**7. Role of furin in skin cancer**

Low doses of UV radiation while they are not inflammatory, can cause mutations in skin tissue [9, 19, 30] or in cultured fibroblasts [150]. Constant exposure to UV radiation will not only result in mutations in p53 [150, 151] but in other genes including p16, PTCH, B-Raf, which will result in these cells becoming carcinogenic [6, 152, 153]. Apart from causing these mutations, UV radiation enhances the release of inflammatory molecules (e.g. TNFα, IL-1α, ROS, chemokines) from the skin (see section 2 for details). The result of which is the creation of an environment that allows for these mutated skin cells to become cancerous over time [15, 19, 21, 46, 56]. As a result of increased levels of MMP activity on the cell membrane, either as a result of UV exposure or other factors, these skin cancer cells may become metastatic as a result of epithelial to mesenchymal transition (EMT) [154].

In normal skin, MMPs are not constitutively expressed but can be induced temporarily in response to exogenous signals such as UVR [10, 25, 143]. Elevated levels of MMP activity in human skin, as a result of prolonged periods of sun exposure, confirm that it plays a major role in photoageing [10, 25, 28]. Onoue *et al*. [144] suggested that MMP-9 secreted from keratino‐ cytes afterUVBirradiationmightresultfromapoptotic events.UVradiationisknowntoelevate the expression of MMP-1, MMP-3 (stromelysin-1) and MMP-9 in human skin *in vivo* [25]. All three MMPs (1, 3 and 9) can degrade most of the proteins found in the extracellular matrix [25]. MMP-1,whichisproducedbybothdermalfibroblastsandepidermalkeratinocytes,cleavestype 1 collagen into specific fragments. These fragments can then be further hydrolysed by MMP-2 and MMP-9 [137, 145]. Steinbrenner *et al*. [137] found that UVA irradiation dose-dependently decreased the steady-state mRNA levels of MMP-2 and MMP-9 and lowered the gelatinolytic activityofbothenzymesincellculturesupernatants.Ofinterestisthat*invivo*,followingexposure to UV radiation only keratinocytes express large levels of MMP on the cell membrane [146], but when fibroblasts grown in culture are irradiated they express higher levels of MMP on their plasmamembrane [147].The reasons forthediscrepanciesbetweenthe responsesofhumanskin

TNFα has been shown to induce proMMP-2 in human dermal fibroblasts [8], while IL-1α induced proMMP-9 levels in fibroblasts and keratinocytes [148]. In mesenchymal cells TNFα was shown to stimulate MMP-2 activity by activating a proteolytic cascade involving furin and MT1-MMP [139]. It is not known if TNFα activates MMP activity in epidermal cells via a similar mechanism. MMP-9 [144], has been shown to play an important role in the patho‐ physiologies of many skin conditions such as wound healing [145], and angiogenesis [87]. Activation of pro-MMP2 takes place at the cell surface and involves interactions with active MT1-MMP, which is itself activated through rapid trafficking to the cell surface and proteolytic processing [139]. Maquoi *et al*. [114] demonstrated that furin-inhibitor reduces the level of mature MT1-MMP, which is paralleled by a decrease in pro-MMP-2 activation as well as in cell invasiveness, confirming the furin plays a role in this process. Direct cleavage of proMMP-2 by furin in the TGN has shown to inactivate this matrix metalloprotease [149]. Therefore changes to the level of MMP-2 activity on the surface of the cell can be directly or indirectly regulated by furin either through cleavage in the TGN to reduce activity or indirectly via MT1- MMP, which increases activity. The mechanism by which this is regulated is not clear.

Low doses of UV radiation while they are not inflammatory, can cause mutations in skin tissue [9, 19, 30] or in cultured fibroblasts [150]. Constant exposure to UV radiation will not only result in mutations in p53 [150, 151] but in other genes including p16, PTCH, B-Raf, which will result in these cells becoming carcinogenic [6, 152, 153]. Apart from causing these mutations, UV radiation enhances the release of inflammatory molecules (e.g. TNFα, IL-1α, ROS, chemokines) from the skin (see section 2 for details). The result of which is the creation of an environment that allows for these mutated skin cells to become cancerous over time [15, 19, 21, 46, 56]. As a result of increased levels of MMP activity on the cell membrane, either as a

cells under *in vivo* and *in vitro* conditions are not known.

282 Highlights in Skin Cancer

**7. Role of furin in skin cancer**

In the progression of a mutated cell to that of a tumorigenic or metastatic cell, PCs have been shown to cleave a range of precursors of growth factors, their receptors, adhesion molecules, proteases and MMPs. Some of these molecules include cadherins, TGFβ, platelet derived growth factor as well as insulin-like growth factor 1 and its receptor [23, 101, 106, 117, 155]. Tumour progression and metastasis may enhanced by a number of factors such as (a) hypoxiainduced upregulation of furin activity within the solid tumour mass [105], (b) changes in cell adhesion through PC cleavage and activation of integrins and related adhesion molecules [156], (c) furin processing of vascular endothelial growth factor resulting in increased angio‐ genic activity [157], and (d) furin enhanced expression of MMPs [84, 158].

Furin/PC expression and processing can increase the incidence and severity of the cancer phenotype [104, 111]. Aberrant furin expression has been observed in a number of tumours including those from the breast [159], ovary [160], liver [125], brain [161], skin [111, 132] and from other tissues [23, 128, 162, 163]. Bassi *et al*. [101] observed that PACE4 transgenic mice were more susceptible to epidermal carcinogenesis and tumour progression compared to controls. These transgenic keratinocytes had higher rates of processing of MT1-MMP and MT2- MMP resulting in increased collagen degradation.

MMP-2 (gelatinase A) and MMP-9 (gelatinase B) have been frequently associated with the invasive and metastatic potential of tumour cells [10, 104, 111, 130, 132, 133, 137]. The expres‐ sion of MMP-2, is regulated independently of MMP-9 [144]. The close correlation observed between MMP-2 activation and metastatic progression in various tumours suggests that it may act as a "master switch" triggering tumour spread [114]. The expression of MMP is low in keratinocytes but elevated in BCC and SCC [49, 164, 165]. In SCC, MMP proteins (1, 2, 3 and MT1-MMP) are expressed both in tumorous and stromal cells [165], while MMP proteins (1, 2, 3 and 9) are observed in BCCs and melanomas [165, 166]. This expression correlates with the progression and the metastatic potentials of these tumours [106, 135, 165, 167]. UVR can participate to the development of skin cancer by the activation of MMPs. Two molecular mechanisms contribute to the UV-induced MMPs expression. First, the activation of cellsurface receptors with subsequent activation of mitogen activated protein kinase (MAPK) cascade that in turn contributes to the transcriptional up-regulation of MMPs [168]. Second, through the expression of pro-inflammatory cytokines, which induce the expression of MMPs [46, 133, 142, 143]. The role of UV in the induction of MMPs is supported by two experimental findings. First, UV-irradiation of SCC cell lines results in an increased secretion of MMPs [49, 132]. Second, UV-induced phosphorylation of extracellular signal-regulated kinase (ERK) and stress kinases precedes the rapid stimulation of MMPs in SCC cells [10]. If there are cells in the skin which become cancerous as a result of DNA damage, some may go onto become metastatic due to increased MMP activity [111]. This increase in levels of activated MMPs on the surface of the cell could be due to either increased expression of proMMP protein and/or increased furin activity. The role furin plays in the development of skin cancer suggests that it could be significant, and as such the development of specific inhibitors may offer a new therapy to treat such tumours.

Recently, Fu *et al*. [106] showed that in transgenic mice overexpressing furin, exposed to chemical carcinogens formed more and larger tumours than did control mice. This suggests that furin enhances skin cancer growth. Of interest was that these tumours were induced by chemical carcinogens and not by UV radiation. While enhanced furin levels were shown to enhance skin tumour development, in studies using cultured melanoma and glioma cells, furin inhibition was shown to reduce the processing of the pro-N-cadherin adhesion molecule, enhancing their migratory and aggressive nature [118]. The result of this study suggests that high furin levels may not enhance the metastasis of some tumours. Huang *et al*. [125] when investigating the expression of furin in hepatocellular carcinoma and surrounding tissue in humans, observed that if this ratio was >3.5 this resulted in higher patient survival rates compared to those who had ratios <3.5. This suggests that furin may play a dual role in cancer development, though the exact nature of which is not clear at this stage. Further study on this is warranted, and may result in a better understanding of its role in cancer development, and from which it may be possible to develop specific furin inhibitors which could be used clinically to treat these tumours.

play an important role in the changes observed in those epidermal cells exposed to UV radiation. The development of specific furin inhibitors has the potential to reduce the carci‐ nogenic effects of sunlight by preventing the activation of TACE and MMPs and their subse‐ quent downstream effects. Such compounds may have the potential to offer new therapies

The Role of Furin in the Development of Skin Cancer

http://dx.doi.org/10.5772/55569

285

We would like to that the School of Medical Sciences, RMIT University, Bundoora, Australia for supporting this study, and for the awarding of a School of Medical Science's Postgraduate

[1] Guy GP, Ekwueme DU. Years of potential life lost and indirect costs of melanoma and non-melanoma skin cancer: a systematic review of the literature. Pharmacoeco‐

[2] Qureshi AA, Laden F, Colditz GA, Hunter DJ. Geographic variation and risk of skin cancer in US women. Differences between melanoma, squamous cell carcinoma, and

[3] Narayanan DL, Saladi RN, Fox JL. Ultraviolet radiation and skin cancer. Internation‐

[4] Thomas-Ahner JM, Wulff BC, Tober KL, Kusewitt DF, Riggenbach JA, Oberyszyn TM. Gender differences in UVB-induced skin carcinogenesis, inflammation, and

basal cell carcinoma. Archives of Internal Medicine. 2008;168(5) 501-507.

both in the prevention and treatment of skin cancer.

and Terrence J Piva2\*

nomics. 2011;29(10) 863-874.

\*Address all correspondence to: terry.piva@rmit.edu.au

1 Endeavour College of Natural Health, Melbourne, Australia

al Journal of Dermatology. 2010;49(9) 978-986.

DNA damage. Cancer Research. 2007;67(7) 3468-3474.

2 School of Medical Sciences, RMIT University, Melbourne, Australia

**Acknowledgements**

Research Scholarship to RR.

**Author details**

Rethika Ravi1

**References**

### **8. Conclusion**

While it has been shown that UVA and UVB radiation cause different effects on the immune response this could be related to the activity of cell surface metalloproteases found on skin cells. Although the effect of TNFα in UV-induced inflammation has been well documented, little is known if the changes in TACE activity are due to increased protein levels or changes in enzyme activity. The inflammatory environment, seen in the skin, following exposure to UV radiation is known to stimulate the development of mutated cells which possess DNA damage caused directly by UV radiation or indirectly through the generation of ROS. Apart from processing TNFα, TACE also cleaves EGF family members, which would stimulate the growth of these mutated cells, which over time may become cancerous. As TNFα is a powerful inflammatory cytokine, considerable research has been under taken to develop specific TACE inhibitors [55, 56, 80]. Such inhibitors may play an important role in preventing the develop‐ ment of UV-induced skin cancers. An increased knowledge of the roles played by metallo‐ proteases in tumour progression, combined with the use of more selective inhibitors, could lead to effective use of these compounds in cancer therapies [55].

Similar to that of TACE, MMPs are also activated by UV radiation and also play a crucial role in skin tumour cell development and metastasis. Furin and other PCs have been shown to play an important role in activating both TACE and many MMPs in skin cells. Whereas the overexpression and activity of furin exacerbates the cancer phenotype, inhibition of its activity decreases or nullifies its effects, and thus, the development and use of specific inhibitors may also be a viable route to cancer therapy [23, 86, 102, 127].

It is known that UV induces furin mRNA in skin cells, though protein levels do not appear to change with respect to time, which suggests a rapid turnover of the enzyme. Further study is needed on how UV radiation activates furin, TACE and MMPs in skin cells. These proteases play an important role in the changes observed in those epidermal cells exposed to UV radiation. The development of specific furin inhibitors has the potential to reduce the carci‐ nogenic effects of sunlight by preventing the activation of TACE and MMPs and their subse‐ quent downstream effects. Such compounds may have the potential to offer new therapies both in the prevention and treatment of skin cancer.
