Abstract. Fibronectin (FN) represents the mixture of a number of structurally different molecules (isoforms) whose make-up varies depending on the FN sources. FN from cultured transformed human cells has a very different isoform composition with respect to its normal counterpart. In fact, SV-40-transformed WI-38VAI3 human fibroblasts produce high levels of a FN isoform (B-FN) which is very poorly expressed in their normal, WI-38, counterpart. We have recently demonstrated that the B-FN isoform derives from a differential splicing pattern of the FN primary transcript which leads, in transformed cells, to a high level expression of the exon ED-B (Zardi, L., B. Carnemolla, A. Siri, T. E. Petersen, G. Paolella, G. Sebastio, and F. E. . EMBO (Eur. Mol. Biol. Organ.) J. 6:2337-2342). Here we report on the production and characterization of a monoclonal antibody (BC-1) which recognizes an epitope within the protein sequence coded for by the ED-B exon. This monoclonal antibody makes it possible to carry out immunohistochemical analysis of the distribution of the ED-B-containing FN isoform (B-FN) in human tissues. The results show that while in normal, adult, human tissues total FN has a widespread distribution, the B-FN isoform is restricted only to synovial cells, to some vessels and areas of the interstitium of the ovary, and to the myometrium. On the contrary, the B-FN isoform has a much greater expression in fetal and tumor tissues. These results demonstrate that, in vivo, different FN isoforms have a differential distribution and indicate that the B-FN isoform may play a role in ontogenesis and oncogenetic processes. It has been previously demonstrated that FN polymorphism is at least partially caused by alternative splicing schemes in three regions of the primary transcript of a single gene which may generate 20 different FN subunit isoforms (18,19,29). One of these regions (IIICS, see Fig. 1 A) is between the last two type lZI homology repeats; a single exon is subdivided to yield five alternative patterns of splicing. Hynes and co-workers (34) showed that inclusion of the IIICS sequence contributed to the differences in size between the larger and smaller subunit of plasma FN. Humphries et At the second region of variation (ED-A), a single exon encoding a complete type III repeat is either included or omitted from the mature mRNA. This variation is tissue specific and the ED-A sequence is absent in the mRNA of liver (10,20,21) which is the source of plasma FN (38). Using a rabbit antiserum to the rat ED-A segment, Paul et al. (31) At the third region of variation (ED-B), a single exon encoding a complete type III repeat is either included or omitted from the mature mRNA (14,33,44). The ED-B sequence presents two interesting peculiarities: (a) it is the more conserved FN region, 100 and 96% homology with rat and chicken FN, respectively (28,44); and (b) this exon is highly expressed in transformed human cells, while it is barely de-

Mutations at the white spotting (w) locus in mice have deleterious effects on germ cells, melanocytes and hematopoietic stem cells. The w locus encodes the c-kit tyrosine-kinase receptor whose ligand is the product of the SI locus. Using monoclonal antibodies (MAb(s)) to the extracellular domain, we have evaluated the expression of c-kit in normal and transformed melanocytes. This cell lineage synthesizes a receptor with a mw of 145 kDa. The gene product is expressed in epidermal melanocytes and in a fraction of nevocytic and blue nevi. In primary melanomas, loss of the receptor is observed in more invasive lesions. Only 30% of the metastatic lesions express detectable levels of the receptor. These findings demonstrate that the c-kit product is down-regulated in melanocytes following malignant transformation. The functional relevance of this modulation remains to be evaluated.

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The human homolog of the rat neu oncogene, HER2 (also termed c-erbB2) has been demonstrated in amplified form in human breast tumors with poor prognosis. Although amplification of the gene correlates with expression of a 185-kDa transmembrane glycoprotein, no extensive information is available regarding the extent of tissue and tumor specificity of this gene product. We have addressed this issue by immunohistochemically evaluating the expression of p185 HER2 in normal tissue and various tumors using monoclonal antibodies (MAbs) to distinct epitopes of its extracellular domain. No detectable levels of p185 HER2 were found in fetal tissues analyzed, with the exception of renal tubules in 2 out of 3 specimens tested and in intestinal epithelium. In adult tissues, detectable levels of this glycoprotein were found in a restricted number of cell types, the expression being heterogeneous among individuals and cell histotypes. Among the neoplasms assayed p185 HER2 was expressed in 46% of primary breast cancers, in 28% of ovarian tumors and in 30% of colon rectum malignancies. No male breast adenocarcinomas were p185-positive. A large number of other tumors tested revealed only a low incidence of expression of the p185. In metastatic breast tumors p185 HER2 was demonstrated homogeneously among multiple autologous lesions and almost invariably (80%) the expression of p185 in the primary lesion correlated with that of the deriving metastases. Our findings indicate that the expression of the p185 HER2 represents a tumor marker of clinical relevance in breast cancer. Whether this holds true for other malignancies remains to be explored.

Tenascin (TN) is a high-molecular-mass oligomeric glycoprotein of the extracellular matrix (ECM) endowed with a programmed expression in embryonic life and a neo-expression in the interstitium of some malignancies. Using monoclonal antibodies (MAbs) which identify human tenascin, we have conducted an extensive immunohistochemical analysis of TN expression in normal fetal and adult human tissues as well as in a wide variety of human tumors. Results of this study demonstrate that TN (1) is detectable in embryonic and fetal tissues at least from the 10th week of gestation; (2) is present in the interstitium of a variety of adult tissues of different embryonic origin; (3) may be neo-expressed in the stroma of benign and malignant tumors; (4) has the ability to accumulate in a highly variable manner in the ECM of tumors of the same and of different histotypes.