Experimental metastasis and differentiation of murine melanoma cells: actions and interactions of factors affecting different intracellular signalling pathways

Concentration-independent MRI of pH with a dendrimer-based pH-responsive nanoprobe

The measurement of extracellular pH (pHe ) has significant clinical value for pathological diagnoses and for monitoring the effects of pH-altering therapies. One of the major problems of measuring pHe with a relaxation-based MRI contrast agent is that the longitudinal relaxivity depends on both pH and the concentration of the agent, requiring the use of a second pH-unresponsive agent to measure the concentration. Here we tested the feasibility of measuring pH with a relaxation-based dendritic MRI contrast agent in a concentration-independent manner at clinically relevant field strengths. The transverse and longitudinal relaxation times in solutions of the contrast agent (GdDOTA-4AmP)44 -G5, a G5-PAMAM dendrimer-based MRI contrast agent in water, were measured at 3 T and 7 T magnetic field strengths as a function of pH. At 3 T, longitudinal relaxivity (r1 ) increased from 7.91 to 9.65 mM(-1) s(-1) (on a per Gd(3+) basis) on changing pH from 8.84 to 6.35. At 7 T, r1 relaxivity showed pH response, albeit at lower mean values; transverse relaxivity (r2 ) remained independent of pH and magnetic field strengths. The longitudinal relaxivity of (GdDOTA-4AmP)44 -G5 exhibited a strong and reversible pH dependence. The ratio of relaxation rates R2 /R1 also showed a linear relationship in a pH-responsive manner, and this pH response was independent of the absolute concentration of (GdDOTA-4AmP)44 -G5 agent. Importantly, the nanoprobe (GdDOTA-4AmP)44 -G5 shows pH response in the range commonly found in the microenvironment of solid tumors.

Uncoupling of epidermal growth factor-dependent proliferation and invasion in a model of squamous carcinoma progression

Cell lines pairs were established from a primary squamous carcinoma of tongue and a lymph node metastasis and their biological behavior characterized. HN12 cells, derived from metastatic SCC, formed tumors upon subcutaneous transplantation to athymic mice, whereas HN4, derived from a primary lesion in the same individual, were non-tumorigenic in this assay. EGF stimulated proliferation of HN4 cells; in comparison, not only were metastatic HN12 cells refractory to the stimulatory effects of this growth factor but showed inhibition at higher growth factor concentrations. However, in contrast to the effects on proliferation, EGF (10 ng/ml) readily induced HN12 cells to invade in Boyden chamber assays whereas HN4 were non-invasive under these conditions. The invasive properties of HN12 cells were apparently independent of MMP-2 activity, as levels of active MMP-2 were higher in the non-invasive cells. However, EGF stimulated MMP-9 activity in invasive cells. Additionally, HN12 cells expressed constitutively high levels of active MMP-7 and MMP-3/10. The pharmacological agents LY294002, PD098059, SP600125, or SB202190 inhibited invasion of HN12 cells, suggesting requirement for phosphoinositide 3-OH kinase- and mitogen activated protein kinase-dependent pathways in the process. The data indicate that distinct biochemical differences distinguish metastatic squamous carcinoma cells from those derived from corresponding primary tumors, resulting in their contrasting biological properties.

MRI of the tumor microenvironment

The microenvironment within tumors is significantly different from that in normal tissues. A major difference is seen in the chaotic vasculature of tumors, which results in unbalanced blood supply and significant perfusion heterogeneities. As a consequence, many regions within tumors are transiently or chronically hypoxic. This exacerbates tumor cells' natural tendency to overproduce acids, resulting in very acidic pH values. The hypoxia and acidity of tumors have important consequences for antitumor therapy and can contribute to the progression of tumors to a more aggressive metastatic phenotype. Over the past decade, techniques have emerged that allow the interrogation of the tumor microenvironment with high resolution and molecularly specific probes. Techniques are available to interrogate perfusion, vascular distribution, pH, and pO(2) nondestructively in living tissues with relatively high precision. Studies employing these methods have provided new insights into the causes and consequences of the hostile tumor microenvironment. Furthermore, it is quite exciting that there are emerging techniques that generate tumor image contrast via ill-defined mechanisms. Elucidation of these mechanisms will yield further insights into the tumor microenvironment. This review attempts to identify techniques and their application to tumor biology, with an emphasis on nuclear magnetic resonance (NMR) approaches. Examples are also discussed using electron MR, optical, and radionuclear imaging techniques.

Role of zyxin in differential cell spreading and proliferation of melanoma cells and melanocytes

Cell spreading, proliferation, and survival are modulated by focal adhesions linking extracellular matrix proteins, integrins, and the cytoskeleton. Zyxin is a focal-adhesion-associated phosphoprotein with one domain involved in the control of actin assembly and three protein-protein adapter domains implicated in the regulation of cell growth and differentiation. We characterized zyxin expression in normal human melanocytes and six melanoma cell lines in relation to cell spreading, growth, and differentiation using Western immunoblotting techniques, image analysis, flow cytometry, and confocal microscopy. We found that zyxin, focal adhesion kinase, and paxillin were significantly upregulated in melanoma cells compared to melanocytes. Zyxin expression directly related to cell spreading and proliferation and inversely related to differentiation, whereas focal adhesion kinase correlated only to cell spreading and paxillin did not significantly correlate with any of the parameters. Treatment of melanoma cells with 12-O-tetradecanoylphorbol-13-acetate downregulated zyxin expression, inhibited cell spreading and proliferation, and promoted differentiation. In contrast, 12-O-tetradecanoylphorbol-13-acetate, a mitogen for melanocytes, induced upregulation of zyxin expression in melanocytes. These findings are consistent with a role of zyxin in modulation of cell spreading, proliferation, and differentiation. Therapies directed at the downregulation of this focal adhesion phosphoprotein in melanoma cells implicate a new approach for controlling melanoma cell growth.

Radio-prevention of micrometastases

In developed countries, the cancer incidence is about 150,000 cases per year and half of people with cancer may die from the extension of the primary tumour in secondary deposits. This disaster costs more than 2 billion euro per year. People with cancer are often treated with surgery and/or radiotherapy of localized primary tumour and chemo-prevention of occult disseminated micrometastases. Since chemotherapy essentially targets cycling tumour cells, quiescent micrometastases which may contain only one cell may escape. We previously reported that human melanoma clones with high metastatic potential and low gangliosides content appeared very radiosensitive to low-dose ionizing radiation both in culture and in immunosuppressed animals. This exquisite radiosensitivity was observed with the highly metastatic single cells which were resting at the time of irradiation. These data are consistent with the dose-response relationship for the radiotherapy of secondary deposits which appears linear with no threshold. Highly metastatic cells at an early stage of growth also appear very sensitive to chemicals and activated immune cells. We propose the medical hypothesis according to which the spread of resting micrometastases should be prevented by a single fraction of total-body irradiation delivered at a dose sufficiently low (below 0.2 Gy) to avoid normal tissue radiotoxicity. Radio-prevention may complement standard treatments for patients with metastases and may be delivered even for patients in whom no distant metastases were detected on tumour diagnosis (M0 stage).

Agouti signaling protein and other factors modulating differentiation and proliferation of immortal melanoblasts

The melanocyte lineage potentially forms an attractive model system for studies in cell differentiation, developmental genetics, cell signaling, and melanoma, because differentiated cells produce the visible pigment melanin. Immortal lines of murine melanoblasts (melanocyte precursors) have been described previously, but induction of differentiation involved a complex culture system with keratinocyte feeder cells. Here we describe conditions for both growth and induced differentiation of the melanoblast line melb-a, without feeder cells, and analyze factors that directly control proliferation and differentiation of these pure melanoblasts. Several active factors are products of developmental and other coat color genes, including stem cell factor (SCF), melanocyte-stimulating hormone (alphaMSH), and agouti signaling protein (ASP), a natural antagonist at the MSH receptor (melanocortin 1 receptor, MC1R) encoded by the agouti gene. A stable analog of alphaMSH (NDP-MSH) stimulated differentiation and inhibited growth. ASP in excess inhibited both effects of NDP-MSH, that is, ASP could inhibit pigmentation and stimulate growth. These effects provide an explanation for the interactions in mice of melanocyte developmental mutations with yellow agouti and Mc1r alleles, and a role for embryonic expression patterns of ASP.

Advances in specific immunotherapy of malignant melanoma

Management of malignant melanoma continues to present a challenge to dermatologists, particularly in advanced cases. In light of the steady increase in the worldwide incidence and mortality rates for melanoma, better understanding of the immune mechanisms regulating melanoma progression and interaction with the host's immune system seems eminently important. New studies on the role of immune mechanisms in the pathogenesis and clinical course of melanoma have recently been published. We review the immune mechanisms involved in tumor progression and ways in which these mechanisms may be applied toward immunotherapeutic management of malignant melanoma.

LEARNING OBJECTIVE

After the completion of this learning activity, participants should be familiar with (1) the immune mechanisms involved in host-tumor interaction and tumor rejection, (2) factors allowing the escape of melanoma cells from immune recognition, and (3) the current rationale for the different types of specific immunotherapy in melanoma. Better understanding of basic mechanisms in tumor immunology should raise awareness of future immunotherapeutic approaches in patients with melanoma, particularly in those who are at high risk of recurrence or who present with advanced disease.

Upregulation of mRNA for the melanocortin-1 receptor but not for melanogenic proteins in macrophage x melanoma fusion hybrids exhibiting increased melanogenic and metastatic potential

Fusion of mouse peritoneal macrophages or human blood monocytes with weakly metastatic mouse Cloudman S91 melanoma cells resulted in hybrids with enhanced metastatic potential (Rachkovsky et al., 1998. Clin. Exp. Metastasis, 16: 299-312). With few exceptions, such hybrids also showed increased basal- and MSH-induced pigmentation, at least in part through increased N-glycosylation of melanogenic proteins (Sodi et al., 1998. Pigment Cell Res., 11: 299-309). Here we report analyses regarding expression of the melanocyte-stimulating hormone (MSH) receptor (melanocortin-1 receptor, MC1-R) and the melanogenic proteins, tyrosinase (E.C. 1.14.18.1), tyrosinase-related protein 1 (TRP-1), and the tyrosinase-related protein 2 (TRP-2, E.C. 5.3.2.3), by a panel of cell lines consisting of parental Cloudman S91 melanoma cells, macrophages from DBA/2J mice, artificially derived macrophage x melanoma hybrids of high and low metastatic potential, and a naturally occurring highly metastatic hybrid between a Cloudman S91 tumor cell and a DBA/2J tumor-infiltrating cell. We show that incubation of cells with MSH/isobutylmethylxanthine (IBMX) resulted in strong melanogenic and morphologic responses in high metastatic hybrids compared to parental cells and the low metastatic hybrid, and that high metastatic hybrids exhibit increased mRNA expression for MC1-R accompanied by increased 125I-alphaMSH binding. Although tyrosinase activity and the protein level for tyrosinase and TRP-2, but not for TRP-1, were increased in the high metastatic hybrids versus the other cells, no significant changes in mRNA either for tyrosinase or for TRPs were observed in them. Furthermore, unlike tyrosinase, the abundance and gel mobility pattern of TRP-2 did not correlate with changes in activity in all hybrids and parental melanoma cells. The results suggest that although the activity MC1-R and tyrosinase correlate with enhanced basal as well as MSH-induced melanogenesis in metastatic/melanotic hybrids, their expression is differentially regulated, i.e., regulation of MC1-R while at transcriptional level, the TRPs are primarily regulated via post-transcriptional mechanisms in high metastatic hybrids.

Regulation of c-met expression in B16 murine melanoma cells by melanocyte stimulating hormone

B16 murine melanoma cells selected in vivo for enhanced liver metastatic ability (B16-LS9) show on the one hand an increased expression and constitutive activation of the proto-oncogene c-met (the receptor for hepatocyte growth factor/scatter factor), and on the other hand a more differentiated phenotype, when compared to the parental cell line, B16-F1. Following this observation, we have tried to establish whether there is a direct relationship between differentiation and c-met expression in B16 melanoma cells. Treatment of these cells with differentiating agents indicated that c-met expression was strongly induced by melanocyte stimulating hormone, while retinoic acid had almost no influence. c-met induction was triggered by engagement of the melanocortin receptor, cAMP elevation and PKA/PKC(α) activation, as respectively shown by the effects of ACTH, cAMP elevating agents and specific PK inhibitors. Regulation of c-met expression via the melanocortin receptor and cAMP raises the intriguing possibility that autocrine and/or paracrine mechanisms acting in vivo on this circuit might influence (through c-met expression and activation) the metastatic behavior of these tumor cells, which we have shown to be dependent on their c-met expression.

Immunoreactivity of alpha-melanocyte-stimulating hormone, adrenocorticotrophic hormone and beta-endorphin in cutaneous malignant melanoma and benign melanocytic naevi

Melanocyte-stimulating hormone (MSH) has been reported to enhance the experimental metastatic behaviour of melanoma cells in the mouse model. alpha-MSH production and MSH receptor (melanocortin 1 receptor gene) expression have been detected in cultured normal human melanocytes and metastasized melanomas. The exact role of MSH in the metastatic behaviour of human melanoma cells is, however, not yet known. To clarify a possible role of proopiomelanocortin (POMC)-derived peptides, including alpha-MSH, in melanoma development and progression, we analysed immunohistochemically the localization of alpha-MSH adrenocorticotrophic hormone (ACTH) and beta-endorphin in various kinds of benign pigmented naevocytic lesions and malignant melanomas. Three of 21 samples of common and dysplastic naevi showed detectable alpha-MSH staining in naevus cells, and five and six of 15 samples were weakly positive for ACTH and beta-endorphin staining, respectively. In melanoma samples, 24 of 45, 23 of 39 and 30 of 42 samples showed positive staining with alpha-MSH, ACTH and beta-endorphin antibodies, respectively. Furthermore, staining for all three antibodies was noted to be more intense and diffuse in samples of nodular melanoma, vertically growing acral lentiginous melanoma and superficial spreading melanoma as well as metastatic lesions compared with those of naevi. Although it is yet to be determined whether or not this strong staining for POMC-derived peptides in advanced melanoma cells indicates a role of autocrine or paracrine regulation, our results suggest a possible involvement of POMC gene products in melanoma progression.

Interleukin-2 increases intracellular glutathione levels and reverses the growth inhibiting effects of cyclophosphamide on B16 melanoma cells

Glutathione (GSH) plays an essential role in the metabolism of melanoma. As changes in intracellular GSH content can modify the processes of cell proliferation and detoxification, this could determine the therapeutic response to some cancer treatment strategies. The purpose of this study was to test the effects of treatment with interleukin-2 (IL-2), alone and in combination with cyclophosphamide (CY), on survival of mice bearing B16 melanoma liver metastases, and to determine the influence of these therapeutic agents on the GSH metabolism of B16 cells. In the in vivo test system, B16 melanoma liver metastases were induced in C57BL/6 mice which were subsequently treated with IL-2, CY and CY plus IL-2. Survival time was used to determine the response to treatment. In the in vitro system, we evaluated the effects of IL-2, acrolein (an active metabolite of CY responsible for GSH depletion) and acrolein plus IL-2 on GSH levels and proliferation of B16 melanoma cells. Results indicated that, in vivo, all treatments increased mouse survival times with respect to control mice. However, the addition of IL-2 to CY therapy decreased survival time compared with treatment with CY alone. In vitro, whereas acrolein produced a GSH depletion and inhibited B16 cell proliferation, IL-2 increased GSH content and cell proliferation rate compared with untreated cells. Moreover, addition of IL-2 to cells preincubated with acrolein increased GSH levels and proliferation with respect to acrolein alone. In summary, the data suggest that GSH plays a critical role in the growth-promoting effects of IL-2 on B16F10 melanoma cells and in the antagonistic effect of IL-2 on CY inhibitory activity on these tumor cells.

Inverse relationship between invasiveness and differentiative capacity in different human neuroblastoma cell lines

In order to clarify the relationship between invasiveness and loss of cellular differentiation in tumor cells, we studied the invasive properties on Matrigel of (a) a series of clones we isolated from human neuroblastoma LaN1 and Platt cell lines inducible to differentiation by adhesion on fibronectin, and (b) SY5Y human neuroblastoma cells inducible to differentiation by retinoic acid. We found that, regardless of the parental line, the more differentiated clones were scarcely invasive, while the less differentiated clones showed a higher degree of invasiveness. Differences in invasiveness between differentiated and non-differentiated neuroblastoma clones did not reflect differences in adhesiveness to laminin, the major component of Matrigel. The retinoic acid-sensitive SY5Y human neuroblastoma cells also reduced their invasiveness on Matrigel after differentiation induced by growth in media supplemented with retinoic acid. These results point to an inverse relationship between differentiative properties and invasiveness in human neuroblastoma cell lines.

Trophic factors and central nervous system metastasis

To metastasize to the central nervous system (CNS) malignant cells must attach to brain microvessel endothelial cells, respond to brain endothelial cell-derived motility factors, respond to CNS-derived invasion factors and invade the blood-brain barrier (BBB), and finally, respond to CNS survival and growth factors. Trophic factors such as the neurotrophins play an important role in tumor cell invasion into the CNS and in the survival of small numbers of malignant cells under stress conditions. Trophic factors promote BBB invasion by enhancing the production of basement membrane-degrading enzymes in neurotrophin-responsive cells. The expression of certain neurotrophin receptors on brain-metastasic neuroendocrine cells occurs in relation to their invasive and survival properties. For example, CNS-metastatic melanoma cells respond to particular neurotrophins (nerve growth factor, neurotrophin-2) that can be secreted by normal cells within the CNS. In addition, a paracrine form of transferrin is important in CNS metastasis, and brain-metastatic cells respond to low levels of transferrin and express high levels of transferrin receptors. CNS-metastatic tumor cells can also produce autocrine factors and inhibitors that influence their growth, invasion and survival in the brain. Synthesis of paracrine factors and cytokines may influence the production of trophic factors by normal brain cells adjacent to tumor cells. Moreover, we found increased amounts of neurotrophins in brain tissue at the invasion front of human melanoma tumors in CNS biopsies. Thus the ability to form metastatic colonies in the CNS is dependent on tumor cell responses to trophic factors as well as autocrine and paracrine growth factors and probably other underdescribed factors.

The role of trophic factors and autocrine/paracrine growth factors in brain metastasis

The brain is a unique microenvironment enclosed by the skull, lacking lymphatic drainage and maintaining a highly regulated vascular transport barrier. To metastasize to the brain malignant tumor cells must attach to microvessel endothelial cells, respond to brain-derived invasion factors, invade the blood-brain barrier and respond to survival and growth factors. Trophic factors are important in brain invasion because they can act to stimulate this process. In responsive malignant cells trophic factors such as neurotrophins can promote invasion by enhancing the production of basement membrane-degradative enzymes (such as type IV collagenase/gelatinase and heparanase) capable of locally destroying the basement membrane and the blood-brain barrier. We examined human melanoma cell lines that exhibit varying abilities to form brain metastases. These melanoma lines express low-affinity neurotrophin receptor p75NTR in relation to their brain-metastatic potentials but the variants do not express trkA, the gene encoding a high affinity nerve growth factor (NGF) tyrosine kinase receptor p140trkA. Melanoma cells metastatic to brain also respond to paracrine factors made by brain cells. We have found that a paracrine form of transferrin is important in brain metastasis, and brain-metastatic cells respond to low levels of transferrin and express high levels of transferrin receptors. Brain-metastatic tumor cells can also produce autocrine factors and inhibitors that influence their growth, invasion and survival in the brain. We found that brain-metastatic melanoma cells synthesize transcripts for the following autocrine growth factors: TGF beta, bFGF, TGF alpha and IL-1 beta. Synthesis of these factors may influence the production of neurotrophins by adjacent brain cells, such as oligodendrocytes and astrocytes. Increased amounts of NGF were found in tumor-adjacent tissues at the invasion front of human melanoma tumors in brain biopsies. Trophic factors, autocrine growth factors, paracrine growth factors and other factors may determine whether metastatic cells can successfully invade, colonize and grow in the central nervous system.