Abstract
Non-melanoma skin cancers, including squamous cell carcinoma (SCC), are the most common malignancy worldwide, with an overwhelming majority of cases resulting from ultraviolet (UV) irradiation. With surgical intervention as the main treatment of SCCs, there is an obvious demand for alternative non-invasive therapies. In keratinocytes, UV-induced DNA damage activates checkpoints that can halt the cell cycle via the inactivation and subsequent degradation of the cell cycle regulator cell division cycle 25 (CDC25) phosphatases. 14-3-3 proteins bind a wide variety of client proteins, including CDC25, to regulate protein stability, localization or activity. CDC25 phosphatases, including CDC25A B and C, and 14-3-3 proteins, a family of seven isoforms (b, e, h, s, q, g and z) have been implicated in various cancers where they often promote cell division or survival. We hypothesized that increased CDC25 levels in skin cancer result from interaction with 14-3-3 family members, leading to increased cell proliferation and skin cancer growth. Immunohistochemistry revealed that CDC25A, B and C as well as 14-3-3e, g, z, h and q were overexpressed in skin cancer, and also that CDC25A, CDC25B, CDC25C, and 14-3-3e and -g were primarily cytoplasmic in skin cancer but nuclear in normal skin. Cytoplasmic accumulation of CDC25A was facilitated by its nuclear export sequence and exportin 1 and 6 proteins. Surprisingly, CDC25A overexpression or silencing did not impact proliferation in SCC cells, but instead, CDC25A suppressed apoptosis in a manner dependent on 14-3-3 and cytoplasmic localization. Both CDC25A and 14-3-3e activated Akt, inhibited BAD and increased Survivin, leading to increased SCC cell survival. Normal keratinocytes, with mostly nuclear CDC25A and 14-3-3e were resistant to the effects of modulating their levels or activity. Targeting of CDC25A or 14-3-3e with inhibitors or by deletion of the gene for 14-3-3e decreased skin tumorigenesis, reduced Akt activity and Survivin levels, and increased apoptosis in skin cancer. Peptides were designed to block CDC25A binding to 14-3-3e or 14-3-3e heterodimerization. These peptides successfully disrupted CDC25A binding to 14-3-3e and 14-3-3e heterodimerization increased SCC apoptosis, suppressed pro-survival signaling and reduced tumor growth. Taken together, these data reveal the importance of CDC25A and 14-3-3e signaling for skin cancer growth and suggest that targeting of CDC25A and 14-3-3e pro-survival signaling may be an effective strategy for skin cancer treatment.