Abstract
Proteasome inhibitors have distinct properties and the biochemical consequences of suppressing ubiquitin E1 enzymes and the proteasome differ. We compared the effects of the proteasome inhibitors bortezomib, ixazomib and carfilzomib and the ubiquitin E1 enzyme inhibitor MLN7243/TAK-243 on cell viability and cell death in normal keratinocytes and cutaneous squamous cell carcinoma (cSCC) cell lines. The effects of both a pulse of treatment and more extended incubation were investigated. This is relevant to directly-delivered therapy (topical treatment/intratumoral injection) where the time of exposure can be controlled and a short exposure may better reflect systemically-delivered inhibitor pharmacokinetics. These agents can selectively kill cSCC cells but there are variations in the pattern of cSCC cell line sensitivity/resistance. Variations in the responses to proteasome inhibitors are associated with differences in the specificity of the inhibitors for the three proteolytic activities of the proteasome. There is greater selectivity for killing cSCC cells compared to normal keratinocytes with a pulse of proteasome inhibitor treatment than with a more extended exposure. We provide evidence that c-MYC-dependent NOXA upregulation confers susceptibility to a short incubation with proteasome inhibitors by priming cSCC cells for rapid BAK-dependent death. We observed that bortezomib-resistant cSCC cells can be sensitive to MLN7243-induced death. Low expression of the ubiquitin E1 UBA1/UBE1 participates in conferring susceptibility to MLN7243 by increasing sensitivity to MLN7243-mediated attenuation of ubiquitination. This study supports further investigation of the potential of proteasome and ubiquitin E1 inhibition for cSCC therapy. Direct delivery of inhibitors could facilitate adequate exposure of skin cancers.
Highlights
Cutaneous squamous cell carcinoma causes significant mortality and morbidity in the general population [1]
The effects of these agents were assessed in untransformed normal keratinocytes from an recessive dystrophic epidermolysis bullosa (RDEB) patient (RDEBK), normal human keratinocytes (NHK) and eight cutaneous squamous cell carcinoma (cSCC)-derived cell lines. cSCCs arise from the malignant transformation of keratinocytes in the epidermis
We compared the effects in normal keratinocytes and cSCC cell lines of a long and short treatment with proteasome inhibitors and the ubiquitin E1 inhibitor MLN7243
Summary
Cutaneous squamous cell carcinoma (cSCC) causes significant mortality and morbidity in the general population [1]. CSCC has a huge impact on patients with the chronic skin blistering disease recessive dystrophic epidermolysis bullosa (RDEB) [2]. The cumulative risk of death from cSCC in patients with severe generalized RDEB is 80% by the age of 55 [3]. Immunosuppressed patients are at high risk of cSCC. Organ transplant recipients are up to 150-fold more likely to develop cSCC than the general population and these tumours tend to be more aggressive [4]. CSCC is responsible for 25% of deaths due to skin cancer in the United Kingdom and in some regions of the United States it is responsible for more deaths than melanoma [1]. There is a need for improved therapy for the minority of cSCCs that are responsible for this substantial health burden [5]
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