Biochemical characterization of Trop-2, a cell surface molecule expressed by human carcinomas: formal proof that the monoclonal antibodies T16 and MOv-16 recognize Trop-2

Cloning of the murine TROP2 gene: conservation of a PIP2-binding sequence in the cytoplasmic domain of TROP-2

Trop-2 is a novel calcium signal transducer expressed at high levels by most human carcinomas. To develop an animal model to study the function of this molecule in vivo, we have cloned the murine Trop2 gene. Using human TROP2 primers, we amplified by PCR a segment of murine Trop2. This was used as a probe to clone a full-length gene by hybridization of a genomic library. The cloned murine Trop2 gene is functional, as indicated by sequencing and by expression after transfection. The murine Trop2 is 87.4% similar to its human homologue, with the highest conservation in the extracellular region between residues 86 and 157. Essentially all cysteines are conserved between the human and the murine genes, suggesting conservation of the Trop2 disulfide bridges and of its overall structure. Intriguingly, the cytoplasmic tail of Trop2 shows a highly conserved phosphatidylinositol 4,5-bisphosphate (PIP2)-binding sequence, which overlaps with a protein kinase C phosphorylation site. Thus, we speculate that PIP2 might regulate the phosphorylation state of Trop2 and play a role in its signal transduction. Murine Trop2 mRNA is detected in normal kidney, lung, ovary and testis, similarly to the human gene. Interestingly, the highest levels of expression are found in immortalized keratinocytes. Since Trop2 is undetectable in undifferentiated spindle cell carcinomas, this suggests a preferential expression at early stages of tumor progression.

Cloning of the gene encoding Trop-2, a cell-surface glycoprotein expressed by human carcinomas

We have cloned by expression the cDNA encoding Trop-2, a cell-surface glycoprotein expressed by most human carcinomas. Formal proof of the identity of the clone is the hybridization to DNA and RNA from genomic TROP2 transfectants. TROP2 is a single-copy gene in human cells, hybridizes to a single 1.8-kb mRNA from expressing sources and encodes a 35,709 Da type-1 transmembrane protein with a single transmembrane domain. TROP2 is essentially identical to GA733-1. Thus, we have proven that GA733-1, for which a protein product had not been identified, is a functional gene. TROP2 is also homologous to TROP1/KSA/GA733-2, confirming the serological similarities between the 2 molecules. The homology between the Trop-1 and Trop-2 peptides is clustered in 2 extracytoplasmic domains and in the transmembrane/cytoplasmic region. Twelve cysteines and a potential cytoplasmic tyrosine phosphorylation site are also conserved. Trop-1 and Trop-2 are homologous to serum IGF-II-binding proteins and appear as signal transducers. Thus, they likely represent novel cell-surface receptors and may play a role in regulating the growth of carcinoma cells. On the other hand, we have found no evidence for a role of Trop-2 and Trop-1 as homophilic adhesion molecules.

Transformation of human keratinocytes is characterized by quantitative and qualitative alterations of the T-16 antigen (Trop-2, MOv-16)

The regulation of synthesis and post-translational processing of the T-16 antigen, a human cell-surface glycoprotein of 50 to 60 kDa, was investigated in normal and transformed human keratinocytes in vitro. Normal keratinocytes of interfollicular and follicular origin were compared with squamous-cell-carcinoma lines, spontaneously immortalized keratinocytes, and SV-40 transformed keratinocytes. FACS analysis and radio-immunoprecipitation showed that the synthesis and expression of T-16 was 3- to 4-fold higher in transformed keratinocytes than in their normal counterparts. In normal keratinocytes, no quantitative differences were observed among freshly prepared cells, primary cultures and sub-cultures. In SDS-PAGE, a single broad band at 50 to 60 kDa was observed in normal keratinocytes, whereas 2 bands at 42 and 45 to 55 kDa were detected after transformation. Tunicamycin treatment of living cells and glycosidase digestion of immunopurified T-16 antigen revealed this molecular heterogeneity to be due to different N-glycosylation in normal and transformed keratinocytes. In pulse-chase experiments, 2 distinct precursor proteins at 38 and 42 kDa were detected in transformed keratinocytes, whereas in normal cells the 38-kDa signal was dramatically decreased. These findings indicate that quantitative and qualitative changes of T-16 mark the transformation process of human keratinocytes, showing similar post-translational alterations in all transformed populations investigated.

DNA methylation prevents the amplification of TROP1, a tumor-associated cell surface antigen gene

We tested the hypothesis that different genes can have different abilities to be amplified after transfection under comparable selection conditions. DNA from human lymphoid or choriocarcinoma cell lines was transfected into L cells. Transfectants for CD5, CD8A, TROP1, and TROP2, genes expressed on lymphocytes or trophoblast and carcinomas, were selected by fluorescence-activated cell sorting. To select for amplification of the transfected gene we cloned twice by fluorescence-activated cell sorting the transfectants with the highest expression. We analyzed a total of 38 families (1768 clones) derived from the original transfectants. We then analyzed by Southern blotting the clones with the highest increase in surface expression and determined the copy number of each transfected gene. CD5, CD8A, and TROP2 were amplified with high frequency and progressively, whereas TROP1 essentially was not amplified at all. We examined the hypothesis that DNA methylation prevents the amplification of the TROP1 gene by treating JAR choriocarcinoma cells with 5-azacytidine to decrease DNA methylation. DNA extracted at different times after the treatment was used for transfection. When DNA that showed demethylation of the TROP1 gene was used, 16 Trop-1 transfectants were obtained and 6 of them were found to contain up to 40 copies of the TROP1 gene per haploid genome. Thus, we showed that transfectants obtained from a demethylated TROP1 gene were amplified efficiently and progressively. We propose that DNA methylation affects DNA amplification either by altering the recognition of methylated DNA sequences or by changing the conformation of the chromatin of methylated segments. We speculate that DNA methylation is a determinant of gene amplification in vivo, for example in tumor cells.