TAF4 inactivation in embryonic fibroblasts activates TGFβ signalling and autocrine growth

Abstract

We have inactivated transcription factor TFIID subunit TBP-associated factor 4 (TAF4) in mouse embryonic fibroblasts. Mutant taf4 À/À cells are viable and contain intact TFIID comprising the related TAF4b showing that TAF4 is not an essential protein. TAF4 inactivation deregulates more than 1000 genes indicating that TFIID complexes containing TAF4 and TAF4b have distinct target gene specificities. However, taf4 À/À cell lines have altered morphology and exhibit serum-independent autocrine growth correlated with the induced expression of several secreted mitotic factors and activators of the transforming growth factor b signalling pathway. In addition to TAF4 inactivation, many of these genes can also be induced by overexpression of TAF4b. A competitive equilibrium between TAF4 and TAF4b therefore regulates expression of genes controlling cell proliferation. We have further identified a set of genes that are regulated both by TAF4 and upon adaptation to serum starvation and which may be important downstream mediators of serum-independent growth. Our study also shows that TAF4 is an essential cofactor for activation by the retinoic acid receptor and CREB, but not for Sp1 and the vitamin D3 receptor.

New Insights into TAFs as Regulators of Cell Cycle and Signalling Pathways

“…TAF4 was then inactivated in these cells by expression of the Cre recombinase. 43 In contrast to what has been observed with other TAFs in proliferating cell lines in vitro, loss of TAF4 did not result in cell cycle arrest and apoptosis. Morover, the taf4 -/-fibroblasts contained intact TFIID.…”

“…43 Amongst the affected genes, are several secreted growth factors including TGFβ1, β3 and CTGF. CTGF is induced by TGFβ, has intrinsic growth factor properties and acts as a secondary mediator of the proliferative response of fibroblasts to TGFβ, in particular during the development of fibrosis.…”

TAF4b and Jun/Activating Protein-1 Collaborate to Regulate the Expression of Integrin α6 and Cancer Cell Migration Properties

“…Our analysis, using luciferase reporter constructs transfected to colorectal cancer cells, supports that TAF4b, but not TAF4, acts as a coactivator on specific AP-1 targets. Furthermore, in our study, we observed that an intracellular reduction of TAF4b might be compensated by induced expression levels of TAF4 (Supplementary Data), reinforcing the previous assumption that TAF4 was limiting the TAF4b-containing TFIID complexes present in cells (36). Interestingly, TAF4 and TAF4b, through their competitive equilibrium in TFIID, have been shown to regulate the expression of genes involved in cell proliferation and transforming growth factor-β signaling, known to drive tumor cells to EMT transformation (41,42).…”

“…Notably, it has been shown that individual TAFs are required for the expression of only a specific subset of genes (35)(36)(37)(38), and that the TFIID, or any of its other forms, was recruited in core promoters by a direct interaction between TAFs and their genespecific activators (39). Given that TAF4b target promoter selectivity is enhanced by activators such as c-Jun and Sp-1 (12), we propose a synergistic function of the two factors, c-Jun and TAF4b, through their interaction in which TAF4b is the coactivator of AP-1 during regulation of AP-1 target genes (Fig.…”

“…On the other hand, TAF4b was shown to regulate integrin α6 in different cell lines: a colon adenocarcinoma cell line, its derivative Caco-H cell line constitutively overexpressing Ha-RAS, and in parallel, in an established colon cell line that has gained some EMT characteristics. Interestingly, in another study (36), integrin α6 was found to be upregulated in mouse fibroblasts after knockdown of TAF4 and its proven replacement by TAF4b.…”