br Introduction br The p protein produced from mRNA
The p53 protein, produced from mRNA encoded by the TP53 gene, plays a perhaps more crucial role than any other tumor suppressor in preventing tumorigenesis and tumor progression. Moreover, p53 is an
essential and powerful transcription factor, acting through the binding of its DNA binding domain (DBD) to corresponding promoter elements, whereby it can transactivate the expression of p53 target genes, ap-proximately 246 human genes, including p21, Bax and PUMA [1,2]. In response to DNA damage stress and other oncogenic stresses, cells
Abbreviations: FB1, fumonisin B1; Gb3, globotriaosylceramide; GCS, glucosylceramide synthase; GOF, gain-of-function; G > A, guanine transited adenosine; m6A, N6-methyladenosine; MeRIP, N6-methyladenosine RNA immunoprecipitation; METTL3, methyltransferase like 3; NPC, neplanocin A; Oxa, oxaliplatin; PARP, poly (ADP-ribose) polymerase (PARP); PDMP, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol; pp53, phosphorylated p53 (Ser15); siRNA, silencing RNA; SRSF, serine/arginine-rich splicing factor; STxB, Shiga toxin 1 B-subunit
Corresponding author at: School of Basic Pharmaceutical and Toxicological Sciences, University of Louisiana at Monroe, 700 University Avenue, Monroe, LA 71201, USA. E-mail address: [email protected] (Y.-Y. Liu).
1 Present address: Center for Drug Evaluation and Research, U.S. Food and Drug Administration, Silver Spring, MD 20993, USA.
Available online 19 December 2018
highly express p53 protein, which, with concomitant upregulation of the expression of p53 target genes, normally enables p53 to trigger cell-cycle arrest, senescence, and cell death by apoptosis or ferroptosis [2–4]. It has been found that TP53 mutation is one of the most common genetic alterations in cancers, appearing in approximately 42% of cases across 12 tumor types carrying mutant TP53, the presence of which correlates to poor prognosis [5,6]. Among all such TP53 mutations, more than 75% are missense point-mutations extant in the region en-coding the DBD, and these produce full-length, missense proteins that function aberrantly with respect to their transactivation of p53 target genes [7,8]. The transition from guanine to Cyclosporin H (G > A) at codons 175, 248, and 273 accounts for 11.2% of all TP53 mutations in cancers appearing in the colon and lungs [9–11] (http://p53.free.fr/Database/ p53_cancer/all_cancer.html). p53 missense proteins that lack the tumor suppression activity of wild-type p53 (wt p53) instead often exhibit oncogenic gain-of-function (GOF) . Knock-in mouse models that express hot-spot mutant alleles R172H or R270H (R175H or R273H in human) manifest GOF by conferring a broader tumor spectrum and more tumor metastases, as compared with wt p53-expressing mice . TP53 mutants appear with increased frequency in tumors diagnosed at advanced stages, or with more metastases, and in recurrences of cancers in colon, ovaries and breasts [14,15]. Missense p53 mutants thus de-serve strong attention with respect to therapeutic targeting aimed at improving cancer treatments. r> Under normal conditions, p53 protein levels are low, owing to feedback regulation by p53-activated MDM2-mediated degradation. In cancer cells, wt p53 can be activated by stress conditions, including oncogenic activation (oncogenic stress) and DNA damage . Mis-sense p53 mutants are expressed at high levels in cancer cells, in part owing to failure of mutant proteins to induce expression of MDM2 . The small molecules PRIMA-1 and APR-246 promote refolding of p53 mutant proteins (R273H, R175H) by the binding of the reactive me-thylene quinuclidinone (MQ) moiety to cysteine, thereby enabling mutant protein to activate p53 target genes, including p21, Bax and PUMA, in tumor cells [17,18]. As an augmentation to effecting the refolding of missense proteins for reactivating p53 function, our recent work indicates that it is possible to eliminate mutant protein while restoring wt p53 expression in cancer cells. Inhibition of glucosylcer-amide synthase (GCS) restores wt p53 protein levels, and abolishes oncogenic GOF, in cells heterozygously carrying a TP53 R273H muta-tion .
Unearthing how cells select pre-mRNA molecules to generate mRNA transcripts coding protein for wt vs. mutant can be expected to reveal therapeutic intervention opportunities for restoring p53 in cells car-rying TP53 missense mutations. DNA sequences determine the se-quences of pre-mRNA; however, further RNA processing, including pre-mRNA splicing and RNA methylation, contributes to the post-transcriptional regulation of protein expression . During pre-mRNA splicing, a dynamically assembled spliceosome of nuclear ribonucleo-protein (snRNP) complexes recognizes splice sites in pre-mRNA and catalyzes two transesterification reactions, so as to excise introns and splice exons together to form a mature and functional mRNA for translation to produce proteins [20,21]. Alternative splicing, in which extrinsic and non-spliceosomal RNA-binding proteins (classical/cano-nical hnRNPs, SR proteins, tissue-specific RNA-binding proteins) are involved in recognizing introns in pre-mRNA, allowing generation of more than one unique mRNA species from a single gene [20,21]. Al-terative splicing can generate mRNAs that differ in their untranslated regions or coding sequences through mechanisms that include exon-skipping, a choice between exons, the use of alternative splice sites, or intron retention . Aberrant RNA splicing has been found, in a growing number of instances, to underlie human diseases, including cancers . Upregulation of epithelial-restricted splicing proteins (ERSP1, ERSP2) and SR proteins (SRSF1, SRSF3) in cancer cells con-tributes to cancer progression [23,24].