Tumorigenicity of the mixed spindle-epithelioid SP6.5 and epithelioid TP17 uveal melanoma cell lines is differentially related to alpha5beta1 integrin expression

PURPOSE

It has been suggested that the epithelioid morphology and high aggressiveness that is typical of the uveal melanoma cell line TP17 is dependent on the loss of alpha5beta1 integrin expression at the cell surface. The purpose of the current study was to test this hypothesis in the TP17 cell line and investigate the role this integrin may play in the tumorigenicity of the SP6.5 cells, a mixed spindle-epithelioid culture-type human uveal melanoma that shows tumorigenic properties clearly distinct from that of TP17 cells.

METHODS

Expression of the alpha5 integrin subunit was restored in the alpha5-TP17 cell line by stably transfecting the cells with a recombinant plasmid encoding the integrin subunit. Flow cytometry and adhesion assays on fibronectin (FN)-coated culture plates were used to monitor alpha5 expression in the cells. The effect of alpha5 expression on both tumorigenicity and cell proliferation was evaluated in vivo in nude mice. In vitro growth properties of the alpha5(+) TP17 cells was evaluated by cell counting and compared with that of the alpha5 parental TP17 cell line. The influence exerted by the alpha5 integrin subunit on the tumorigenic and proliferative properties of the SP6.5 cells was evaluated in vivo in nude mice by exposing the cells to increasing doses of a blocking antibody directed against the alpha5-subunit before subcutaneous injection, and compared with the results obtained with untreated SP6.5 cells.

RESULTS

Expression of the alpha5 integrin subunit in the alpha5-TP17 cells was successfully achieved, as evidenced by both flow cytometry and adhesion assays on FN-coated culture plates. Restoring expression of alpha5 in TP17 cells enhanced epithelioid cell morphology and increased the growth properties of this cell line in vivo. The ability of the SP6.5 cells to yield subcutaneous tumors was found to be concentration dependent and was reduced in a dose-dependent manner when the cells were exposed to the anti-alpha5 blocking antibody.

CONCLUSIONS

Restoring expression of alpha5 in the alpha5-negative TP17 uveal melanoma cell line influenced the proliferative properties of these cells but did not alter its tumorigenic potential. In contrast, the ability of the SP6.5 cells to yield tumors in vivo in nude mice appeared to be related to expression of this integrin.

Expression of integrin alpha5beta1 and MMPs associated with epithelioid morphology and malignancy of uveal melanoma

PURPOSE

Altered expression of the alpha5beta1 integrin and matrix metalloproteinases (MMPs) is recognized as a hallmark of invasive tumor cells. The purpose of the present study was to investigate the expression of integrin subunit alpha5, its corresponding ligand fibronectin (FN), and the expression pattern for MMPs in four highly proliferative human choroidal melanomas (TP17, TP31, SP8.0, and SP6.5) to evaluate whether any correlation can be established between these markers and cell tumorigenicity.

METHODS

Cell tumorigenicity was evaluated by subcutaneous injection of uveal melanoma cell lines in immunodeficient nude mice. Anchorage dependency was evaluated by growth assays in soft agar. The invasive ability of each cell type was also determined using a modified Boyden chamber. Expression of both the alpha5 integrin subunit and FN was determined at the mRNA level by RT-PCR. The protein level (for alpha5) was determined by flow cytometry and inhibition of adhesion assays by using an antibody directed against the alpha5 subunit. Expression of MMPs was determined by standard gelatin zymography.

RESULTS

Assays in nude mice provided evidence that the cell lines possess a range of tumorigenic ability of TP17>TP31>SP8.0>SP6.5. Antibody inhibition of cell adhesion and flow cytometry demonstrated that TP17 cells have no detectable membrane-bound alpha5beta1, whereas low levels are found in primary cultured melanocytes, as well as in SP6.5, SP8.0, and TP31 cells. RT-PCR analyses provided evidence that both FN and alpha5 expression may be regulated at the transcriptional level. Gelatin zymography revealed that all cell lines, as well as normal melanocytes, express MMP-2 at varying levels but that only the highly invasive TP17 cell line secretes a distinctive MMP with a high molecular weight of 117 kDa.

CONCLUSIONS

Among the four melanoma cell lines selected for the completion of this study, TP17 exhibited the most aggressive phenotype, which also correlated with the mostly epithelioid morphology of these cells. The cell morphology of the TP17 cell line could be related to the loss of alpha5beta1, whereas its invasive properties are more likely related to the expression of the 117-kDa MMP.

Multiple cloning sites from mammalian expression vectors interfere with gene promoter studies in vitro

When performing transcriptional analyses, reporter gene-expression vectors are used to insert promoter fragments through the selected use of a multiple cloning site (MCS) located upstream of the reporter gene. The MCS from pBluescript has frequently been transferred into reporter plasmids (usually bearing the chloramphenical acetyltransferase reporter gene) and used to subclone various promoter fragments from diverse genes. Analyses in electrophoretic mobility shift assay using this MCS as labeled probe revealed that it specifically binds multiple nuclear proteins from a whole array of widely used cell types. Moreover, the presence of the MCS sequence dramatically altered promoter activity in a totally unpredictable fashion that depends on the distance between the MCS and the basal promoter start site of the gene, leading to severe misinterpretation of the transfection data. Finally, we provide evidence that the BamHI/SmaI/PstI restriction site combination is likely one of the major binding site for nuclear proteins on the pBluescript MCS, therefore suggesting that this particular combination of restriction sites should be avoided in the MCS from plasmids that are to be used in promoter studies.

Transcriptional regulation of the alpha 4 integrin subunit gene in the metastatic spread of uveal melanoma

Recently, expression of the alpha 4 integrin subunit has been shown to be inversely correlated with the invasive potential of B16 mouse epidermal melanoma. The purpose of this study was to establish whether expression of the human alpha 4 integrin subunit gene might be similarly regulated in human uveal melanoma which has varying degrees of invasiveness, and whether such modifications are determined by alterations in the transcriptional activity directed by the alpha 4 gene promoter. Two metastatic variants (MH5 and MH10) derived from a human uveal melanoma (SP6.5) were used. Expression studies were performed by transiently transfecting each of these cell lines with recombinant plasmids bearing various lengths of the alpha 4 promoter fused to the CAT reporter gene, and were further validated by Northern blot analyses of the alpha 4 transcript. Both transient transfection and mRNA analyses provided evidence that the transcriptional activity directed by the alpha 4 promoter sequences extending up to position -76 and -120 was indeed inversely correlated to the potential of uveal melanoma to yield metastasis. Experiments in electrophoretic mobility shift assay (EMSA) demonstrated that binding of the nuclear proteins that likely account for transcription of the alpha 4 gene to alpha 4.1 (namely Bp1, Bp2, Bp4, and Bp5) was dramatically reduced in uveal melanoma, but not in normal uveal melanocytes. These results highlight the fundamental function the alpha 4 integrin subunit may play in the ability of tumor cells to evade the primary tumor and form metastasis.