displays a dominant genetic linkage to multiple types of cutaneous adnexal tumors that often develop in bulky clusters in the head, neck, trunk and pubic areas 10,37
. Although predominantly benign 10
, these tumors are painful and disfiguring, can undergo malignant transformation with metastasis over time, and eventually lead to mortality 38–41
. Thus, the malignant features of the tumors developed on K14-CYLDm
transgenic mice are in line with the clinical manifestations seen in patients. Our transgenic tumor models allowed us to define JNK/AP1 signaling cascade as a key regulator in CYLDm
-driven epidermal malignancy.
loss-of-function is not only relevant to cutaneous adnexal tumors but also to many other cancers, including SCC 22,42
. It is worth noting that cyld
−-− mice are sensitive to chemically induced carcinogenesis, but the tumors developed on these mice are not more malignant than those of WT mice 22
. We predict that the differential tumor growth phenotypes observed in cyld
−-− and CYLDm
-transgenic mice could be explained by multiple possibilities. First, CYLDm
might have dominant negative effects such that the N-terminus of CYLD possesses oncogenic functions that are independent of the C-terminal catalytic function. Such a scenario is in line with the fact that every patient-relevant cyld
mutation characterized so far produces a catalytically deficient CYLD mutant 10
. Second, CYLD is required for endothelia cell migration 43
; thus, its absence in endothelial cells of cyld
−-− mice might result in an impairment of angiogenesis, and consequently affect tumor progression. In contrast, expression of CYLDm
is limited to epidermal cells in the transgenic mice. Third, the differences in mice genetic backgrounds might also contribute to the differential sensitivity to carcinogenesis, which can be addressed by cross-breeding of the transgenic and knockout mice in future studies.
transgenic mice did not develop spontaneous skin tumors, indicating that other genetic or environmental challenges are required to promote tumorigenesis. Because cutaneous adnexal tumors are frequently located on the exposed areas, UV irradiation has been considered as the major cause of tumor initiation. However, recent studies have demonstrated that the pubic area is also susceptible to cylindromatosis, a phenomenon that has been previously underreported. This datum suggests that hormonal factors might be involved in tumor induction in patients 37
. Future efforts are necessary to determine how UV, hormonal factors and LOH of the WT cyld
allele contribute to CYLDm
-driven epidermal malignancy.
Bcl3 is a direct substrate of CYLD; and upon activation via K63-ubiquitination, it forms hetero-dimers with p50/p52 to induce expression of cyclin D1. Thus, Bcl3 is recognized as an important regulator in skin carcinogenesis of cyld
−-− mice 22
. Interestingly, despite the inhibitory role of CYLD on NF-κB, neither Bcl3 nor RelA displayed increased induction in the CYLDm
-transgenic tumors. It is possible that this is due to the negative cross-talk from JNK/AP1 as described in our previous studies 34
. These findings implicate that NF-κB is unlikely the sole key regulator in the malignant tumor phenotype developed on transgenic mice. To this end, we found that JNK and its downstream c-Jun and c-Fos proteins were highly activated in both primary and metastatic tumors from the transgenic mice. Additionally, CYLDm
increased the basal levels of c-Jun and c-Fos, and sustained their activation status in response to EGF-treatment. Moreover, both CYLDWT
interacted with c-Jun and c-Fos, but with opposite effects; the latter increased c-Fos/c-Jun K63-ubiquitination and potentiated their transcriptional activity. Presumably, K63-ubiquitination excludes the degradation-targeting K48-ubiqutination, and thereby increases c-Jun/c-Fos protein stability. Findings to date indicate that signals transmitted through membrane receptors are subjected to CYLD-regulation at multiple levels. Specifically, CYLD not only suppresses IKK/NF-κB and MKKK7/JNK/AP1 signaling through TRAF/TRADD 1–5
, but also directly regulates IKKγ, Bcl3 and c-Jun/c-Fos ubiquitination 3,22
. In contrast to the canonical NF-κB pathway which suppresses epidermal growth and neoplasia 24,26,44
, Bcl3 and the JNK/AP1 signaling cascades support epidermal growth and tumorigenesis 22,25,32
. Taken together, our data established an important and broad role for CYLD in malignant and metastatic tumor development and identified c-Jun and c-Fos as novel CYLD-downstream regulators. These findings provide mechanistic insights to therapeutic targeting of the JNK/AP1 pathway for cancers associated with CYLD loss-of-function.