Human tumor spontaneous metastasis in immunosuppressed newborn rats. II. Multiple selections of human melanoma metastatic clones and variants

DNA methylome combined with chromosome cluster-oriented analysis provides an early signature for cutaneous melanoma aggressiveness

Aberrant DNA methylation is a well-known feature of tumours and has been associated with metastatic melanoma. However, since melanoma cells are highly heterogeneous, it has been challenging to use affected genes to predict tumour aggressiveness, metastatic evolution, and patients’ outcomes. We hypothesized that common aggressive hypermethylation signatures should emerge early in tumorigenesis and should be shared in aggressive cells, independent of the physiological context under which this trait arises. We compared paired melanoma cell lines with the following properties: (i) each pair comprises one aggressive counterpart and its parental cell line and (ii) the aggressive cell lines were each obtained from different host and their environment (human, rat, and mouse), though starting from the same parent cell line. Next, we developed a multi-step genomic pipeline that combines the DNA methylome profile with a chromosome cluster-oriented analysis. A total of 229 differentially hypermethylated genes was commonly found in the aggressive cell lines. Genome localization analysis revealed hypermethylation peaks and clusters, identifying eight hypermethylated gene promoters for validation in tissues from melanoma patients. Five Cytosine-phosphate-Guanine (CpGs) identified in primary melanoma tissues were transformed into a DNA methylation score that can predict survival (log-rank test, p=0.0008). This strategy is potentially universally applicable to other diseases involving DNA methylation alterations.

Tspan8 Drives Melanoma Dermal Invasion by Promoting ProMMP-9 Activation and Basement Membrane Proteolysis in a Keratinocyte-Dependent Manner

Melanoma is the most aggressive skin cancer with an extremely challenging therapy. The dermal-epidermal junction (DEJ) degradation and subsequent dermal invasion are the earliest steps of melanoma dissemination, but the mechanisms remain elusive. We previously identified Tspan8 as a key actor in melanoma invasiveness. Here, we investigated Tspan8 mechanisms of action during dermal invasion, using a validated skin-reconstruct-model that recapitulates melanoma dermal penetration through an authentic DEJ. We demonstrate that Tspan8 is sufficient to induce melanoma cells’ translocation to the dermis. Mechanistically, Tspan8⁺ melanoma cells cooperate with surrounding keratinocytes within the epidermis to promote keratinocyte-originated proMMP-9 activation process, collagen IV degradation and dermal colonization. This concurs with elevated active MMP-3 and low TIMP-1 levels, known to promote MMP-9 activity. Finally, a specific Tspan8-antibody reduces proMMP-9 activation and dermal invasion. Overall, our results provide new insights into the role of keratinocytes in melanoma dermal colonization through a cooperative mechanism never reported before, and establish for the first time the pro-invasive role of a tetraspanin family member in a cell non-autonomous manner. This work also displays solid arguments for the use of Tspan8-blocking antibodies to impede early melanoma spreading and therefore metastasis.

Gene expression profiles of human melanoma cells with different invasive potential reveal TSPAN8 as a novel mediator of invasion

Background:

Metastatic melanoma requires early detection, being treatment resistant. However, the earliest events of melanoma metastasis, and especially of dermal invasion, remain ill defined.

Results and methods:

Gene expression profiles of two clonal subpopulations, selected from the same human melanoma cell line, but differing in ability to cross the dermal–epidermal junction in skin reconstructs, were compared by oligonucleotide microarray. Of 26 496 cDNA probes, 461 were differentially expressed (>2-fold; P< 0.001), only 71 genes being upregulated in invasive cells. Among them, TSPAN8, a tetraspanin not yet described in melanoma, was upregulated at mRNA and protein levels in melanoma cells from the invasive clone, as assessed by RT–PCR, flow cytometry and western blot analysis. Interestingly, TSPAN8 was the only tetraspanin in which overexpression correlated with invasive phenotype. Flow cytometry of well-defined melanoma cell lines confirmed that TSPAN8 was exclusively expressed by invasive, but not non-invasive melanoma cells or normal melanocytes. Immunohistochemistry revealed that TSPAN8 was expressed by melanoma cells in primary melanomas and metastases, but not epidermal cells in healthy skin. The functional role of TSPAN8 was demonstrated by silencing endogenous TSPAN8 with siRNA, reducing invasive outgrowth from tumour spheroids within matrigel without affecting cell proliferation or survival.

Conclusion:

TSPAN8 expression may enable melanoma cells to cross the cutaneous basement membrane, leading to dermal invasion and progression to metastasis. TSPAN8 could be a promising target in early detection and treatment of melanoma.

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%).

Prediction of outcome for patients with cutaneous melanoma

Most patients with primary melanoma are cured by local surgery, but a significant minority develop fatal metastases. The ability to identify patients with progressive disease is central to efficient management: permitting optimal deployment of adjunctive therapy and sparing the non-progressing majority the morbidity of aggressive therapy. Accurate prediction on an individual patient basis is the ideal, but the best current prognosticators permit only assignment to risk categories. Formulaic combinations of well tried correlates of outcome (gender, ulceration, depth, thickness, and mitotic rate increase accuracy of prediction, but not to personalised level. The use of large data bases against which the attributes of individual patients may be compared is useful and amalgamation of data bases will increase the availability of this approach. The development of markers of proliferation fraction (PCNA and MIB-1) and of the metastatic phenotype (PNA-receptor status) will further refine the process. Staging of disease is critical. Accuracy of staging is improved by mapping the (sentinel) lymph nodes likely to contain early tumor by lymphoscintigraphy and dye/radiomarker localisation. The application of exquisitely sensitive immunohistochemical and molecular biological techniques to biopsies from tissues likely to be the site of metastases permit assessment of clinical stage with a previously impossible degree of accuracy (ultrastaging).

Selection of adenoid cystic carcinoma cell clone highly metastatic to the lung: an experimental study

Lung metastasis is very common in patients with adenoid cystic carcinoma (ACC). The purpose of the present study is to establish a model for an experimental study of ACC metastatic mechanisms and antimetastatic procedures. After five repeated selection in vivo, combined with an in vitro cloning technique and analysis of platelet aggregation activity, a clone (Acc-M) highly metastatic to the lung was selected from an ACC cell line (Acc-2). The metastatic rate was 96% vs 18% for Acc-M and the Acc-2 cell line. The metastatic rate and the weight of the lung with metastases correlated positively with platelet aggregation activity. The present study might be important for research into mechanisms of ACC metastases.

The gangliosides as a possible molecular coupling factor between the proportion of radiosensitive cells in vitro and the metastatic potential in vivo within a human melanoma cell line

With an experimental model of spontaneous lung metastases in immunosuppressed newborn rats, seven clones and variants with different metastatic potential and gangliosides expression were derived from a single parental human melanoma cell line M4Be. The cellular radiosensitivity of M4Be and its seven sublines was estimated using an in vitro colony assay. The total amount of gangliosides in M4Be and its seven sublines was determined by cell extraction and thin-layer chromatography, while the expression of GD3 gangliosides was estimated by flow cytometry with a monoclonal antibody. The radiation-cell survival curves of most clones and variants derived from M4Be showed a zero dose extrapolation clearly lower than 100%, suggesting that two populations of cells of very different radiosensitivity coexist within each of these clones and variants. Although the proportion of radiosensitive cells could be estimated from the shape of the survival curve, its radiosensitivity is too high to be properly evaluated by the colony assay. The eight survival curves differ essentially in the proportion of radiosensitive cells--which varied from 0% to 40% among M4Be and its seven sublines--whereas the cellular radiosensitivity of the radioresistant population was similar among them. The metastatic potential in vivo of M4Be and its seven sublines was not significantly related to the cellular radiosensitivity of their corresponding radioresistant population, but significantly increased with the fraction of radiosensitive cells. This relationship is valid only when the highly metastatic cells are cultured for no more than five passages in vitro as the fraction of radiosensitive cells is rapidly lost during subcultures. The relationship remains valid in vivo as metastatic melanoma-bearing newborn rats whole body irradiated with 20 cGy show no lung metastasis compared with controls. The radiosensitive cell fraction is inversely correlated with both the total ganglioside content (r = 0.84, P < 0.02) and the number of cells positively labelled with the monoclonal antibody directed to GD3 (r = 0.92, P < 0.001). The incubation of a radiosensitive clone with the exogenous bovine brain ganglioside GM1 significantly increases the proportion of radioresistant cells and suppresses its metastatic potential, while the inhibition of the endogenous gangliosides synthesis in the radioresistant cell line M4Be increases the proportion of radiosensitive cells. This study provides a possible explanation for the correlation between the metastatic potential and the proportion of radiosensitive cells within the seven sublines derived from a single parental human melanoma cell line.

Gangliosides protect human melanoma cells from ionizing radiation-induced clonogenic cell death

With an experimental model of spontaneous lung metastases of melanoma developed in this laboratory, a range of sublines (variants and clones) with different metastatic potential and ganglioside expression was established from a single human melanoma cell line M4Be. Using an in vitro clonogenic assay and provided that cells were cultured for no more than five passages, variations in cellular radioresistance of M4Be and seven sublines derived from M4Be were detected. This study shows a positive correlation between the cell intrinsic radioresistance of M4Be and its seven sublines and their total ganglioside content. More precisely, the proportion of radioresistant cells in M4Be and the seven sublines correlated with the number of cells determined by flow cytometry that were positively labelled with a monoclonal antibody directed to GD3 disialoganglioside. Blocking the cellular biosynthesis of gangliosides with the inhibitor Fumonisin B1 or cleaving with Vibrio cholerae neuraminidase the cell surface ganglioside-bound sialic acid in a radioresistant poorly metastatic subline increased its radiosensitivity in vitro. In contrast, enrichment of a radiosensitive metastatic subline with exogenous bovine brain GM1 increased its radioresistance in vitro. These results suggest that, in the radiation dose range important for radioprotection (0-1 Gy), membrane gangliosides radioprotect human melanoma cells in vitro.

Deficiency of ganglioside biosynthesis in metastatic human melanoma cells: relevance of CMP-NeuAc:LacCer alpha 2-3 sialyltransferase (GM3 synthase)

The glycosphingolipid patterns were analyzed on two clones derived from a human melanoma cell line and selected for their respectively high and low metastatic ability in immunosuppressed newborn rats. Conversely to the weakly metastatic cells which exhibited a pattern similar to that of the parental cell line, highly metastatic human melanoma cells appeared to be deficient in ganglioside biosynthesis. An accumulation of lactosylceramide was found in the latter cells, with low amounts of GM3 as the only ganglioside detected and a fourfold decreased activity of GM3 synthase (EC 2.4.99.9). After subcutaneous injection of metastatic cells in newborn rats, the cells proliferating in the tumor induced at the injection site re-expressed the four common gangliosides of melanoma: GM3, GM2, GD3 and GD2, whereas the cells growing in the lungs as metastatic nodules were deficient in ganglioside synthesis and showed an accumulation of lactosylceramide. Taken together, our results suggest that the human melanoma cells which are able to escape from the primary tumor and invade the lungs have an impaired ganglioside biosynthesis with a deficient GM3 synthase.

Correlation between the radiosensitivity in vitro of clones and variants derived from a human melanoma cell line and their spontaneous metastatic potential in vivo

With an experimental model of spontaneous lung metastases of human melanoma in immunosuppressed newborn rats, a large panel of clones and variants with different metastatic potential were derived from a single human melanoma parental cell line (M4Be). Seven clones and variants from M4Be were selected, respectively, for their low (parental, clone 1), intermediate (clones 2 and 3, subvariant 1-) and high (variant 1, subvariant 1+, clone 4) metastatic potential. This paper investigates the relationship between the in vivo metastatic potential of the eight cell lines and their sensitivity to ionizing radiation in vitro (range 0.05-7 Gy). The radiosensitivity was estimated from the mean inactivation dose, a parameter equal to the area under the survival curve plotted in linear coordinates. Examination of the eight survival curves, obtained with cells cultured for no more than five passages after defrost, shows that clone 1, subvariant 1- and the M4be parental line are the most radioresistant cells, clone 4 and subvariant 1+ are the most radiosensitive cells, while clones 2 and 3 and variant 1 showed an intermediate response to radiation. The metastatic potential in vivo of the parental line and the seven sublines is significantly correlated to their radiosensitivity in vitro: the higher the metastatic potential, the higher the radiosensitivity.

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 the galactose binding protein Mac-2 by human melanoma cell-lines

The Mac-2 protein is a lectin specific for galactose-containing glycoconjugates. Present in some normal cells, it was also associated with the metastatic potential of some carcinoma cells. We studied Mac-2 expression in three human melanoma cell lines and in five variants and clones from one of them. By using the M3/38 rat monoclonal antibody, Mac-2 was demonstrated on cell surface by flow cytometry as well as in the cytoplasm and in the nucleus by confocal microscopy. The expression of Mac-2 was not correlated with that of terminal unsialylated Gal beta 1-3 GalNac structures on metastatic melanoma cell lines. However, the presence of extracellular Mac-2 containing vesicles was observed in cell lines with metastatic potency. Western blot analysis of cell lysates, in reducing or non-reducing conditions, revealed two bands of 34-36 and 93-98 kDa apparent M(r), also found in HL60 and P388.D1 cell lines used as positive controls.

Two human melanoma cell-line variants with enhanced in vivo tumor growth and metastatic capacity do not express the beta 3 integrin subunit

The alpha v beta 3 integrin complex is thought to play an important role in in vivo melanoma tumor growth and metastasis. However, not all human metastatic melanomas, present in lymph node biopsies, express alpha v beta 3. In this study, we have investigated the possibility that certain melanoma cell lines can grow aggressively in vivo in the absence of alpha v beta 3 expression. Established human melanoma cell lines (M3Da., M4Beu.) were isolated from an achromic skin metastasis or lymph nodes. Two stable variants (7GP, T1P26), derived from a poorly metastatic M4Beu. melanoma cell line, were isolated by sequential selection for spontaneous metastasis formation in an immunosuppressed newborn rat model. Flow-cytometry analysis shows an absence of the beta 3 integrin subunit (less than 2% of parental levels) in the two variant melanoma cell lines (7GP, T1P26) compared to M3Da. and M4Beu. cell lines which express a relatively high number of beta 3 subunits. The expression levels of the integrin subunits beta 1, beta 5, beta 6 and alpha v were found to be similar for all four melanoma cell lines. Northern blot analysis confirmed the absence of beta 3 in 7GP or T1P26 cell lines and its presence in M3Da. and M4Beu. Moreover, similar levels of alpha v transcript were present in the four melanoma cell lines. The functional effect of the absence of beta 3 was investigated by subcutaneously implanting the variants and the melanoma cell lines in nude mice. Variant 7GP and T1P26 cell lines yielded tumors which were larger and grew at a faster rate than tumors in M3Da. or M4Beu. cell lines. The beta 3 integrin subunit was not detectable on the surface of cells harvested from tumors after 20 or 35 days. Similarly, subcutaneous innoculation of the two variants into immunosuppressed newborn rats gave rise to extensive spontaneous lung metastases compared to the M4Beu. cell line. These results provide evidence that a population of melanoma cells can grow aggressively in vivo and metastasize in the absence of beta 3 or alpha v beta 3 integrin complex. Our results may have clinical relevance and suggest that certain types of melanomas in patients may grow and spread in the absence of the alpha v beta 3 integrin complex.

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.

Accumulation of heparan sulfate in the culture of human melanoma cells with different metastatic ability

Glycosaminoglycans were metabolically labeled in subconfluent cultures of highly metastatic 7Gp122 and poorly metastatic IC8 variants and of the low metastatic parental M4Be human melanoma cell line. Proteoglycans were separated by DEAE Trisacryl chromatography from the culture medium, from the heparin extract of the cell layer and from the heparin-extracted cell residue lyzed with detergents. Glycosaminoglycans were released from the proteoglycans by reductive alkaline hydrolysis and heparan sulfate (HS) was detected by deaminative cleavage with nitrous acid. Expressed on cell protein basis, the labeled HS content in the medium and in the cell layer decreased with increasing metastatic ability. The extraction of HS with heparin from the 7Gp122 cells indicated that this variant was enriched in (polypeptide bound) HS non inserted into the plasma membrane, compared with the low metastatic IC8 and M4Be cells. The HS fraction in heparin extract and in the heparin-extracted cell residue exhibited molecular mass heterogeneity on gel permeation chromatography and it contained HS fragments. Scission with nitrous acid followed by molecular sieve chromatography of the degradation products indicated that the tetra- and disaccharide repeats separated by the N-sulfated glucosamine residue were present in about equal amounts and constituted 60% of the HS chains in the IC8 and M4Be cells. HS from 7Gp122, IC8 and M4Be cells did not bind antithrombin III with high affinity but it was capable of binding bFGF in in vitro assay.

Monoclonal antibody accumulation in experimental and spontaneous lung metastases of human malignant melanoma in rodents

Monoclonal antibody (mAb) uptake in metastatic lung lesions was depicted and evaluated by digital autoradiography. The models examined were experimental metastases of a human melanoma in nude mice and spontaneous metastases of human melanoma in immunocompromised young rats. By comparing uptake patterns in local (s.c.) tumours and in lung processes of various sizes it was found that patterns were essentially similar in both types of malignant tissue. From the point of view of visualization, however, the high blood content of lung tissue resulted in high background and low contrast. This could be overcome by the use of rapidly cleared antibody fragments.