Identification of RPL15 60S Ribosomal Protein as a Novel Topotecan Target Protein That Correlates with DAMP Secretion and Antitumor Immune Activation.

Damage-associated molecular patterns (DAMPs) contribute to antitumor immunity during cancer chemotherapy. We previously demonstrated that topotecan (TPT), a topoisomerase I inhibitor, induces DAMP secretion from cancer cells, which activates STING-mediated antitumor immune responses. However, how TPT induces DAMP secretion in cancer cells is yet to be elucidated. Here, we identified RPL15, a 60S ribosomal protein, as a novel TPT target and showed that TPT inhibited preribosomal subunit formation via its binding to RPL15, resulting in the induction of DAMP-mediated antitumor immune activation independent of TOP1. TPT inhibits RPL15-RPL4 interactions and decreases RPL4 stability, which is recovered by CDK12 activity. RPL15 knockdown induced DAMP secretion and increased the CTL population but decreased the regulatory T cell population in a B16-F10 murine melanoma model, which sensitized B16-F10 tumors against PD-1 blockade. Our study identified a novel TPT target protein and showed that ribosomal stress is a trigger of DAMP secretion, which contributes to antitumor immunotherapy.

Positive interactions between STAP-1 and BCR-ABL influence chronic myeloid leukemia cell proliferation and survival.

Chronic myeloid leukemia (CML) is a clonal disease characterized by the presence of the Philadelphia chromosome and its oncogenic product, BCR-ABL, which activates multiple pathways involved in cell survival, growth promotion, and disease progression. We recently reported that signal-transducing adaptor protein 1 (STAP-1) is upregulated in CML stem cells (LSCs) and functions to reduce the apoptosis of CML LSCs by upregulating the STAT5-downstream anti-apoptotic genes. In this study, we demonstrate the detailed molecular interactions among BCR-ABL, STAP-1, and signal transducer and activator of transcription 5 (STAT5). Studies with deletion mutants have revealed that STAP-1 interacts with BCR-ABL and STAT5a through its SH2 and PH domains, respectively, suggesting the possible role of STAP-1 as a scaffold protein. Furthermore, the binding of STAP-1 to BCR-ABL stabilizes the BCR-ABL protein in CML cells. Since STAP-1 is highly expressed in CML cells, we also analyzed the STAP-1 promoter activity using a luciferase reporter construct and found that NFATc1 is involved in activating the STAP-1 promoter and inducing STAP-1 mRNA expression. Our results demonstrate that STAP-1 contributes to the BCR-ABL/STAT5 and BCR-ABL/Ca2+/NFAT signals to induce proliferation and STAP-1 mRNA expression in CML cells, respectively.

CD47 promotes T-cell lymphoma metastasis by up-regulating AKAP13-mediated RhoA activation.

CD47, a 50 kDa transmembrane protein, facilitates integrin-mediated cell adhesion and inhibits cell engulfment by phagocytes. Since CD47 blocking promotes engulfment of cancer cells by macrophages, it is important to clarify the mechanism of CD47 signaling in order to develop treatments for diseases involving CD47-overexpressing cancer cells, including breast cancer and lymphoma. Here, we show that CD47 plays an essential role in T-cell lymphoma metastasis by up-regulating basal RhoA activity independent of its anti-phagocytic function. CD47 interacts with AKAP13, a RhoA-specific guanine nucleotide exchange factor (GEF), and facilitates AKAP13-mediated RhoA activation. Our study shows that CD47 has a novel function on the AKAP13-RhoA axis and suggests that CD47-AKAP13 interaction would be a novel target for T-cell lymphoma treatment.