Phosphorylation mediates Sp1 coupled activities of proteolytic processing, desumoylation and degradation

Abstract

Cell signaling pathways induce Sp1 phosphorylation, which allows for the upregulation of Sp1-dependent genes that control cell growth, cell cycle progression, survival and tumorigenesis. Sp1 activity is under constitutive repression through the sumoylation of Lysine-16, and Lysine-16 dependent N-terminal cleavage relieves this repression. The present investigation probes further into the mechanisms of Sp1 processing, desumoylation and degradation to reveal that phosphorylation is the major driving force behind these coupled activities. The first 7 amino acid residues of Sp1 enhance the accessibility of Lysine-16 to the homologous modifiers SUMO-1 and ubiquitin; and Serine-7 specifically enhances ubiquitinylation. Our data show that Serine-59 regulates Sp1 proteolytic processing, and thereby provides a mechanism for the upregulation of Sp1-dependent transcription by CyclinA/cdk2 phosphorylation of Serine-59. Sp1 activators, forskolin and PMA, enhance Sp1 processing in MCFE cells through distinct signaling pathways. PKC, ERK and ERBB2 kinase inhibitors suppress PMA induction of Sp1 and the specific isozyme PKCα enhances Sp1 cleavage. Sp1 contains several NFκB2-like proteolytic processing components including a functional phosphorylation-dependent β-TrCP binding motif. From these data, we propose a model by which cell cycle and mitotic kinases induce Sp1 proteolytic processing resulting in a desumoylated, derepressed and unstable Sp1 product.

A potent enhancer element in the 5′-UTR intron is crucial for transcriptional regulation of the human ubiquitin C gene

“…It will be now of great importance to determine if the Sp proteins, besides controlling basal gene expression, play also a pivotal role in the molecular mechanism(s) involved in the transcriptional regulation of the UbC promoter. Since several reports described the implications of ubiquitin-dependent proteolytic processing in controlling both the level and the transactivation potential of Sp1 (Spengler et al, 2008;Wang et al, 2008), our findings raised the intriguing hypothesis that Sp1-mediated expression of the ubiquitin substrate could, in turn, regulate the level of the transcription factor, thus establishing a regulatory loop to fine-tune UbC gene activity. In addition, we recently demonstrated that fluctuations in intracellular ubiquitin content may actively modulate the level of the regulatory protein p53 (Crinelli et al, 2008).…”

SUMOylation Inhibits SF-1 Activity by Reducing CDK7-Mediated Serine 203 Phosphorylation

“…Studies of multiple sequence-specific transcription factors indicate that phosphorylation of residues in the vicinity of SUMOylation sites can alter SUMOylation (17,29,37,59,63,66,67). Thus, a subset of SUMOylation motifs in factors such as HSFs (29) and ERR ␣ and ␥ (63, 67) conform to the recently described phosphorylation-dependent SUMOylation In this regard, phosphorylation may provide an additional negative charge, which appears to favor SUMOylation in certain contexts (72).…”

Okadaic acid inhibits the trichostatin A-mediated increase of human CYP46A1 neuronal expression in a ERK1/2-Sp3-dependent pathway

“…An interesting hypothesis is that Sp3 phosphorylation by ERK1/2 favors other post-translational modifi cations, such as acetylation or sumoylation, which can also be responsible for Sp3-mediated repression. Indeed, it has been reported that sumoylation and ubiquitination of Sp1 transcription factor can be mediated by phosphorylation ( 48 ), supporting a novel mechanism of Sp-dependent gene regulation. This is also a feature of the ERK1/2 MAPK family to control transcriptional regulation as players between phosphorylation-dependent post-translational modifi cations that can affect protein's transcriptional activity from ubiquitination to sumoylation and acetylation ( 49 ).…”