Candidate of Metastasis 1;  Homo sapiens nuclear protein 1;  NUPR1;  p8

The p8 gene, first described as overexpressed in the pancreas during the acute phase of pancreatitis, encodes a ubiquitous nuclear and cytoplasmic stress protein. Expression of the p8 mRNA is rapid, strong, and transient in response to several stresses. The human p8 gene was assigned to chromosome 16, at position p11.2, and the gene is organized in three exons interrupted by two introns. The sizes of exons I, II, and III are 214, 150, and 329 nucleotides, respectively, and the complete mRNA sequence comprises 693 nucleotides (exclusive of the poly A tail) and has only one open reading frame.

The p8 gene was cloned from human, rat, mouse, and Xenopus laevis, conceptually translated from the Drosophila melanogaster genome or deduced from EST libraries (Bos taurus, Xenopus tropicalis, Zebrafish, Oryzias latipes, Bombyx mori, and Paralichthys olivaceus). p8 is a highly basic 82-aa polypeptide, with a theoretical molecular mass of about 8 kDa, containing a canonical bipartite domain of positively charged amino acids typical of nuclear localization signals (NLS), and a nuclear/cytoplasmic location has been demonstrated for human p8. Importantly, the nuclear or cytoplasmic localization of p8 depends on growth conditions. When cells are growing, p8 is nuclear, whereas it is at the cytoplasm when cell growth is arrested. The transport to the nucleus is ATP dependent, and localization is also regulated by its p300-dependent acetylation.

The p8 protein contains an N-terminal PEST region (Pro/Glu/Ser/Thr rich), suggesting a regulation of p8 expression by the ubiquitin (Ub)/proteasome system. Homology searching in databases yielded no homology of p8 with other proteins of known function. Biochemical properties of the mammalian p8 proteins are a high isoelectric point (9.6–10.4), 14% of acidic amino acids, 20–24% of basic amino acids, 14–17% of phosphorylatable amino acids (serine, threonine, and tyrosine), and a high abundance of proline (6–9%) and glycine (5–6%). The negatively charged residues appear located at the amino-terminal region, and all the positive residues accumulate in the carboxy-terminal portion of the molecule. All these features are shared by some of the high-mobility-group (HMG) proteins, particularly by the HMG-I/Y, family. The overall identity of human p8 with human HMG-I/Y is only around 35%, but the molecular mass; the isoelectric point; the percentage of Arg + Lys, Glu + Asp, Ser + Thr, and Gly + Pro; the hydrophilicity plot, and the charge separation (despite a reverse orientation) are very similar. A characteristic property of these HMG proteins, also shared by p8, is that they are neither denatured by heating at 100 °C nor precipitated by 2% trichloroacetic acid. NMR and CD analyses of p8 showed absence of stable secondary structure. The protein binds DNA weakly and is a substrate for protein kinase A (PKA). The phosphorylated p8 (PKAp8) has a higher content of secondary structure than the nonphosphorylated protein, and PKAp8 binds DNA strongly. Moreover, secondary structure prediction methods indicated that within the high homology region of other proteins, there is a basic helix-loop-helix secondary structure motif, characteristic of some classes of transcription factors. An architectural role in transcription is proposed, and several apparently unrelated functions have been ascribed to p8.

p8 is described as a stage-specific component of the gonadotrope transcriptome that may play a functional role in the initiation of LHβ gene expression during embryonic cellular differentiation. p8 is also a transcriptional regulator critical to two key cellular events in heart failure: cardiomyocyte hypertrophy and cardiac fibroblast matrix metalloprotease (MMP) expression. Furthermore, p8 is an important component of the defense program. For example, inactivation of the p8 gene increases liver sensitivity to CCl4 or increases sensitivity to systemic LPS treatment. Thus, p8 is an important element in the  stress response. Finally, p8 and p53 are involved in an autoregulatory loop, p8 regulating p53 transactivation activity and p53 acting as a strong repressor of p8 expression. Also, p8 is involved in two major mechanisms, cell cycle regulation and  apoptosis. To account for these various functions, it is suggested that the small size of the protein, its lack of specific tridimensional structure, and its nuclear-cytoplasm localization allow its interaction with several partners to target different signaling pathways. Several partners have been identified by yeast two-hybrid screening cDNA libraries.

p8 and Apoptosis

p8 expression has been inversely correlated to apoptosis in samples obtained from human pancreatic and breast cancers. According to this antiapoptotic effect, the interaction of p8/prothymosin alpha (ProTα), one of the molecular partners of p8, is very exciting. This natively unstructured protein was originally considered as a thymic hormone, but like p8, it was eventually attributed to several other functions. p8 and ProTα are two small proteins without stable secondary structure in solution, showing opposite electrostatic charges at neutral pH. They interact and promote mutual stabilization of their structures in a particular conformation, the resulting p8/ProTα complex becoming able to block staurosporine-induced apoptosis. In other words, two natively unfolded proteins, p8 and ProTα, which had been attributed an antiapoptotic function, are in fact inactive if alone and require interacting with each other to exert that function, probably because the active complex has acquired stable secondary structure. Moreover, p8 is involved in the effect of  gemcitabine, the only chemotherapeutic treatment presently available for pancreatic cancer. It was demonstrated that in pancreatic cancer cells, a large part of gemcitabine-induced apoptosis results from the inhibition of the constitutive antiapoptotic activity of p8 (Fig. 1).

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p8 can also act as a proapoptotic protein. In fibroblast, the presence of p8 sensitizes cells to apoptosis induced by DNA damage. Moreover, recent works demonstrate a link between p8 and the antitumoral effects of  cannabinoids. 9-tetrahydrocannabinol (THC) is the most abundant compound of Cannabis sativa, with potential therapeutic applications in patients with cancer. This antitumoral action of THC relies, at least in part, on its ability to induce p8 expression and subsequent p8-mediated apoptosis of tumor cells.