2A is an oligopeptide sequence mediating a ribosome ‘skipping’ effect, producing an apparent ‘cleavage’ of polyproteins. First identified and characterized in picornaviruses, ‘2A-like’ sequences are found in other mammalian viruses and a wide range of insect viruses. Databases were analysed using a motif conserved amongst 2A/2A-like sequences. The newly identified 2A-like sequences (30 aa) were inserted into a reporter polyprotein to determine their cleavage activity. Our analyses showed that these sequences fall into two categories. The majority mediated very high (complete) cleavage to separate proteins and a few sequences mediated cleavage with lower efficiency, generating appreciable levels of the uncleaved form. Phylogenetic analyses of 2A-like sequences and RNA-dependent RNA polymerases (RdRps) indicated multiple, independent, acquisitions of these sequences at different stages during virus evolution. Within a virus family, 2A sequences are (probably) homologous, but diverge due to other evolutionary pressures. Amongst different families, however, 2A/2A-like sequences appear to be homoplasic.

Insertion of picornaviral 2A sequences into mRNAs causes ribosomes to skip formation of a peptide bond at the junction of the 2A and downstream sequences, leading to the production of two proteins from a single open reading frame. Adenoviral protein IX is a minor capsid protein that has been used to display foreign peptides on the surface of the capsid. We have used 2A sequences from the foot-and-mouth disease virus (FMDV) and porcine teschovirus 1 (PTV-1) to express protein IX (pIX) and green fluorescent protein (GFP) from pIX-2A-GFP fusion genes in an oncolytic virus derived from human adenovirus 5. GFP was efficiently expressed by constructs containing either 2A sequence. Peptide bond skipping was more efficient with the 58 aa FMDV sequence than with the 22 aa PTV-1 2A sequence, but the virus with the FMDV 2A sequence showed a reduction in plaque size, cytopathic effect, viral burst size and capsid stability. We conclude that ribosome skipping induced by 2A sequences is an effective strategy to express heterologous genes in adenoviruses; however, careful selection or optimization of the 2A sequence may be required if protein IX is used as the fusion partner. INTRODUCTIONReplication-competent oncolytic adenoviruses have been extensively tested in animals, and a virus lacking the E1B 55K gene was recently approved for cancer therapy in China (reviewed by Alemany, 2007). The main factor limiting widespread application of oncolytic adenoviruses is their low efficacy. Many groups have attempted to develop more active viruses, for example by inserting genes for toxic proteins or prodrug-activating enzymes (reviewed by Alemany, 2007). We and others have previously used internal ribosome entry sites (IRESs) to initiate the translation of prodrug-activating enzymes in adenoviruses but the results were disappointing (reviewed by de Felipe, 2002;Fuerer & Iggo, 2004;Lukashev et al., 2005). Ribosome skipping is a recently described mechanism that allows translation of multiple proteins from a single mRNA. It is based on the use of a picornaviral 2A sequence that causes the ribosome to continue translation after skipping the formation of one peptide bond. This process was first characterized in foot-and-mouth disease virus (FMDV) (Ryan & Drew, 1994). The process was originally termed 'cleavage' by analogy with the proteasemediated cleavages occurring at other sites in the FMDV polyprotein but this is misleading because the 'cleavage' results from failure to form a peptide bond during translation. Unlike reinitiation of translation by IRESs, skipping induced by 2A sequences gives approximately equal expression of the proteins upstream and downstream of the 2A site (de Felipe et al., 2006). 2A and 2A-like sequences have previously been used in biotechnology applications, but not in adenoviruses (reviewed by de Felipe et al., 2006). Protein IX is a small cement protein located between the hexons in the capsid of the adenovirus (Furcinitti et al., 1989). It is essential for the packaging of full-length viral genomes (Ghosh-Cho...

The secreted growth factor Activin-A of the transforming growth factor β family and its receptors can promote or inhibit several cancer hallmarks including tumor cell proliferation and differentiation, vascularization, lymphangiogenesis and inflammation. However, a role in immune evasion and its relationship with tumor-induced muscle wasting and tumor vascularization, and the relative contributions of autocrine versus paracrine Activin signaling remain to be evaluated. To address this, we compared the effects of truncated soluble Activin receptor IIB as a ligand trap, or constitutively active mutant type IB receptor versus secreted Activin-A or the related ligand Nodal in mouse and human melanoma cell lines and tumor grafts. We found that although cell-autonomous receptor activation arrested tumor cell proliferation, Activin-A secretion stimulated melanoma cell dedifferentiation and tumor vascularization by functional blood vessels, and it increased primary and metastatic tumor burden and muscle wasting. Importantly, in mice with impaired adaptive immunity, the tumor-promoting effect of Activin-A was lost despite sustained vascularization and cachexia, suggesting that Activin-A promotes melanoma progression by inhibiting antitumor immunity. Paracrine Activin-A signaling emerges as a potential target for personalized therapies, both to reduce cachexia and to enhance the efficacy of immunotherapies.

The secreted growth factor Activin-A of the TGFβ family and its receptors can promote or inhibit several cancer hallmarks including tumor cell proliferation and differentiation, vascularization, lymphangiogenesis and inflammation. However, a role in immune evasion and its relationship with tumor-induced muscle wasting and tumor vascularization, and the relative contributions of autocrine versus paracrine Activin signaling remain to be evaluated. To address this, we compared the effects of truncated soluble Activin receptor II B as a ligand trap, or constitutively active mutant type IB receptor versus secreted Activin-A or the related ligand Nodal in mouse and human melanoma cell lines and tumor grafts. We found that while cell-autonomous receptor activation arrested tumor cell proliferation, Activin-A secretion stimulated melanoma cell dedifferentiation and tumor vascularization by functional blood vessels, and it increased primary and metastatic tumor burden and muscle wasting. Importantly, in mice with impaired adaptive immunity, the tumor-promoting effect of Activin-A was lost despite sustained vascularization and cachexia, suggesting that Activin-A promotes melanoma progression by inhibiting anti-tumor immunity. Paracrine Activin-A signaling emerges as a potential target for personalized therapies, both to reduce cachexia and to enhance the efficacy of immunotherapies.

No abstract