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We have studied the effects of two new synthetic triterpenoids, 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid (CDDO) and its derivative, 1-(2-cyano-3,12-dioxooleana-1,9-dien-28-oyl) imidazole (CDDO-Im), on transforming growth factor (TGF)-beta/Smad signaling. These agents, at nanomolar concentrations, increase the expression of TGF-beta-dependent genes, such as those for plasminogen activator inhibitor 1 and the type II TGF-beta receptor, and they synergize with TGF-beta in this regard. They prolong the activation of Smad2 induced by TGF-beta and markedly enhance the ability of Smad3 to activate a Smad binding element, CAGA-luciferase. In transfection assays, they reverse the inhibitory effects of Smad7. CDDO and CDDO-Im also enhance Smad signaling in the pathways of two other members of the TGF-beta superfamily, namely, activin and bone morphogenetic protein. Finally, these triterpenoids induce expression of the transcriptional coactivator p300-CBP-associated factor and synergize with TGF-beta in this regard. These are the first studies to report enhancement of Smad signaling by synthetic triterpenoids and should further their optimal use for applications in prevention or treatment of diseases in which there is aberrant function of TGF-beta.

SNIP1 is a 396-amino acid nuclear protein shown to be an inhibitor of the TGF-beta signal transduction pathway and to be important in suppressing transcriptional activation dependent on the co-activators CBP and p300. In this report we show that SNIP1 potently inhibits the activity of NF-kappa B, which binds the C/H1 domain of CBP/p300, but does not interfere with the activity of transcription factors such as p53, which bind to other domains of p300, or factors such as VP16, which are independent of these co-activators. Inhibition of NF-kappa B activity is a function of the N-terminal domain of SNIP1 and involves competition of SNIP1 and the NF-kappa B subunit, RelA/p65, for binding to p300, similar to the mechanism of inhibition of Smad signaling by SNIP1. Immunohistochemical staining shows that expression of SNIP1 is strictly regulated in development and that it colocalizes, in certain tissues, with nuclear staining for RelA/p65 and for p300, suggesting that they may regulate NF-kappa B activity in vivo in a spatially and temporally controlled manner. These data led us to suggest that SNIP1 may be an inhibitor of multiple transcriptional pathways that require the C/H1 domain of CBP/p300.

X-linked inhibitor of apoptosis protein (XIAP) is a potent suppressor of apoptotic cell death, which functions by directly inhibiting caspases, the principal effectors of apoptosis. Here we report that XIAP can also function as a cofactor in the regulation of gene expression by transforming growth factor-beta (TGF-beta). XIAP, but not the related proteins c-IAP1 or c-IAP2, associated with several members of the type I class of the TGF-beta receptor superfamily and potentiated TGF-beta-induced signaling. Although XIAP-mediated activation of c-Jun N-terminal kinase and nuclear factor kappa B was found to require the TGF-beta signaling intermediate Smad4, the ability of XIAP to suppress apoptosis was found to be Smad4-independent. These data implicate a role for XIAP in TGF-beta-mediated signaling that is distinct from its anti-apoptotic functions.

Here we describe a protein product of the human septin H5/PNUTL2/CDCrel2b gene, which we call ARTS (for apoptosis-related protein in the TGF-beta signalling pathway). ARTS is expressed in many cells and acts to enhance cell death induced by TGF-beta or, to a lesser extent, by other apoptotic agents. Unlike related septin gene products, ARTS is localized to mitochondria and translocates to the nucleus when apoptosis occurs. Mutation of the P-loop of ARTS abrogates its competence to activate caspase 3 and to induce apoptosis. Taken together, these observations expand the functional attributes of septins previously described as having roles in cytokinesis and cellular morphogenesis.