Differential response of adherent and unanchored melanoma cells to bromodeoxyuridine evidenced by specific lectin-binding protein changes

Changes in cytoarchitecture and mobility in B16F1 melanoma cells induced by 5-Br-2'-dU coincide with Rock2, miRNAs 138-5p and 455-3p reciprocal expressions

ROCK2 is a protein involved in the restructuring of the cytoskeleton in cell adhesion and contractibility processes. miR-138-5p and miR-455-3p regulate Rock2 expression, cell proliferation, migration, and invasion in different experimental cell models. However, their participation in the cytoarchitecture and mobility of B16F1 melanoma cells exposed to 5-Br-2'-dU is partially known. This work aimed to analyze ROCK2 and miRs 138-5p and 455-3p expression associated with morphological and mobility changes of B16F1 mouse melanoma cells exposed to the thymidine analog 5-Bromo-2'-deoxyuridine (5-Br-2'-dU). We observed an increase (2.2X n = 3, p < 0.05) in the cell area, coinciding with an increase in cell diameter (1.27X n = 3, p < 0.05), as well as greater cell granularity, capacity for circularization, adhesion, which was associated with more significant polymerization of F-actin, collapsed in the intermediate filaments of vimentin (VIM), and coinciding with a decrease in migration (87%). Changes coincided with a decrease in Rock2 mRNA expression (2.88X n = 3, p < 0.05), increased vimentin and a reciprocal decrease in miR-138-5p (1.8X), and an increase in miR-455-3p (2.39X). The Rock2 kinase inhibitor Y27632 partially rescued these changes. These results suggest ROCK2 and VIM regulate the morphological and mobility changes of B16 melanoma cells after exposure to 5-Br-2'-dU, and its expression may be reciprocally regulated, at least in part, by miR-138-5p and miR-455-3p.

Reciprocal Changes in miRNA Expression with Pigmentation and Decreased Proliferation Induced in Mouse B16F1 Melanoma Cells by L-Tyrosine and 5-Bromo-2'-Deoxyuridine

Background: Many microRNAs have been identified as critical mediators in the progression of melanoma through its regulation of genes involved in different cellular processes such as melanogenesis, cell cycle control, and senescence. However, microRNAs’ concurrent participation in syngeneic mouse B16F1 melanoma cells simultaneously induced decreased proliferation and differential pigmentation by exposure to 5-Brd-2′-dU (5’Bromo-2-deoxyuridine) and L-Tyr (L-Tyrosine) respectively, is poorly understood. Aim: To evaluate changes in the expression of microRNAs and identify which miRNAs in-network may contribute to the functional bases of phenotypes of differential pigmentation and reduction of proliferation in B16F1 melanoma cells exposed to 5-Brd-2′-dU and L-Tyr. Methods: Small RNAseq evaluation of the expression profiles of miRNAs in B16F1 melanoma cells exposed to 5-Brd-2′-dU (2.5 μg/mL) and L-Tyr (5 mM), as well as the expression by qRT-PCR of some molecular targets related to melanogenesis, cell cycle, and senescence. By bioinformatic analysis, we constructed network models of regulation and co-expression of microRNAs. Results: We confirmed that stimulation or repression of melanogenesis with L-Tyr or 5-Brd-2′-dU, respectively, generated changes in melanin concentration, reduction in proliferation, and changes in expression of microRNAs 470-3p, 470-5p, 30d-5p, 129-5p, 148b-3p, 27b-3p, and 211-5p, which presented patterns of coordinated and reciprocal co-expression, related to changes in melanogenesis through their putative targets Mitf, Tyr and Tyrp1, and control of cell cycle and senescence: Cyclin D1, Cdk2, Cdk4, p21, and p27. Conclusions: These findings provide insights into the molecular biology of melanoma of the way miRNAs are coordinated and reciprocal expression that may operate in a network as molecular bases for understanding changes in pigmentation and decreased proliferation induced in B16F1 melanoma cells exposed to L-Tyr and 5-Brd-2′-dU.

Thymidine dinucleotides induce S phase cell cycle arrest in addition to increased melanogenesis in human melanocytes

Although the induction of pigmentation following exposure of melanocytes to ultraviolet light in vivo and in vitro is well documented, the intracellular mechanisms involved in this response are not yet fully understood. Exposure to UV-B radiation leads to the production of DNA damage, mainly cyclobutane pyrimidine dimers, and it was recently suggested that the thymidine dinucleotide pTpT, mimicking small DNA fragments released in the course of excision repair mechanisms, could trigger melanin synthesis. We now report that the thymidine dinucleotide pTpT induces melanogenesis both in human normal adult melanocytes and in human melanoma cells. Thus, the SOS-like response suggested by Gilchrest's work to be evolutionary conserved, based primarily on work in murine cells and guinea pigs, is also apparently present in the human. Thymidine dinucleotide is nontoxic to melanoma cells and does not induce apoptosis in these cells, but induces S phase cell cycle arrest and a proliferation slow down. Because thymidine excess in culture medium leads to the synchronization of cells in S phase, we investigated whether this phenomenon was involved in the increase in melanin synthesis. We show that melanin synthesis is specifically triggered by the dimeric form of the thymidine and not by the monomeric form pT. Thus, our data strongly support that thymidine dinucleotides pTpT mimic at least part of the effects of ultraviolet irradiation, and may hence represent an invaluable model in the study of the molecular events involved in melanogenesis induction triggered through DNA damage.

Bromodeoxyuridine-induced expression of endothelin A in A375 human melanoma cells

Expression of endothelin (ET) receptor subtypes was examined in an experimental model of A375 human melanoma cell differentiation using the pyrimidine analog bromodeoxyuridine (BUdR). BUdR (10 microM)-treated cells had an increased surface area and an increased dendricity, were contact-inhibited and lacked tumorigenecity in athymic nude mice. The untreated A375 cells exclusively expressed ETB and BUdR-induced phenotypical changes were accompanied by induction of ETA expression as evidenced by northern blotting, [125I]ET-1 binding assay and [Ca2+]i measurement. Thus, BUdR-induced differentiation of A375 melanoma cells may provide a model system to study the receptor subtype switch in melanocyte development.

Spontaneous and induced differentiation of human melanoma cells

Malignant melanoma cells can differentiate spontaneously in vivo and in vitro into cells with a finite lifespan. Analysis of differentiating cells from primary melanomas in culture revealed a flat, fibroblast-like morphology and expression of the fibroblast-associated marker leucine aminopeptidase (LAP). Differentiation was also observed in a minor sub-population of permanent cell lines derived from metastatic lesions. An experimental model of melanoma cell differentiation was then developed, using the pyrimidine analog bromodeoxyuridine (BUdR). BUdR-treated cells had a flat morphology, were contact-inhibited, had up to 20-fold increased surface area, expressed LAP, no longer proliferated anchorage-independently in soft agar, and 3 out of 4 cell lines were non-tumorigenic in athymic nude mice. Our results show that models of differentiation of melanoma cells can be established that help to define pathways of differentiation.

Cell adhesion regulates melanoma specific differentiation and interactions with the 3' region of the tyrosinase gene

Decreased attachment to substratum has now been found to increase melanosome formation and cell-cell interaction in B16 melanoma. Since melanosome formation involves tyrosinase gene expression, we assayed for differential RNA expression by hybridization with probes from the distal ends of this gene, detecting unequal reactivity only with the 3' end probe. The same DNA showed binding of 2 nuclear proteins of 50 and 60 kd in unanchored cells, in contrast with a decreased binding of the 60 kd species, in nuclear extracts from attached cells. No comparable differences were detected with a gamma-actin DNA of identical length, suggesting that the changes observed are sequence-specific. Our studies suggest that the adhesion-mediated modulation of pigmentation in B16 melanoma correlates with differential macromolecular interactions with the 3' end of the tyrosinase gene.

Tumor hypersensitive DNA is enriched in c-myc sequences and reacts differentially with normal and malignant genomic DNA

We now show that exposure of B16 melanoma cells to bromodeoxyuridine increases cell-substratum interactions concurrent with an increase in genome susceptibility to nucleases. Hypersensitive DNA was isolated after mild nicking of nuclei with DNase I followed by repair with DNA polymerase I in the presence of biotin-19-SS-dUTP and affinity chromatography on streptavidin-agarose. Dot blot studies showed that the hypersensitive DNA is enriched in c-myc sequences compared to total tumor genomic DNA, and hybridizes preferentially to the latter, compared to normal genomic DNA, particularly when prepared from BrdU-treated cells. Since hypersensitive DNA can hybridize with multiple Alu sequences in the genome, we postulate that one of the mechanisms for its differential reactivity may be by recognition of an unequal number of Alu repeats in normal and tumor genomic DNA.