Sp1 and Sp3 physically interact and co-operate with GABP for the activation of the utrophin promoter¹

Abstract

The utrophin gene codes for a large cytoskeletal protein closely related to dystrophin which, in the absence of dystrophin, can functionally substitute it. Utrophin is transcribed by two independently regulated promoters about 50 kb apart. The upstream promoter is TATA-less and contains a functional GABP binding site which, in muscle, restricts the promoter activity to post-synaptic nuclei. Transient transfections analysis of mutant promoters in rhabdomyosarcoma cells showed that the upstream promoter contains three functional GC elements that are recognised by Sp1 and Sp3 factors in vitro. Co-transfections of the promoter with Sp1, Sp3 and GABP factors in Drosophila SL2 Schneider cells, which lack of endogenous Sp factors, demonstrated that both Sp1 and Sp3 are positive regulators of the utrophin promoter and that they activate transcription synergistically with GABP. Consistent with this result, we observed physical interaction of both Sp factors with the GABPα subunit in vitro. Functional domain interaction analysis of Sp1 and Sp3 revealed that both factors interact with GABPα through their DNA binding zinc finger domain. The modulation and correct interaction between Sp1, Sp3 and GABP in muscle cells may be critical for the regulation of the utrophin promoter, and provide new targets for therapies of Duchenne muscular dystrophy.

Introduction

The utrophin gene (also named dystrophin-related gene) is an autosomal homologue of dystrophin (Love et al., 1989), which when mutated is responsible for Duchenne and Becker muscular dystrophies (DMD and BMD, respectively). Utrophin is transcribed in a large mRNA of 13 kb coding for a 395 kDa protein, with up to 73% of amino acid identity with dystrophin in important functional domains Grady et al 1997, Pearce et al 1993. Functional substitution of utrophin with dystrophin in mice has demonstrated that a cure of DMD and BMD up-regulating the utrophin gene in patients is conceivable Campbell and Crosbie 1996, Deconinck et al 1997, Grady et al 1997, Rafael et al 1998, Tinsley et al 1996, Tinsley et al 1998. Utrophin is expressed ubiquitously, although in adult skeletal muscle its expression is mainly restricted to neuromuscular junctions Gramolini et al 1997, Khurana et al 1990, Schofield et al 1993. Utrophin is transcribed by two independently regulated promoters Burton et al 1999, Dennis et al 1996. The upstream promoter is a TATA-less promoter associated with a CpG island. It contains several GC sequences which are putative Sp factor binding sites, and a functional N box (Dennis et al., 1996). This promoter is also under the control of a downstream utrophin enhancer (DUE) localised at about 9 kb within the second intron (Galvagni & Oliviero, 2000). A second promoter is localised about 50 kb further downstream, which gives rise to a utrophin with a different N-terminal domain (Burton et al., 1999). Both promoters drive a wide distribution of utrophin transcripts with overlapping expression in most tissues. The upstream utrophin promoter is mostly expressed in skeletal muscle, while the intronic promoter is more active in heart muscle Burton et al 1999, Love et al 1991.

The upstream utrophin promoter in cultured muscle cells responds by two- to threefold induction to treatment with heregulin or by the transfection of the Ets-related GABP factor Gramolini et al 1999, Khurana et al 1999. GABP has been reported to activate several viral and cellular promoters Fromm and Burden 1998, LaMarco et al 1991, Thompson et al 1991, Triezenberg et al 1988, Virbasius et al 1993, Watanabe et al 1993. In response to heregulin, the GABPα protein level is increased and both α and β subunits are phosphorylated (Schaeffer et al., 1998). GABP activates transcription synergistically with several factors, including Sp1, and has been demonstrated to interact directly with ATF and HCF factors Ding et al 1999, Dittmer et al 1994, Gegonne et al 1993, Rosmarin et al 1998, Sawada et al 1999, Vogel and Kristie 2000.

The Sp family of transcription factors is composed of four zinc finger proteins Sp1-4, which in addition to the conserved DNA-binding domain, contain a glutamine-rich activation domain at the N-terminal region. Sp1, Sp3 and Sp4 recognise the consensus GC box element with identical affinity Hagen et al 1992, Hagen et al 1994. Sp4 expression is most abundant in neuronal tissues (Supp et al., 1996), while Sp1 and Sp3 are both ubiquitously expressed Dynan and Tjian 1983a, Dynan and Tjian 1983b, Hagen et al 1994, Kingsley and Winoto 1992. Sp1-deficient embryos die after day 10 of embryonic development, while Sp3-deficient mice die at birth as a result of respiratory failure Bouwman et al 2000, Marin et al 1997. Knockout mice phenotypes suggest that both factors have functional redundancy during early embryo development but exert distinct functions at later developmental stages. Sp1 binding to G+C-rich sequences are found in close proximity of transcriptional start sites and in enhancers (Pugh & Tjian, 1990). Accordingly, Sp1 has been shown to be associated both with general coactivators and with several promoter-specific transcription activators Hoey et al 1993, Kardassis et al 1999, Lee et al 1993, Rotheneder et al 1999, Ryu et al 1999, Seto et al 1993. Sp3 has been shown to activate several promoters. It also seems to act as a repressor, since it also contains an inhibitory domain Hagen et al 1994, Liang et al 1996, Majello et al 1997, Udvadia et al 1995, Zhao and Chang 1997.

Here, report the identification and characterisation of functional GC sites present on the upstream utrophin promoter. Using in vitro binding experiments and transient transfections, we demonstrated that both Sp1 and Sp3 act as activators and co-operate with GABP to activate the utrophin promoter. We propose that the synergistic transcriptional activation observed is due to direct physical interaction of GABP with both Sp1 and Sp3, and mapped it to the α-subunit of GABP and the DNA-binding zinc finger domain of both Sp1 and Sp3 factors.