Tripartite motif protein 32, Trim32, mRNA and protein expression was elevated in independently transformed and tumorigenic keratinocytes of a mouse epidermal carcinogenesis model, in ultraviolet B (UVB)-induced squamous cell carcinomas (SCC), and in approximately 20-25% of chemically induced mouse papillomas and human head and neck SCCs. This suggests that elevated Trim32 expression occurs frequently in experimental epidermal carcinogenesis and is relevant to human cancer. Transduced Trim32 increased colony number in an epidermal in vitro transformation assay and epidermal thickening in vivo when skin-grafted to athymic nu/nu mice. These effects were not associated with proliferation and were not sufficient for tumorigenesis, even with 12-O-tetradecanoylphorbol-13-acetate treatment or defects in the tumor suppressor p53. However, transduced Trim32 inhibited the synergistic effect of tumor necrosis factor alpha (TNFalpha) on UVB-induced apoptosis of keratinocytes in vitro and the apoptotic response of keratinocyte grafts exposed to UVB-light in vivo. Consistent with its RING domain, Trim32 exhibited characteristics of E3-ubiquitin ligases, including being ubiquitylated itself and interacting with ubiquitylated proteins, with increases in these properties following treatment of cultured keratinocytes with TNFalpha/UVB. Interestingly, missense point mutation of human TRIM32 has been reported in Limb-Girdle Muscular Dystrophy Type 2H, an autosomal recessive disease. We propose a model in which Trim32 activities as an E3-ubiquitin ligase favor initiated cell survival in carcinogenesis by blocking UVB-induced TNFalpha apoptotic signaling.

Protein inhibitors of activated STATs (PIAS) family members are ubiquitin-protein isopeptide ligase-small ubiquitin-like modifier ligases for diverse transcription factors. However, the regulation of PIAS protein activity in cells is poorly understood. Previously, we reported that expression of Trim32, a RING domain ubiquitin-protein isopeptide ligase-ubiquitin ligase mutated in human limb-girdle muscular dystrophy type 2H (LGMD2H) and Bardet-Biedl syndrome, is elevated during mouse skin carcinogenesis, protecting keratinocytes from apoptosis induced by UVB and tumor necrosis factor-␣ (TNF␣). Here we report that Trim32 interacts with Piasy and promotes Piasy ubiquitination and degradation. Ubiquitination of Piasy by Trim32 could be reproduced in vitro using purified components. Their interaction was induced by treatment with UVB/ TNF␣ and involved redistribution of Piasy from the nucleus to the cytoplasm, where it accumulated in cytoplasmic granules that colocalized with Trim32. Piasy destabilization and ubiquitination required an intact RING domain in Trim32. The LGMD2H-associated missense point mutation prevented Trim32 binding to Piasy, and human Piasy failed to colocalize with human Trim32 in fibroblasts isolated from an LGMD2H patient. Trim32 expression increased the transcriptional activity of NFB in epidermal keratinocytes, both under basal treatment and after UVB/TNF␣ treatment. Conversely, Piasy inhibited NFB activity under the same conditions and sensitized keratinocytes to apoptosis induced by TNF␣ and UVB. Our results indicate that, by controlling Piasy stability, Trim32 regulates UVB-induced keratinocyte apoptosis through induction of NFB and suggests loss of function of Trim32 in LGMD2H.Protein ubiquitination is a fundamental process in eukaryotic cells, controlling the degradation of proteins through the 26 S proteasome. E3 4 -ubiquitin ligases catalyze the last step of the process and provide substrate specificity. The activity of many cellular factors is controlled by their abundance and stability in the cell, properties that depend on the rate of ubiquitination. By providing substrate specificity, E3 ligases become a point of control for ubiquitination and stability of many cellular factors. A variety of otherwise structurally unrelated E3 ligases contain a common RING domain that provides interaction with the E2 ubiquitin-conjugating enzyme (1). In Trim32, the RING domain is present in the amino terminus of the protein, as part of the RBCC (Ring/B-Box/Coiled-coil) or tripartite motif that defines the TRIM family of proteins (2). Members of the TRIM family may be located in the cytoplasm or nucleus and are thought to form large multimeric complexes, forming subcellular structures resembling spheres, speckles, or ribbons. Different TRIM family members are characterized by a specific carboxyl-terminal domain. In Trim32, the carboxyl terminus contains six repeats of the NHL (NCL-1, HT2A and LIN-41) motif, thought to mediate protein/protein interactions (3). TRIM family members have been implicated...