Immune selection drives the evolution of tumor cells toward an immune-resistant and cancer stem cell (CSC)-like phenotype. We reported that apoptosis inhibitor-5 (API5) acts as an immune escape factor, which has a significant role in controlling immune resistance to antigen-specific T cells, but its functional association with CSC-like properties remains largely unknown. In this study, we demonstrated for the first time that API5 confers CSC-like properties, including NANOG expression, the frequency of CD44-positive cells and sphere-forming capacity. Critically, these CSC-like properties mediated by API5 are dependent on FGFR1 signaling, which is triggered by E2F1-dependent FGF2 expression. Furthermore, we uncovered the FGF2-NANOG molecular axis as a downstream component of API5 signaling that is conserved in cervical cancer patients. Finally, we found that the blockade of FGFR signaling is an effective strategy to control API5high human cancer. Thus, our findings reveal a crucial role of API5 in linking immune resistance and CSC-like properties, and provide the rationale for its therapeutic application for the treatment of API5+ refractory tumors.

Contents Spermatogonial stem cells (SSCs) are an important tool for fertility preservation and species conservation. The ability to expand SSCs by in vitro culture is a crucial premise for their use in assisted reproduction. Because SSCs represent a small proportion of the germ cells in the adult testis, culture success is aided by pre‐enrichment through sorting techniques based on cell surface‐specific markers. Given the importance of the domestic cat as a model for conservation of endangered wild felids, herein we sought to examine culture conditions as well as molecular markers for cat SSCs. Using a cell culture medium for mouse SSCs supplemented with glial cell‐derived neurotrophic factor (GDNF), germ cells from prepuberal cat testes remained viable in culture for up to 43 days. Immunohistochemistry for promyelocytic leukaemia zinc finger (PLZF) protein on foetal, prepuberal and adult testis sections revealed a pattern of expression consistent with the labelling of undifferentiated spermatogonia. Fluorescence‐activated cell sorting (FACS) with an antibody against epithelial cell adhesion molecule (EPCAM) was used to sort live cells. Then, the gene expression profile of EPCAM‐sorted cells was investigated through RT‐qPCR. Notably, EPCAM (+) cells expressed relatively high levels of CKIT (CD117), a surface protein typically expressed in differentiating germ cells but not SSCs. Conversely, EPCAM (‐) cells expressed relatively high levels of POU domain class 5 transcription factor 1 (POU1F5 or OCT4), clearly a germ line stem cell marker. These results suggest that cat SSCs would probably be found within the population of EPCAM (–) cells. Future studies should identify additional surface markers that alone or in combination can be used to further enrich SSCs from cat germ cells.

N G -Methylation of arginine residues in many nucleic-acid-binding proteins are formed post-translationally, catalysed by Sadenosylmethionine : protein-arginine N-methyltransferase in their glycine-rich and arginine-rich motifs. The amino acid sequences of the stimulator of HIV-1 TAR (Tat-responsive element) RNA-binding protein (SRB) and fibronectin also show the presence of the internal -Gly-Arg-Gly-(-GRG-) sequence, which is potentially methylatable by the methyltransferase. To investigate the sequence requirement for methylation of these proteins, several synthetic oligopeptides with different chain lengths and sequences similar to the -GRG-regions of SRB and fibronectin were synthesized. Whereas the heptapeptide AGGRGKG (residues 16-22 in SRB) served as the methyl acceptor for the methyltransferase with a K m of 50 µM, the 19-mer peptide (residues 10-28 in SRB) was methylated with a K m of 8.3 µM, indicating that a greater peptide chain length yields a better methyl acceptor. Product analysis of the methylated [methyl-"%C]SRB-peptide by HPLC indicated the formation of N G -monomethylarginine and N G ,N G -dimethyl(asymmetric)-

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