Expression of cell surface sialic acid and galactose by normal adult human melanocytes in culture

Recombinant adeno-associated virus serotype 6 efficiently transduces primary human melanocytes

The study of melanocyte biology is important to understand their role in health and disease. However, current methods of gene transfer into melanocytes are limited by safety or efficacy. Recombinant adeno-associated virus (rAAV) has been extensively investigated as a gene therapy vector, is safe and is associated with persistent transgene expression without genome integration. There are twelve serotypes and many capsid variants of rAAV. However, a comparative study to determine which rAAV is most efficient at transducing primary human melanocytes has not been conducted. We therefore sought to determine the optimum rAAV variant for use in the in vitro transduction of primary human melanocytes, which could also be informative to future in vivo studies. We have screened eight variants of rAAV for their ability to transduce primary human melanocytes and identified rAAV6 as the optimal serotype, transducing 7-78% of cells. No increase in transduction was seen with rAAV6 tyrosine capsid mutants. The number of cells expressing the transgene peaked at 6-12 days post-infection, and transduced cells were still detectable at day 28. Therefore rAAV6 should be considered as a non-integrating vector for the transduction of primary human melanocytes.

Cytoplasmic accumulation of peanut agglutinin-binding glycoconjugates in the cells of primary melanoma correlates with clinical outcome

In an experimental model, human melanoma cell lines enriched for cells that express the glycoconjugate B-D galactose N-acetyl-D-galactosamine, which reacts with the peanut agglutinin lectin (PNA), are associated with an increase in the frequency of metastases. We previously showed that this glycoconjugate is expressed on the cells of some primary melanomas in humans and that such cells are found selectively in melanomas with a high risk for developing metastases and causing death. Using fixed archival tissues from 99 primary melanomas and lectin histochemistry, we found 65 tumors that contained melanoma cells that were PNA-positive. PNA-reactive cells were not identified in normal melanocytes or in the nevocytes of 24 nevi. PNA-reactive material accumulates adjacent to the nucleus in the area of the Golgi apparatus, initially as a tiny dot, but later in quantities sufficient to displace and indent the nucleus, producing a signet ring cell-like appearance. Tumor cells containing PNA-reactive material were associated with more evolved, deeper, and thicker tumors. Two melanomas up to Clark level II were PNA positive (20%), compared with 60% of level III, 76% of level IV, and 100% of level V. Five of 13 tumors less than 0.76 mm thick (39%) were positive, compared with 50% of tumors 0.76 to 1.49 mm thick, 64% of tumors 1.5 to 2.99 mm thick, and 85% of tumors 3 mm thick or thicker. PNA-reactivity was negatively correlated with disease-free survival (PNA-negative, 49.2+/-23 months; PNA-positive grade 1, 41.6+/-26 months and PNA-positive grade 2, 24.4+/-23 months), survival rate 5 years after initial treatment (PNA-negative, 84.8%; PNA-positive grade 1, 63.8%; and PNA-positive grade 2, 31.3%) and disease-free survival at 5 years after initial treatment (PNA-negative, 69.7%; PNA-positive grade 1, 53.2%; and PNA-positive grade 2, 25%).

Selective expression of PNA-binding glycoconjugates by invasive human melanomas: a new marker of metastatic potential

Alterations of cell-surface glycoconjugates have been associated with invasiveness and metastatic capacity in a number of experimental and human tumors (bladder and colon cancer). We have recently shown that human melanoma cells from variants selected for high metastatic potential in an animal model bind the lectin peanut agglutinin (PNA), and that human melanoma cell populations enriched for PNA binding cells generated a higher frequency of metastases when xenografted into immune suppressed neonatal rats. We have therefore sought cells binding PNA in biopsied human melanocytic tumors and compared frequencies of PNA binding by cells from benign nevi, early and late primary melanomas, and metastatic melanomas. Sections of conventionally processed tissues were deparaffinised and exposed to biotinylated PNA; PNA fixation was revealed by the avidine/peroxidase/AEC technique. In 51 specimens tested, PNA appears to react electively with invasive tumors, since only one of the 7 early primary melanomas (Clark I-II) reacted while 13/23 late primary melanomas (Clark III-V), and 4/21 melanoma metastases were reactive. In addition, only 1/17 benign nevi bound PNA. In primary tumors, the reactive cells were exclusively invasive tumors cells in the dermis. PNA reactive material was observed in the cytoplasm and plasma membrane of reactive cells. Hence, alterations in composition and cellular localisation of glycoconjugates detectable by lectin histochemistry in melanoma cells may be markers of metastatic potential that may be applicable on an individual patient basis.

Expression of PNA-binding sites on specific glycoproteins by human melanoma cells is associated with a high metastatic potential

Lectin-binding patterns of seven human melanoma clones and variants selected from the same parental cell line and differing in their spontaneous metastatic potential in an animal model were compared by flow cytometry and Scatchard analysis. Human melanoma clones and variants with high and low metastatic potential could be distinguished by their peanut agglutinin (PNA)-binding patterns, but not by their wheat germ agglutinin (WGA)-, Ulex europaeus agglutinin I (UEA I)-, and soybean agglutinin (SBA)-binding patterns. Low metastatic clones and variants proved to be made up of single poorly peanut agglutinin-binding cell population (2.20-3.52 x 10(6) sites/cell, Ka = 2.48-2.75 x 10(6) M-1). By contrast, highly metastatic variants were found to be constituted by two cellular subpopulations, exhibiting respectively a moderate 2.62-3.72 x 10(6) sites/cell) and a high peanut agglutinin staining (17.68-18.76 x 10(6) sites/cell). One highly metastatic clone was found to be homogeneously constituted by a single population of cells strongly binding this lectin (18.86 x 10(6) sites/cell) with an association constant of 4.06 +/- 10(6) M-1. Using an EPICS V cytometer, these two subpopulations were sorted from a highly metastatic variant and tested for their metastatic abilities: cells with high PNA binding generated a higher frequency of metastases than did moderately PNA-binding cells. Following treatment with Vibrio cholerae neuraminidase, all cells from all variants and clones were brightly labeled by PNA, collecting in a single peak with similar fluorescence intensities. Electrophoresis of total cellular proteins and subsequent detection with labeled PNA om Western blots show two major PNA-reactive glycoproteins with apparent molecular weights of 140 and 110 kDa (MAGP1 and MAGP2), expressed only in highly metastatic cells, but which can be strongly labeled by PNA in slightly metastatic cells following a treatment with neuraminidase. These results provide evidence that the expression of terminal galactose (beta 1-3)N-acetyl galactosamine structure, positioned on MAGP1 and MAGP2 glycoproteins, is associated with the metastatic potential of human melanoma cells.