ERK-Dependent Phosphorylation of the Transcription Initiation Factor TIF-IA Is Required for RNA Polymerase I Transcription and Cell Growth

Abstract

Phosphorylation of transcription factors by mitogen-activated protein kinase (MAPK) cascades links cell signaling with the control of gene expression. Here we show that growth factors induce rRNA synthesis by activating MAPK-dependent signaling cascades that target the RNA polymerase I-specific transcription initiation factor TIF-IA. Activation of TIF-IA and ribosomal gene transcription is sensitive to PD98059, indicating that TIF-IA is targeted by MAPK in vivo. Phosphopeptide mapping and mutational analysis reveals two serine residues (S633 and S649) that are phosphorylated by ERK and RSK kinases. Replacement of S649 by alanine inactivates TIF-IA, inhibits pre-rRNA synthesis, and retards cell growth. The results provide a link between growth factor signaling, ribosome production, and cell growth, and may have a major impact on the mechanism of cell transformation.

RNA polymerase III transcription and cancer

“…Striking experiments using mutants of the pol I-specific factor TIF-IA suggest that rRNA synthesis is indeed limiting in HEK293 T cells. Thus, a constitutively active TIF-IA mutant accelerated proliferation by 43%, whereas dominant negative TIF-IA produced a 33% slowing (Zhao et al, 2003). Similarly, the hypertrophic growth of rat cardiac myocytes can be blocked by specifically preventing a 37% increase in rRNA synthesis (Brandenburger et al, 2001).…”

“…Hepatitis B virus can stimulate pol I transcription by raising TBP levels in a Ras-dependent manner . A more immediate effect of Ras can be mediated through Erk, which can bind, phosphorylate and activate two components of the pol I machinery (Stefanovsky et al, 2001;Zhao et al, 2003). Synthesis of large rRNA is also stimulated by CK2, which binds and phosphorylates both UBF and pol I itself (Belenguer et al, 1989;Voit et al, 1992Voit et al, , 1995Hannan et al, 1998;Voit and Grummt, 2001).…”

MAP kinases as structural adaptors and enzymatic activators in transcription complexes

“…For example, ERK-mediated phosphorylation of TIF-IA and the BRF1 subunit of TFIIIB induces transcription by pol I and pol III, respectively (Felton-Edkins et al, 2003;Zhao et al, 2003). Both ERK2 and the downstream kinase RSK interact with TIF-IA in vivo, ERK2 association being dependent on mitogenic stimulation (Zhao et al, 2003).…”

Akt activation enhances ribosomal RNA synthesis through casein kinase II and TIF-IA

“…TIF-IA interacts with the TBP-containing factor TIF-IB/SL1 and both are required to recruit Pol I to the rDNA promoter and generate a productive transcription initiation complex (4,5). TIF-IA is phosphorylated at multiple sites by a variety of protein kinases, with both positive and negative effects on its ability to initiate transcription (6)(7)(8), and is ubiquitinated and degraded by the proteasome (9). TIF-IA expression is also essential to maintaining nucleolar architecture and cell viability (10).…”