The product of the cph oncogene is a truncated, nucleotide-binding protein that enhances cellular survival to stress

Abstract

Cph was isolated from neoplastic Syrian hamster embryo ®broblasts initiated by 3-methylcholanthrene (MCA), and was shown to be a single copy gene in the hamster genome, conserved from yeast to human cells, expressed in fetal cells and most adult tissues, and acting synergistically with H-ras in the transformation of murine NIH3T3 ®broblasts. We have now isolated Syrian hamster full-length cDNAs for the cph oncogene and proto-oncogene. Nucleotide sequence analysis revealed that cph was activated in MCA-treated cells by a point-mutational deletion at codon 214, which caused a shift in the normal open reading frame (ORF) and brought a translation termination codon 33 amino acids downstream. While proto-cph encodes a protein (pcph) of 469 amino acids, cph encodes a truncated protein (cph) of 246 amino acids with a new, hydrophobic C-terminus. Similar mechanisms activated cph in other MCA-treated Syrian hamster cells. The cph and proto-cph proteins have partial sequence homology with two protein families: GDP/GTP exchange factors and nucleotide phosphohydrolases. In vitro translated, gel-puri®ed cph proteins did not catalyze nucleotide exchange for H-ras, but were able to bind nucleotide phosphates, in particular ribonucleotide diphosphates such as UDP and GDP. Steady-state levels of cph mRNA increased 6.7-fold in hamster neoplastic cells, relative to a 2.2-fold increase in normal cells, when they were subjected to a nutritional stress such as serum deprivation. Moreover, cphtransformed NIH3T3 cells showed increased survival to various forms of stress (serum starvation, hyperthermia, ionizing radiation), strongly suggesting that cph participates in cellular mechanisms of response to stress.

Cellular function and molecular structure of ecto-nucleotidases

“…The cellular mechanisms underlying these changes are not fully understood, but it is of interest that ENTPD5 has also been identified as the PCPH proto-oncogene [67]. The PCPH oncogene is a truncated form of ENTPD5 [112]. Furthermore, expression of the normal NTPDase5 protein is deregulated or lost in some human cancers and in a wide variety of malignant cells, consistent with a role as a tumor suppressor [111].…”

PCPH expression is an early event in the development of testicular germ cell tumors

“…The mechanism of oncogenic activation of the PCPH gene is known to involve the deletion of a single base pair within its open reading frame (ORF) (2). This point-mutational event shifts the normal ORF, causing the translation of the mutated protein to terminate 33 amino acid residues downstream of the mutated codon.…”

“…3b and d) in glandular embryonal carcinoma samples may suggest that PCPH might be secreted to the extracellular compartment by the neoplastic cells. The presence of a putative signal peptide in the primary sequence of PCPH, deduced by translation of its nucleotide sequence (2,26,27), may satisfy the necessary structural/ functional requirements to allow for PCPH secretion.…”

“…Indeed, promotion of resistance to stress stimuli such as nutritional starvation, ionizing radiation or chemotherapeutic agents contributes to the neoplastic transformation activity of the PCPH oncogene (2). The survival-promoting function of the PCPH oncoprotein is mediated by its ability to antagonize proapoptotic signaling pathways activated by exposure to ionizing radiation (4) or chemotherapeutic drugs such as cisplatin (Tirado et al, Proc Am Assoc Cancer Res 45: 1158, abs.…”

“…PCPH is a gene involved in the regulation of cell proliferation, as evidenced by its initial identification as an oncogene, formerly termed cph (1), and of the cellular response to stress (2,3). Indeed, promotion of resistance to stress stimuli such as nutritional starvation, ionizing radiation or chemotherapeutic agents contributes to the neoplastic transformation activity of the PCPH oncogene (2).…”