done in a quantitiative manner. Immunoblotting directed against the activated form of XBP-1 confirmed that there is increased protein present in KFs. We demonstrated that the increased UPR activation we see was specifically due to up-regulation of XBP-1 activation, and there was no effect on PERK or ATF6 activation. Furthermore, we show that there was a specific contribution from hypoxia that leads to this increase; treatment with tunicamycin, an activator of the UPR through inhibition of glycosylation, does not lead to increased XBP-1 activation. Our findings suggest that the UPR is indeed heightened in KFs compared to NFs and the possibility of it serving as link between the aforementioned aberrant signaling pathways in KFs, deserves further consideration ().
Figure 6 Schematic of the activated Unfolded Protein Response (UPR) signaling cascade. After hypoxia exposure, misfolded proteins in the lumen of the endoplasmic reticulum lead to the activation of the three UPR transducers pancreatic ER kinase (PERK), inositol-requiring-1α/X-box (more ...)
Analysis of KFs have revealed significant up-regulation of a variety of cellular stress signaling pathways including p38 kinase27
, FAK/ERK28, 29
, c-Jun N-terminal kinase (JNK)30, 31
A common upstream link between these various aberrant signaling pathways involved in keloid pathogenesis have yet to be ascertained. The UPR represents a more proximal cell response to stress and is capable of up-regulating several of the signaling pathways mentioned including both JNK and p38 kinase.
There is evidence that KFs have key pathways that are altered when compared to NFs during a number of stress responses. The UPR is a stress response pathway that has been found to be up-regulated in professional secretory cells, and may have a role in keloid’s abnormal stress response signaling. What is not demonstrated by our data is whether the increased activation of IRE-1/XBP-1 is a causative event in keloid formation or secondary to the hypoxic environment of the keloid. Future experiments will use direct agonists of IRE-1/XBP-1 to see if NFs can be made to behave like KFs as a way to address this question.
Another question that is not addressed by this study is when IRE-1/XBP-1 might be up regulated during keloid formation. This would also help in determining whether its up regulation is causative or a consequence of keloids. If IRE-1/XBP-1 is causative, it might be expected that we would see increased activity very early in keloid formation. Unfortunately, we do not have access to keloid specimens timed from their date of onset, but it would be an intriguing experiment to perform is specimens were available.
Many wound healing researchers, including us, believe that keloids are a distinct clinical condition from hypertrophic scars. Although the initial appearance may be similar, the behavior is significantly different with hypertrophic scars limited to the original scar area and spontaneously resolving over time, but with keloids showing progressive growth over time beyond the original scar area. It would be equally informative to study UPR regulation in hypertrophic scars. Our prediction would be that there is no difference between NFs and fibroblasts from hypertrophic scars, but the experiments need to be done to demonstrate this.
There continues to be a clear difference between normal and keloid fibroblast biology that was once again displayed with our UPR data. Continuing the evaluation of stress response signaling in normal and keloid fibroblasts will be imperative to further define keloid behavior and to establish possible targets for more effective therapeutic interventions.