Higher levels of melanin and inhibition of cdk2 activity in primary human melanoma cells WM115 overexpressing nPKCdelta

Functional features of cancer stem cells in melanoma cell lines

BACKGROUND

Recent evidence suggests a subset of cells within a tumor with "stem-like" characteristics. These cells are able to transplant tumors in immunodeficient hosts. Distinct from non-malignant stem cells, cancer stem cells (CSC) show low proliferative rates, high self-renewing capacity, propensity to differentiate into actively proliferating tumor cells, and resistance to chemotherapy or radiation. They are often characterized by elevated expression of stem cell surface markers, in particular CD133, and sets of differentially expressed stem cell-associated genes. CSC are usually rare in clinical specimens and hardly amenable to functional studies and gene expression profiling. In this study, a panel of heterogenous melanoma cell lines was screened for typical CSC features.

METHODS

Nine heterogeneous metastatic melanoma cell lines including D10 and WM115 were studied. Cell lines were phenotyped using flow cytometry and clonogenic assays were performed by limiting dilution analysis on magnetically sorted cells. Spheroidal growth was investigated in pretreated flasks. Gene expression profiles were assessed by using real-time rt-PCR and DNA microarrays. Magnetically sorted tumor cells were subcutaneously injected into the flanks of immunodeficient mice. Comparative immunohistochemistry was performed on xenografts and primary human melanoma sections.

RESULTS

D10 cells expressed CD133 with a significantly higher clonogenic capacity as compared to CD133- cells. Na8, D10, and HBL cells formed spheroids on poly-HEMA-coated flasks. D10, Me39, RE, and WM115 cells expressed at least 2 of the 3 regulatory core transcription factors SOX2, NANOG, and OCT4 involved in the maintenance of stemness in mesenchymal stem cells. Gene expression profiling on CD133+ and CD133- D10 cells revealed 68 up- and 47 downregulated genes (+/-1.3 fold). Two genes, MGP and PROM1 (CD133), were outstandingly upregulated. CD133+ D10 cells formed tumors in NSG mice contrary to CD133- cells and CD133 expression was detected in xenografts and primary human melanoma sections using immunohistochemistry.

CONCLUSIONS

Established melanoma cell lines exhibit, to variable extents, the typical features of CSCs. The tumorigenic cell line D10, expressing CD133 and growing in spheroids and might qualify as a potential model of melanoma CSCs.

AQP1 is not only a water channel: it contributes to cell migration through Lin7/beta-catenin

Background

AQP1 belongs to aquaporins family, water-specific, membrane-channel proteins expressed in diverse tissues. Recent papers showed that during angiogenesis, AQP1 is expressed preferentially by microvessels, favoring angiogenesis via the increase of permeability In particular, in AQP1 null mice, endothelial cell migration is impaired without altering their proliferation or adhesion. Therefore, AQP1 has been proposed as a novel promoter of tumor angiogenesis.

Methods/Findings

Using targeted silencing of AQP1 gene expression, an impairment in the organization of F-actin and a reduced migration capacity was demonstrated in human endothelial and melanoma cell lines. Interestingly, we showed, for the first time, that AQP1 co-immunoprecipitated with Lin-7. Lin7-GFP experiments confirmed co-immunoprecipitation. In addition, the knock down of AQP1 decreased the level of expression of Lin-7 and β-catenin and the inhibition of proteasome contrasted partially such a decrease.

Conclusions/Significance

All together, our findings show that AQP1 plays a role inside the cells through Lin-7/β-catenin interaction. Such a role of AQP1 is the same in human melanoma and endothelial cells, suggesting that AQP1 plays a global physiological role. A model is presented.

Melanoma contains CD133 and ABCG2 positive cells with enhanced tumourigenic potential

The failure to eradicate most cancers and in particular melanoma may be as fundamental as a misidentification of the target. The identification of cancer stem/initiating cells within the tumour population with a crucial role for tumour formation may open new pharmacological perspectives. Our data show three main novelties for human melanoma: firstly, melanoma biopsy contains a subset of cells expressing CD133 (CD133+) and the latter is able to develop a Mart-1 positive tumour in NOD-SCID mice. Secondly, the WM115, a human melanoma cell line, has been found to express both CD133 and ABCG2 markers. This cell line grows as floating spheroids, expresses typical progenitors and mature neuronal/oligodendrocyte markers and is able to transdifferentiate into astrocytes or mesenchymal lineages under specific growth conditions. As in xenografts generated with CD133+ biopsy melanoma cells, those produced by the cell line displayed lower levels of CD133 and ABCG2. Thirdly, the WM115 cells express the most important angiogenic and lymphoangiogenic factors such as notch 4, prox1 and podoplanin which can cooperate in the development of the tumourigenic capability of melanoma in vivo. Therefore, in this study, we demonstrate the presence of stem/initiating subsets in melanoma both in biopsy and in an established melanoma cell line grown in vitro and in xenografts. Interestingly, considering that melanoma gives metastasis primarily through lymphatic vessels, herein, we demonstrated that a melanoma cell line expresses typical lymphoangiogenic factors.

Protein kinase C delta inhibits Caco-2 cell proliferation by selective changes in cell cycle and cell death regulators

PKC-delta is a serine/threonine kinase that mediates diverse signal transduction pathways. We previously demonstrated that overexpression of PKC-delta slowed the G1 progression of Caco-2 colon cancer cells, accelerated apoptosis, and induced cellular differentiation. In this study, we further characterized the PKC-delta dependent signaling pathways involved in these tumor suppressor actions in Caco-2 cells overexpressing PKC-delta using a Zn2+ inducible expression vector. Consistent with a G1 arrest, increased expression of PKC-delta caused rapid and significant downregulation of cyclin D1 and cyclin E proteins (50% decreases, P<0.05), while mRNA levels remained unchanged. The PKC agonist, phorbol 12-myristate 13-acetate (TPA, 100 nM, 4 h), induced two-fold higher protein and mRNA levels of p21(Waf1), a cyclin-dependent kinase (cdk) inhibitor in PKC-delta transfectants compared with empty vector (EV) transfected cells, whereas the PKC-delta specific inhibitor rottlerin (3 microM) or knockdown of this isoenzyme with specific siRNA oligonucleotides blocked p21(Waf1) expression. Concomitantly, compared to EV control cells, PKC-delta upregulation decreased cyclin D1 and cyclin E proteins co-immunoprecipitating with cdk6 and cdk2, respectively. In addition, overexpression of PKC-delta increased binding of cdk inhibitor p27(Kip1) to cdk4. These alterations in cyclin-cdks and their inhibitors are predicted to decrease G1 cyclin kinase activity. As an independent confirmation of the direct role PKC-delta plays in cell growth and cell cycle regulation, we knocked down PKC-delta using specific siRNA oligonucleotides. PKC-delta specific siRNA oligonucleotides, but not irrelevant control oligonucleotides, inhibited PKC-delta protein by more than 80% in Caco-2 cells. Moreover, PKC-delta knockdown enhanced cell proliferation ( approximately 1.4-2-fold, P<0.05) and concomitantly increased cyclin D1 and cyclin E expression ( approximately 1.7-fold, P<0.05). This was a specific effect, as nontargeted PKC-zeta was not changed by PKC-delta siRNA oligonucleotides. Consistent with accelerated apoptosis in PKC-delta transfectants, compared to EV cells, PKC-delta upregulation increased proapoptotic regulator Bax two-fold at mRNA and protein levels, while antiapoptotic Bcl-2 protein was decreased by 50% at a post-transcriptional level. PKC-delta specific siRNA oligonucleotides inhibited Bax protein expression by more than 50%, indicating that PKC-delta regulates apoptosis through Bax. Taken together, these results elucidate two critical mechanisms regulated by PKC-delta that inhibit cell cycle progression and enhance apoptosis in colon cancer cells. We postulate these antiproliferative pathways mediate an important tumor suppressor function for PKC-delta in colonic carcinogenesis.

Protein kinase C in melanoma

Protein kinase C (PKC) is activated by diacylglycerol generated by receptor-mediated hydrolysis of membrane phospholipids to mediate signals for cell growth and plays as a target of tumor-promoting phorbol esters in malignant transformation. PKC is a family of enzymes and their expression profiles have been examined in the normal melanocytes and melanoma cells, and studies have been carried out on the functions of PKC isoforms in proliferation, transformation, and metastasis of melanoma cells. Here, we summarize current knowledge of the expression and possible roles of the PKC family in melanoma in comparison with those of normal melanocytes.

Nuclear sphingomyelin-synthase and protein kinase C δ in melanoma cells

The aim of this research is to study the influence of protein kinase C delta on the nuclear phospholipids metabolism. Murine and human melanoma cells, in which overexpression of protein kinase delta was induced, were used. After purification of the nuclei, the phosphatidylcholine-dependent phospholipase C, sphingomyelin-synthase, and sphingomyelinase activities were measured. The results showed that the nuclear sphingomyelin-synthase activity increased and sphingomyelinase activity decreased in the protein kinase C delta overexpressive cells with respect to the controls. As a consequence, the ceramide pool decreased and diacylglycerol pool increased; this effect was not due to the phosphatidylcholine-dependent phospholipase C activity that did not change. The inhibition of sphingomyelinase could be due to protein kinase C delta as well as to existence of a sort of nuclear self-regulation between sphingomyelin-synthase and sphingomyelinase. The possible role of nuclear sphingomyelin-synthase in cell proliferation is discussed.

Effect of glucose stress conditions in BL6T murine melanoma cells

Elevated expression of stress proteins can be a characteristic of human cancer and may be involved in the development of resistance to some types of chemotherapeutic agent. In this paper, the effect of physiological stress conditions, such as glucose deprivation, was investigated in overexpressing nPKCdelta murine melanoma BL6 (BL6T) cells. Glucose stress conditions decreased the proliferative capacity, increasing the percentage of BL6T cells in the G0/G1 phase of the cell cycle. Furthermore, under such conditions, nPKCdelta, whose subcellular localization is cell cycle dependent, showed a cytoplasmic and perinuclear localization by immunohistochemistry, this being typical for cells in G0/G1 phase. Moreover, these cells expressed GRP-78, a known stress protein. On the other hand, glucose depletion enhanced intracellular melanin as well as tyrosinase activity and expression. In summary, these data demonstrate that stress conditions can modify the biological characteristics of BL6T cells, and therefore can select a quiescent cellular population.