The Homeobox Protein MSX2 Interacts with Tax Oncoproteins and Represses Their Transactivation Activity

Abstract

Bovine leukemia virus (BLV)taxHuman T-cell leukemia virus type 1 (HTLV-1) 1 and bovine leukemia virus (BLV) are members of the Deltaretrovirus genus in the Retroviridae family (1-4). In addition to the structural proteins Gag, Pol, and Env, these viruses also encode a series of regulatory proteins: Tax, Rex, p12/R3, and p13/G4. The Tax protein is a transcriptional activator, which increases the synthesis of viral proteins acting on a triplicate 21-bp element located in the 5Ј long terminal repeat (LTR) (5-7). Tax does not interact directly with DNA but rather acts via cellular factors, such as members of the CREB/ATF family of basic leucine zipper proteins (8 -11). The tax gene is believed to be essential because its presence is absolutely required for infectivity in vivo (12). Besides its role in the regulation of transcription, the Tax protein also exhibits an oncogenic potential (13). Tax behaves as an immortalizing oncogene because it is able to cooperate with the Ha-ras oncoprotein to fully transform primary rat embryo fibroblasts (14, 15). Both transactivation and immortalizing functions of Tax can be dissociated by mutations in specific regions of the protein. For example, mutation of the phosphorylation sites at serines 106 and 293 of BLV Tax abrogates immortalization potential in vitro but maintains transcriptional activity and viral oncogenicity in vivo (16,17). Conversely, the transactivation of the LTR promoter is not required for Tax to transform primary cells in vitro (14). The Tax protein of HTLV-1 (Tax1) is known to activate several cellular genes including IL-2, IL-2R␣, IL-3, TNF-␣, and . Tax1 is also involved in cell cycle regulation by direct activation of cyclin D3 and cyclin kinases cdk4 and cdk6 (21), or by inactivating the cyclin-dependent kinase inhibitor p16 INK4A (22). In fact, protein-protein interactions with cellular factors are crucial for Tax1 to perturb the regulation of many cellular pathways (23). These HTLV-1 Tax binding factors include the human mitotic checkpoint protein HsMAD1 (24), MEKK1 (25), the IB kinase (26), or the PCAF protein (27). In contrast, little is known about the cellular partners of the BLV Tax protein. In this study, we describe a functional interaction between the homeobox protein MSX2 and the BLV Tax oncoprotein. EXPERIMENTAL PROCEDURESPlasmids-Plasmids pSGMSX2 and pEGFPMSX2 were constructed by subcloning the human MSX2-cDNA (kindly offered by Dr. T. Iimura, Tokyo Medical and Dental University, Japan) into pSG5 (Stratagene) and pEGFP-C1 (Clontech), respectively. Plasmids pcDNAMSX2-c-Myc, pcDNAMSX2⌬N-c-Myc, and pcDNAMSX2⌬C-c-Myc, which express respectively, C-terminal c-Myc-tagged full-length, amino acids 80 -267 and amino acids 1-200 of the human MSX-2 protein, were constructed by inserting the human MSX2 cDNA, or PCR-derived MSX2 truncation mutants into pcDNA3.1/myc-HisB (Invitrogen). Plasmid pcDNARAP74-flag was obtained by subcloning the human RAP74 cDNA (kindly offered by Dr. Z. Burton, Michigan State University) into pcDNA3.1Flag (Invitrogen).The pLTRL...

XBP-1, a Novel Human T-Lymphotropic Virus Type 1 (HTLV-1) Tax Binding Protein, Activates HTLV-1 Basal and Tax-Activated Transcription

“…All three cellular proteins have been shown to associate with HTLV-1 Tax. CREB1 and CREB2 are positive regulators of HTLV-1 Taxactivated transcription, while MSX2 represses Tax transactivation (35,44,45). In the absence of Tax, XBP-1S and XBP-1U stimulated ninefold and fivefold increases in HTLV-1 transcription in Jurkat cells, respectively ( Fig.…”

MSX2 mediates entry of human pluripotent stem cells into mesendoderm by simultaneously suppressing SOX2 and activating NODAL signaling

“…We next generated another MSX2 mutant containing a deletion (∆132-148; Figure 5A) within the homeodomain that carries core suppressor function of MSX2 [28,58]. In contrast to the T147A and P148H mutants, the MSX2-∆132-148 mutant failed to bind to MBS2 and MBS3 (Supplementary information, Figure S5C) and could not inhibit SOX2 promoter activity or induce mesendoderm differentiation of hESCs ( Figure 5B-5D).…”

Mechanisms of leukemogenesis induced by bovine leukemia virus: prospects for novel anti-retroviral therapies in human

“…This immortalization process correlates with increased Bcl-2 protein levels, nuclear accumulation of NFκB and a series of intracellular pathways which remain to be characterized [203]. Tax also inhibits base-excision DNA repair of oxidative damage, potentially increasing the accumulation of ambient mutations in cellular DNA [204]. …”