Targeting the SKP2 Axis for Anti-melanoma Therapy - ABSTRACT Melanoma is one of the most aggressive skin cancers that develops from melanocytes. Although melanoma is less frequent than other types of skin cancer, it has a rapid growth rate and a tendency to spread quickly, leading to a high mortality rate and accounting for almost 75% of all skin cancer-related deaths. Genome sequencing analyses identified critical target molecules for melanoma, including BRAF and NRAS, providing opportunities for targeted therapies for these altered signaling pathways. Additionally, recent advances in cancer immunotherapy have markedly improved the prognosis of patients with melanoma. However, despite this progress, the responses to these therapies are still restricted to specific tumor subtypes. Therefore, further in- depth analyses are required to identify alternative targetable signaling pathways to improve treatment outcomes. A growing body of evidence has implicated the ubiquitin-proteasome system (UPS) in the pathogenesis of various malignancies, including melanomas. Recent genome-wide sequencing analyses revealed that gene alterations in UPS factors, such as BAP1, FBXW7, and PARK2, contribute to melanomagenesis. Our research mainly focuses on the regulation and physiological function of a major oncogenic E3 ubiquitin ligase, SKP2. While SKP2 is among the best-characterized E3s, relatively little is known about the regulatory mechanisms and pathological roles of SKP2 in melanoma. In this proposal, we aim to address two mechanistic questions: 1) how SKP2 protein stability is regulated by its upstream E3 ligase, and how deficiency of SKP2 stability control leads to melanomagenesis, and 2) how SKP2-directed degradation of a downstream target, HHIP, functions as a potential melanoma tumor suppressor and promotes melanoma tumorigenesis. Our unbiased screening identified RNF12 as an upstream E3 of SKP2. Through a database search, we found patient-derived RNF12 catalytic dead mutations in melanoma. Furthermore, we identified HHIP, an emerging tumor suppressor with the highest mutation rate in melanoma, as a potential SKP2 downstream ubiquitin substrate. To rigorously examine our central hypothesis, we propose two complementary Aims: (1) to define the molecular mechanisms by which the dysregulated RNF12-SKP2 E3 ligase cascade promotes melanoma tumorigenesis and (2) to investigate how SKP2-mediated HHIP protein degradation plays an essential role in melanomagenesis. Considering the pivotal oncogenic role of SKP2, it is critical to investigate how SKP2 E3 ligase activity is elevated in melanoma cells, and how SKP2 oncogenic activity promotes melanoma tumorigenesis. The successful completion of the proposed studies could provide robust translational strategies for anti-melanoma therapeutics targeting the RNF12/SKP2/HHIP signaling pathway. Melanoma-derived RNF12 mutations are potential biomarkers of SKP2 overactivation, providing a rationale for anti-SKP2 targeted therapy in melanoma. Furthermore, elucidating the HHIP degradation mechanism offers an innovative approach to restore HHIP protein abundance, thereby suppressing melanoma tumor growth and improving anti-melanoma immunotherapy outcomes.
