Constitutive Epidermal Osteopontin Expression in Osteopontin-Null Mice Develop Ultraviolet B-Induced Cutaneous Squamous Cell Carcinomas Comparable to Wild-Type Mice

Studies of osteopontin (OPN)-null mice have supported the role of OPN as a critical factor in the promotion of skin tumorigenesis. OPN is a highly inducible integrin- and CD44-interacting acidic glycoprotein with pleiotropic functions. In various types of cancer, elevated levels of OPN derived from cancer and from inflammatory cells are secreted into the microenvironment and the bloodstream. To determine whether OPN expressed by keratinocytes without any contribution from activated resident and infiltrating inflammatory cells promotes cutaneous tumor development, transgenic mice with constitutive epidermal expression of OPN in an OPN-null background, tg(K14-OPN) were generated. In photocarcinogenesis studies, tumor multiplicity and the incidence of cutaneous squamous cell carcinoma (cSCC) was similar between tg(K14-OPN) and wild-type (WT) mice. Consistent with our previous findings, constitutive epidermal expression of OPN enhanced the survival of acute ultraviolet B (UVB)-induced cell death compared with OPN-null mice and did not enhance UVB-induced epidermal hyperplasia compared to OPN-null or WT mice. Additionally, tg(K14-OPN) and OPN-null mice irradiated with long-term low dose UVB had significantly lower numbers of mutated p53 keratinocytes than WT mice. RNA-sequencing data from the epidermis of acute UVB-irradiated tg(K14-OPN) versus OPN-null mice compared to UVB-irradiated WT versus OPN-null mice suggests the importance of inflammation, Wnt, integrin and gonadotropin-releasing hormone receptor signaling in cutaneous tumorigenesis and implicates UVB irradiation and its induction of OPN in driving those pathways. In summary, the constitutive epidermal expression of OPN in OPN-null mice facilitates the development of cSCC comparable to WT mice.