Demonstration of histamine H2 receptors on human melanoma cells

Metabolic signature of breast cancer cell line MCF-7: profiling of modified nucleosides via LC-IT MS coupling

Background

Cancer, like other diseases accompanied by strong metabolic disorders, shows characteristic effects on cell turnover rate, activity of modifying enzymes and DNA/RNA modifications, resulting also in elevated amounts of excreted modified nucleosides. For a better understanding of the impaired RNA metabolism in breast cancer cells, we screened these metabolites in the cell culture supernatants of the breast cancer cell line MCF-7 and compared it to the human mammary epithelial cells MCF-10A. The nucleosides were isolated and analyzed via 2D-chromatographic techniques: In the first dimension by cis-diol specific boronate affinity extraction and subsequently by reversed phase chromatography coupled to an ion trap mass spectrometer.

Results

Besides the determination of ribonucleosides, additional compounds with cis-diol structure, deriving from cross-linked biochemical pathways, like purine-, histidine- and polyamine metabolism were detected. In total, 36 metabolites were identified by comparison of fragmentation patterns and retention time. Relation to the internal standard isoguanosine yielded normalized area ratios for each identified compound and enabled a semi-quantitative metabolic signature of both analyzed cell lines.

13 of the identified 26 modified ribonucleosides were elevated in the cell culture supernatants of MCF-7 cells, with 5-methyluridine, N²,N²,7-trimethylguanosine, N⁶-methyl-N⁶-threonylcarbamoyladenosine and 3-(3-aminocarboxypropyl)-uridine showing the most significant differences. 1-ribosylimidazole-4-acetic acid, a histamine metabolite, was solely found in the supernatants of MCF-10A cells, whereas 1-ribosyl-4-carboxamido-5-aminoimidazole and S-adenosylmethionine occurred only in supernatants of MCF-7 cells.

Conclusion

The obtained results are discussed against the background of pathological changes in cell metabolism, resulting in new perspectives for modified nucleosides and related metabolites as possible biomedical markers for breast carcinoma in vivo.

Phenotypic profiling of engineered mouse melanomas with manipulated histamine production identifies histamine H2 receptor and rho-C as histamine-regulated melanoma progression markers

In the present study, the impact of acquired neoplastic L-histidine decarboxylase (HDC) expression, and its direct consequence, the release of histamine in the tumor environment, was assessed on melanoma tumor progression. B16-F10 mouse melanoma cells were manipulated via stable transfection, and nine novel transgenic variants were generated in triplicates, constitutively expressing the full-length sense mouse HDC mRNA, a mock control, and an antisense HDC RNA segment, respectively. Establishing both primary skin tumors and lung metastases in C57BL/6 mice, the nine variants with different histamine-releasing capacities were subjected to a comprehensive comparative progression profiling in vivo. Our analyses showed trends of markedly accelerated tumor growth (P < 0.001), and moderately increased metastatic colony-forming potential (P = 0.010) along with rising levels of local histamine production. Using RNase protection assay for screening of the melanoma progression profile, and Western blotting for subsequent result validation, we looked for molecular progression markers affected by melanoma histamine secretion. Investigation of 21 functionally clustered markers associated with tumor proliferation, angiogenesis, invasivity, metastasis formation, local or systemic immunomodulation, and histamine signaling revealed positive correlations between histamine production, tumor histamine H2 receptor and rho-C expression (P < 0.001, P = 0.002, respectively). These observations confirm the involvement of histamine in the molecular machinery of melanoma progression.

Concentration of histamine in serum and tissues of the primary ductal breast cancers in women

The aim of this study was to evaluate the concentration of histamine (HA) and the activities of their enzymes, namely histidine decarboxylase (HDC) and diaminooxydase (DAO) in 95 women with ductal breast cancer and in healthy women. The control group comprised 60 women without any pathological changes in their breasts, in whom mammoplasties were performed. In women with breast cancer the concentration of HA in serum was significantly higher than in healthy controls (9.1+/-3.2 vs. 5.9+/-3.1 nmol/l; P<0.001). The concentration of HA was significantly higher in neoplasmatic tissues of women with breast cancers than in unchanged tissues of healthy subjects in the control group (14.2+/-5.1 vs. 6.3+/-9.1 nmol/g; P<0.001). HDC activity was significantly elevated in cancerous tissues of women with breast cancer relative to unchanged tissues of healthy subjects (54.7+/-17.1 vs. 39.3+/-26.9 pmol/min per mg; P<0.01). However, the activity of DAO was significantly lower (14.0+/-0.4 vs. 36.1+/-9.7 pmol/min per mg; P<0.001) in neoplasmatic tissues than in normal tissues of healthy women. The adjacent healthy tissue of cancer revealed higher concentrations of HA than were found in unchanged tissues of healthy subjects (6.3+/-9.1 vs. 7.5+/-5.4 pmol/min per mg), but this difference did not reach statistical significance. The activity of HDC did not show any significant difference between the healthy tissues adjacent to cancer foci of women with breast cancer and normal tissues obtained from healthy subjects (39.3+/-26.9 vs. 34.5+/-24.3 pmol/min per mg). However, the activity of DAO was markedly lower than in unchanged tissues of healthy women in the control group (36.1+/-9.7 vs. 14.4+/-10.9 pmol/min per mg; P<0.001). The concentration of HA in cancerous tissues was significantly higher than in adjacent healthy tissues (14.2+/-5.1 vs. 7.5+/-5.4 nmol/g; P<0.001). The activity of HDC was significantly higher in cancerous tissues than in adjacent healthy tissues (54.7+/-17.1 vs. 34.5+/-24.3 pmol/min per mg; P<0.001), but there was no difference in the activity of DAO (14.0+/-6.4 vs. 14.4+/-10.9 pmol/min per mg). The significant elevation of HA concentration in cancerous tissues of women with the ductal breast cancers is caused by the increased synthesis and decreased inactivation of HA.

Melanin pigmentation in mammalian skin and its hormonal regulation

Cutaneous melanin pigment plays a critical role in camouflage, mimicry, social communication, and protection against harmful effects of solar radiation. Melanogenesis is under complex regulatory control by multiple agents interacting via pathways activated by receptor-dependent and -independent mechanisms, in hormonal, auto-, para-, or intracrine fashion. Because of the multidirectional nature and heterogeneous character of the melanogenesis modifying agents, its controlling factors are not organized into simple linear sequences, but they interphase instead in a multidimensional network, with extensive functional overlapping with connections arranged both in series and in parallel. The most important positive regulator of melanogenesis is the MC1 receptor with its ligands melanocortins and ACTH, whereas among the negative regulators agouti protein stands out, determining intensity of melanogenesis and also the type of melanin synthesized. Within the context of the skin as a stress organ, melanogenic activity serves as a unique molecular sensor and transducer of noxious signals and as regulator of local homeostasis. In keeping with these multiple roles, melanogenesis is controlled by a highly structured system, active since early embryogenesis and capable of superselective functional regulation that may reach down to the cellular level represented by single melanocytes. Indeed, the significance of melanogenesis extends beyond the mere assignment of a color trait.

Inhibition of human primary melanoma cell proliferation by histamine is enhanced by interleukin-6

BACKGROUND

Interleukin-6 (IL-6) is a bifunctional growth factor in malignant melanoma; its expression increases during the malignant progression of the disease. Histamine, detected in large amounts in normal and pathological proliferating tissues, is an important paracrine and autocrine regulator of normal and tumour cell proliferation as well.

MATERIALS AND METHODS

We investigated the presence and function of IL-6 and histamine in the WM35 primary human melanoma cell line with respect to their direct role in cell proliferation and their regulatory interactions.

RESULTS

IL-6 inhibited the proliferation of WM35 melanoma cells and increased significantly the expression of histidine decarboxylase as well as histamine production. It had dose-dependent effects on the proliferation: high concentration (10-5 M) was inhibitory through H1 histamine receptors while low histamine concentration acting on H2 receptors, with a simultaneous increase of cAMP, enhanced colony formation in the monolayer. Furthermore, IL-6 increased the H1- but decreased the H2-histamine receptor expression of the melanoma cells. On the other hand, histamine was locally synthesized by the WM35 melanoma cells.

CONCLUSION

We suggest that the growth arrest induced by IL-6 is in part mediated by its dual action on histamine: a shift toward H1 receptor predominance and an elevation of locally produced histamine with prevalent action on the inhibitory response triggered through the H1 receptor. These findings suggest a local cross-talk between histamine and IL-6 in the regulation of melanoma growth.

Histamine induces melanogenesis and morphologic changes by protein kinase A activation via H2 receptors in human normal melanocytes

Hyperpigmentation frequently accompanies chronic or acute inflammation. A number of inflammatory mediators have been shown to stimulate melanin synthesis in human melanocytes. Although histamine is ubiquitous as an inflammatory factor, its involvement in pigmentation remains obscure. In this work, we examined the effects of histamine on cultured human melanocytes. Treatment of human melanocytes with 0.1-10 microM histamine evoked morphologic changes and increases in tyrosinase activity. The concomitant increases in melanin content of the histamine-treated melanocytes indicated an elevation of melanin synthesis by tyrosinase activation. These stimulatory effects of histamine were completely inhibited by an H2 antagonist, famotidine, whereas H1 and H3 antagonists had no inhibitory effect whatsoever. In addition, an H2 agonist, dimaprit, induced the same degree of melanogenesis as histamine at concentrations of 0.1-10 microM. We observed an increase in the intracellular cAMP contents of human melanocytes induced by histamine via the H2 receptors. We know that this cAMP accumulation and subsequent protein kinase A activation plays a critical role in histamine-induced melanogenesis, because a specific protein kinase A inhibitor, H-89, completely suppressed these stimulatory effects of histamine, and because dibutylic cAMP, a specific protein kinase A activator, stimulated human melanocytes as potently as histamine. Taken together, we show here that histamine induces melanogenesis of human cultured melanocytes by protein kinase A activation via H2 receptors.

Histamine as an autocrine growth factor: an unusual role for a widespread mediator

The involvement of histamine in cancer growth represents an old controversy and direct experimental evidence proving this hypothesis is not still available. In this paper we review the most relevant mechanisms referring to the role of histamine receptors, histidine decarboxylase and histamine release in the onset of an autocrine loop, that enables histamine to act as an autocrine growth factor. We postulate that this autocrine loop, that has been studied in an experimental mammary carcinoma model induced in rats, may be present in different human neoplasias. Therefore, the better understanding of this novel regulatory pathway that is controlled by histamine may contribute to identifying new therapeutic targets.

H2-antagonists in the treatment of colon and breast cancer

Several clinical trials have now been carried out with histamine type 2 receptor antagonists in cancer patients often as an adjunct to surgical resection of the primary tumour While promising results have been obtained in some groups of patients with gastrointestinal cancer, with increased survival and evidence of increased immunological recognition of tumour, results are less encouraging for breast cancer. This may be due to differences in the levels of histamine or the role of histamine in growth of these different tumour types.

Histamine in human breast cancer

BACKGROUND

Histamine inhibits lymphocyte function in vitro at concentrations of greater than 10(-6) mol/l. The aim of this study was to determine whether histamine concentrations in breast cancers were sufficient to produce an immunological effect.

METHODS

Tumour and adjacent normal breast content of histamine was measured using a radioenzymatic assay in 29 patients having surgery for breast cancer.

RESULTS

The median content of histamine in breast cancer tissue was 5.4 (range 0.9-27.3) microg/g (median concentration 4.5 x 10(-5) mol/l), and was significantly greater than that in adjacent breast tissue (P = 0.007).

CONCLUSION

The concentration of histamine in breast cancer was sufficient to inhibit lymphocyte function and could be locally immunosuppressive.

Novel insights into histamine H2 receptor biology

Histamine exerts multiple biological actions through one of three receptor subtypes (H1, H2, and H3). This review focuses on new developments regarding the structure and function of the H2 receptor. In addition to the important role this receptor plays in stimulating gastric acid secretion, recent studies have demonstrated that it is also involved in regulating gastrointestinal motility and intestinal secretion. The potential role of the H2 receptor in regulating cell growth and differentiation has also been added to the list of actions this biogenic amine may exert in both normal and transformed tissues. Molecular cloning of the gene indicates that it has the structural characteristics of a heptahelical G protein-linked receptor. Site-directed mutagenesis studies of this receptor reveal the presence of key amino acids within the third and fifth transmembrane domains that are critical for ligand recognition. Molecular approaches have also shed light on the structural components of the H2 receptor important in regulating desensitization and internalization. Although the H2 receptor was classically thought to couple to the adenylate cyclase pathway, recent work with the cloned receptor indicates that it can also activate the phosphoinositide signaling cascade through an independent G protein-dependent mechanism. The novel observation that histamine may stimulate c-fos gene expression lends further support to the possible role of this receptor in regulating cell growth and differentiation.

Pilot study--cimetidine enhances lymphocyte infiltration of human colorectal carcinoma: results of a small randomized control trial

BACKGROUND

Cimetidine preserves postoperative immune function and inhibits the growth of some cancers. In this study, the effect of cimetidine on the local immune response to colorectal carcinoma was investigated.

METHODS

Forty-two patients scheduled for elective resection of colorectal carcinoma were randomized either to receive cimetidine for 1 week perioperatively or to act as controls. A lymphocyte density of 50 cells per high-power field (approximately 50% of the tumor/tissue interface) was considered a positive response. Patient survival was determined by Kaplan-Meier life table analysis. The effects of histamine and cimetidine on normal subject lymphocyte function was determined in a mitogen-stimulated proliferation assay.

RESULTS

A positive lymphocyte response was observed in 5 of 24 control carcinoma patients (21%) and 10 of 18 cimetidine-treated carcinoma patients (56%) (P = 0.03). The presence of a lymphocyte response correlated with a better survival (P = 0.02). Histamine had an inhibitory effect on lymphocyte proliferation with a median effective dose of 5 x 10(-7) M. Cimetidine antagonized this effect with a negative logarithm of the cimetidine molar concentration required to reduce the effect of histamine in half of 6.55.

CONCLUSIONS

Histamine inhibits normal lymphocyte function, antagonized by cimetidine at a histamine type 2 receptor. Cimetidine increases lymphocyte infiltration of primary colorectal carcinoma, possibly by overcoming the immunosuppressive effects of high local histamine concentrations. The presence of a local lymphocyte response correlates with an improved 3-year survival.

Comparison of antiproliferative effects of 1-histamine-2 receptor antagonists, cimetidine, ranitidine, and famotidine, in gastric cancer cells

In the immune system, histamine is known to suppress cytotoxic T-lymphocytes and nitrogen induced lymphocyte thymidine uptake, down-regulate some cytokines, and activate suppressor T-lymphocytes, and in the gastrointestinal system, histamine was reported to have trophic effects on gastrointestinal epithelial cells. Enhanced rates of cell proliferation by histamine are implicated in the pathogenesis. This study was designed since there is a lack of comparative data about the cell proliferations of histamine-2 receptor antagonist (H2-RA), cimetidine, ranitidine, and famotidine, in gastric cancer. KATO-III and AGS cell lines were used in this experiment. The concentrations of the histamine and cimetidine were 10(-5)-10(-8) M, respectively and those of ranitidine and famotidine were 10(-6)-10(-9)M, respectively. Cell proliferation after drug treatment was evaluated by direct cell counting, [3H]thymidine incorporation, and MTT assay. Activities of ornithine decarboxylase (ODC), a rate limiting enzyme in polyamine synthesis, were measured after each drug treatment. Protein kinase A, a cAMP-dependent protein kinase system, was assayed using [alpha-32P]ATP. Histamine showed statistically significant cell proliferating effects in a dose-dependent manner (P < 0.001), the maximal effect in 10(-5) M concentration. ODC activities were increased in accordance with the increment of cell numbers after histamine treatment. Cimetidine reversed the histamine-stimulated cell proliferation significantly, the maximal effect in 10(-5) M concentration (P < 0.01). Although ranitidine showed the tendency to attenuate the cell proliferation dose-dependently, but without statistical significance, famotidine did not show such an effect at all. cAMP-dependent protein kinase activities were significantly increased following 10(-5) M histamine treatment, also reversed significantly by cimetidine co-administration (P < 0.01). Beneficial clinical outcomes could be anticipated from cimetidine treatment in patients with gastric cancer by anti-proliferating effects against gastric cancer cells. These effects of H2-RA are likely to be mediated by specific interactions at the H2-receptor.

Histamine-2 receptor antagonists as immunomodulators: new therapeutic views?

Considerable evidence has emerged to suggest that histamine participates in the regulation of the inflammatory response, immune reaction, coagulation cascade, and cardiovascular function. Furthermore, histamine may play a major role in the growth of normal and malignant tissue as a regulator of proliferation and angiogenesis. Specific histamine receptors have been identified on the surface of bone marrow cells, immune competent cells, endothelial cells, fibroblasts, and also on malignant cells. This has prompted research in regulation by specific histamine receptor agonists and antagonists. Results from such studies are currently accumulating and suggest that the histamine-2 receptor antagonists have potential beneficial effects in the treatment of certain malignant, autoimmune and skin diseases, either alone or in combination with other drugs. The beneficial effect of histamine-2 receptor antagonists as adjuvant single drugs to reduce trauma-, blood transfusion- and sepsis-induced immunosuppression has led to research in combined treatment regimens in major surgery, particularly, of patients operated on for malignant diseases.

Ranitidine and cimetidine differ in their in vitro and in vivo effects on human colonic cancer growth

Histamine has recently been shown to be a growth factor for some gastric and colorectal cancer cells. Previous studies have shown that cimetidine blocks in vitro and in vivo histamine-stimulated growth and cAMP release from the human colonic cancer cell line, C170. In this study, ranitidine, another H2 receptor antagonist, did not affect either basal or histamine-stimulated in vitro proliferation of C170, and failed to prevent cAMP release in vitro. Ranitidine did not inhibit in vivo growth of C170 at a dose of 1, 10, 25, 50 or 100 mg/kg, in contrast to 50 mg/kg/day cimetidine, which produced 39.3% inhibition of tumour volume (p<0.01) after 23 days' treatment. Ranitidine did not inhibit in vivo histamine-stimulated growth of C170 cells . LIM2412, another colonic cancer cell line, was significantly stimulated by both cimetidine and ranitidine in vivo. Ranitidine had no effect on in vitro cell proliferation.

H1 and H2 histamine receptors in N-nitroso-N-methylurea (NMU)-induced carcinomas with atypical coupling to signal transducers

Two specific binding sites for histamine were characterized in the cell membrane of N-nitroso-N-methylurea (NMU)-induced tumors. The first one, with higher affinity (Kd = 4 +/- 2 nM), was further identified as an H2 type, while the lower affinity one (35 +/- 10 nM) corresponded to an H1 receptor. Histamine concentrations up to 50 nM, as well as H2 agonists, significantly enhanced the phosphoinositide turnover by acting through higher affinity H2 receptors. On the other hand, histamine at concentrations over 50 nM and H1 agonists produced a 100% increase in cAMP levels in a response specifically blocked by mepyramine. These H1 and H2 histamine receptors that exhibit different linkages to second messenger systems may prove to be a characteristic of cells with a high proliferating capacity, such as undifferentiated or transformed cells.

Human, recombinant interleukin-2 induces in vitro histamine release in a dose-dependent manner

We previously observed that human, recombinant interleukin-2 in a pharmacologic dose (200 u/ml) induced histamine release from monocyte-depleted peripheral blood mononuclear cells in vitro. Therefore, we studied the role of various pharmacologic doses of rIL-2 on in vitro histamine release. Peripheral blood mononuclear cells (5 x 10(6) cells/ml), which also contain basophils, from 13 patients scheduled for elective colorectal cancer surgery and 10 age and sex matched healthy volunteers were stimulated with rIL-2 in concentrations of 0, 50, 100, 200, 450, 900, 1,800 and 3,600 u/ml, respectively, for 1, 24 and 48 hours under standard conditions. Histamine was analysed in supernatants using the glass fiber method. Simultaneously, total cell-bound histamine was analysed in lysate from 5 x 10(6) mononuclear cells from all patients and volunteers, thus allowing determination of percent histamine release. Supernatant histamine concentration from unstimulated cells was 17.2 +/- 1.5 ng/ml in patients compared to 7.9 +/- 1.0 ng/ml in volunteers (#p < 0.05) after 1 hour stimulation, and no further increase was observed after 24 and 48 hours, respectively. Histamine concentration increased significantly in the supernatant from cells stimulated by rIL-2 in a dose-dependent manner both in patients and volunteers. Total cell-bound histamine was 49.3 +/- 4.1 ng/ml in patients compared to 78.5 +/- 7.7 ng/ml in volunteers (p < 0.05). Therefore, both spontaneous and rIL-2-induced histamine release was significantly enhanced in cancer patients compared to volunteers (*p < 0.05). These data suggest that rIL-2 in high pharmacologic doses stimulates in vitro histamine release in a dose-dependent manner in both cancer patients and volunteers. This may in part explain the severe toxicity observed during high-dose rIL-2 therapy.

Cimetidine inhibits in vivo growth of human colon cancer and reverses histamine stimulated in vitro and in vivo growth

The effect of histamine and cimetidine on the growth of four human colon cancer cell lines was studied. Histamine significantly stimulated the uptake of tritiated thymidine in vitro in a dose dependent manner, to a maximum of 120% and 116% of controls for C170 and LIM2412, respectively. This effect was antagonised by cimetidine, but not diphenhydramine. Histamine also stimulated a dose dependent increase in cyclic adenosine monophosphate accumulation in C170 cells, antagonised by cimetidine. When grown as subcutaneous xenografts in Balb/c nu/nu mice, cimetidine had a significant inhibitory effect on the same two cell lines. The final volume of C170 tumours in animals given cimetidine was 44% of controls. This response was dose dependent, plateauing at a cimetidine dose of 50 mg/kg/day. The final volume of LIM2412 tumours in animals given cimetidine was 60% of controls. Histamine administered locally by a mini-osmotic pump stimulated C170 tumour growth to 164% of controls, was antagonised by cimetidine at a dose of 200 mg/kg/day, but not by lower concentrations. Histamine has a trophic effect on at least two colorectal cancer cell lines in vivo and in vitro. As this effect is antagonised by cimetidine, it may be mediated via tumour histamine type 2 receptors.

Pharmacological characterization of the human histamine H2 receptor stably expressed in Chinese hamster ovary cells

1. The gene for the human histamine H2 receptor was stably expressed in Chinese hamster ovary (CHO) cells and characterized by [125I]-iodoaminopotentidine binding studies. In addition, the coupling of the expressed receptor protein to a variety of signal transduction pathways was investigated. 2. After cotransfection of CHO cells with pCMVhumH2 and pUT626, a phleomycine-resistant clonal cell line (CHOhumH2) was isolated that expressed 565 +/- 35 fmol kg-1 protein binding sites with high affinity (0.21 +/- 0.02 nM) for the H2 antagonist, [125I]-iodoaminopotentidine. 3. Displacement studies with a variety of H2 antagonists indicated that the encoded protein was indistinguishable from the H2 receptor identified in human brain membranes and guinea-pig right atrium. The Ki-values observed in the various preparations correlated very well (r2 = 0.996-0.920). 4. Displacement studies with histamine showed that a limited fraction (32 +/- 6%) of the binding sites showed a high affinity for histamine (2 +/- 1.2 microM); the shallow displacement curves were reflected by a Hill-coefficient significantly different from unity (nH = 0.58 +/- 0.09). The addition of 100 microM Gpp(NH)p resulted in a steepening of the displacement curve (nH = 0.79 +/- 0.02) and a loss of high affinity sites for histamine. 5. Displacement studies with other agonists indicated that the recently developed specific H2 agonists, amthamine and amselamine, showed an approximately 4-5 fold higher affinity for the human H2 receptor than histamine. 6. Stimulation of CHOhumH2 cells with histamine resulted in a rapid rise of the intracellular cyclic AMP levels. After 10 min an approximately 10 fold increase in cyclic AMP could be measured. TheEC50 value for this response was 7 +/- 1 nM for histamine. This response was effectively blocked by tiotidine and cimetidine, resulting in Ki values of 8 +/- 1 nM and 0.56 +/- 0.24 MicroM respectively.7. Stimulation of CHOhumH2 cells with histamine neither inhibited the A23187-induced release of[3H]-arachidonic acid nor changed the intracellular IP3 levels.8. These results show that the cloned human gene encodes a histamine H2 receptor that is indistinguishable from the H2 receptor identified in human brain tissue. This receptor is functionally coupled to the adenylate cyclase in CHO cells, but does not influence the inositolphosphate turnover or arachidonic acid release.

Histamine receptors in an experimental mammary carcinoma

An experimental mammary carcinoma was induced in Sprague-Dawley rats by the ip administration of N-nitroso-N-methylurea (NMU) in three doses of 50 mg/kg. In order to study the expression of histamine receptors in these experimental tumors, the presence of specific binding sites for histamine was studied. Using [3H]-histamine as a radioligand, two specific binding sites were characterized on the cell membrane. The first site, of high affinity, Kd = 4 +/- 2 nM, was further characterized as an H2 type using [3H]-cimetidine and [3H]-tiotidine as radioligands and by displacement experiments with different histamine agonists and antagonists. The second one of low affinity, Kd = 35 +/- 14 nM, needs further characterization. The determination of cAMP levels showed that histamine and the H2 agonist dimaprit, produced a significant decrease in the nucleotide concentration 6 minutes after stimulation, in a response that was specifically abolished by H2 antagonists. Based on these results, we conclude that neoplastic cells from NMU induced tumors express H2 histamine membrane receptors which are coupled to a transductional pathway different from cAMP production, which may be involved in the regulation of tumor growth.

Histamine stimulates normal human melanocytes in vitro: one of the possible inducers of hyperpigmentation in urticaria pigmentosa

Isolated normal human melanocytes became enlarged and more dendritic in association with an increase in the activity of tyrosinase and the amount of b-locus protein when they were cultured with 0.1-10 microM histamine in vitro. However, histamine did not exert a proliferative effect on them. The stimulatory effect of histamine was observable even 6 h after starting the treatment. This stimulation seems not to be pharmacologically mediated through histamine receptors, because it was inhibited neither by pyrilamine, a histamine H-1 antagonist, nor by cimetidine, a H-2 antagonist. Imidazole derivatives that are rapidly metabolized from histamine in vivo and in vitro also stimulated the melanocytes. We propose that high concentrations of histamine and its imidazole metabolites continuously produced in the lesions of urticaria pigmentosa are probable causative factors of its characteristic skin pigmentation.

Antiproliferative and depigmenting effects of the histamine (H2) agonist dimaprit and its derivatives on human melanoma cells

Human melanoma cells were treated in culture with the histamine (H2) agonist S-(3-(N-N-dimethylamino)propyl)isothiourea (dimaprit), a partial agonist, S-(2-(N,N-dimethylamino)ethyl)-isothiourea (nordimaprit), and two analogues of nordimaprit, S-(2-(N,N-diethylamino)ethyl)isothiourea (DENOR) and S-(2-(N,N-diisopropylamino)ethyl)isothiourea (DINOR), to investigate the effects on toxicity and tyrosinase activity. Cell survival studies showed highest toxicity in the constitutively pigmented human melanoma cell line MM418, DINOR being the most effective agent. Toxicity was not blocked by the H2 antagonist cimetidine. Dimaprit and its derivatives decreased tyrosinase activity in the amelanotic human melanoma cell line MM96E and inhibited expression of a melanosomal antigen. Loss of tyrosinase activity could be prevented by cimetidine and ranitidine, an H2 antagonist. Although the tyrosinase activity in MM418 cells was much more resistant to inhibition by these agents compared with that in MM96E cells, prolonged growth in the presence of non-toxic levels of DINOR caused a decrease in tyrosinase activity and subsequent depigmentation. Ultrastructural examination of the depigmented cells showed a decrease in the number of melanized melanosomes and the appearance of premelanosomes. These results indicate that bulky substituents on the tertiary amine group in nordimaprit significantly enhance potency for depigmentation and cell killing but only the former effect is mediated by the H2 receptor.

Regulation of tyrosinase expression and activity in human melanoma cells via histamine receptors

In cultured human melanoma cells, histamine H1 (mepyramine) and H2 receptor antagonists (cimetidine, ranitidine, impromidine) increased tyrosinase activity, whereas H2 agonists (dimaprit, nordimaprit) decreased activity. Mixtures of agonist and antagonist either decreased or increased tyrosinase activity, depending on the relative concentrations of each drug. Nordimaprit, the most effective inhibitor, lowered tyrosinase activity significantly within 36 h and caused a slower loss of tyrosinase protein as judged by reactivity with two monoclonal antibodies. Prolonged treatment of a melanotic cell line with nordimaprit led to complete loss of pigment, with no loss of the 56-kDa melanosomal antigen 1C11. Cells remained amelanotic for 8 weeks after removal of the drug and, even after 26 weeks, melanin content and tyrosinase expression and activity had not fully recovered. Nordimaprit increased the rate of degradation of tyrosinase and of nordimaprit binding proteins. Whereas nordimprit did not directly inhibit tyrosinase, lysates of treated cells contained an inhibitory activity that partitioned approximately equally across a 10-kDa ultrafiltration membrane. Overall, these results showed that melanogenesis can be controlled via histamine receptors, the mechanism for the H2 agonist nordimaprit consisting of three components: induction of a tyrosinase inhibitor, increased degradation of tyrosinase, and long-term down-regulation of tyrosinase expression.

The effects of histamine H2 receptor antagonists on melanogenesis and cellular proliferation in melanoma cells in culture

B16-C3 murine melanoma, A375P human melanotic melanoma, and C32 human amelanotic melanoma cells were incubated in the presence of (0-4 mM) H2-antagonists, ranitidine and cimetidine. Cell proliferation, tyrosinase activity and melanin content were monitored. H2-antagonists stimulated tyrosinase activity and melanin accumulation in B16-C3 cells in a dose- and time-dependent manner. Stimulation of enzyme activity and pigment production was accompanied by inhibition of cellular proliferation in B16-C3 cells. The inhibitory concentration of cimetidine was approximately 2-fold higher than that of ranitidine. H2-antagonists failed to stimulate melanogenesis in A375P or C32 cells, but inhibited cellular proliferation in both cell lines. These results are the first demonstration of H2-antagonist induced phenotypic changes in malignant melanoma cells in vitro, and represent a novel mechanism for the previously described in vivo antitumor effects of these agents.

The regulation of cyclic AMP production and the role of cyclic AMP in B16 melanoma cells of differing metastatic potential

The nature of the relationship between agonist-stimulated cyclic AMP production and metastatic potential was examined in detail for four B16 melanoma cell lines of varying metastatic potential. Highly metastatic cells (B16 F10C1) appeared to differ from cells of low metastatic potential (B16 F1C29) in the degree to which cyclic AMP production in intact cells was stimulated by protein kinase C activation. No significant difference was found in the adenylate-cyclase enzyme activities of the broken cells, irrespective of the agonist used, or in the distribution of cyclic AMP between the intracellular and extracellular compartment. Although B16F1, F10 and F10C1 cells all produced equally pigmented tumors in vivo, the cells differed in their melanogenic response to cyclic AMP elevating agents in vitro: the least metastatic cells produced least agonist-induced cyclic AMP but this induced greatest tyrosinase activation and melanin production in vitro; conversely, the more metastatic cells produced more cyclic AMP but less tyrosinase activation and melanin production in response to agonist stimulation. Thus, agonist-stimulated cyclic AMP production does not appear to be coupled to the differentiated function of melanogenesis for highly metastatic B16 melanoma cells.