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

Anatomic distribution of melanocytes in normal nail unit: an immunohistochemical investigation

Very few histologic reports describe normal melanocytes of the nail unit. Previous studies predominantly address the distal nail matrix melanocytes; we found no review of nail-bed melanocytes in the literature. The proximal nail matrix melanocytes are difficult to identify; the cells cannot be identified by L-DOPA staining. More recently, their scarcity was confirmed by immunohistochemistry with a large panel of antibodies directed against melanocytes. We wished to detect the proximal nail matrix dormant melanocytes and compare their density and distribution with that of the other melanocytes in the distal matrix and nail bed and to establish criteria of normality that may help clarify the pathologic features of benign nevoid melanonychia in the nails of whites. A panel of five monoclonal antibodies (MoAbs), including HMB45 and TRP1 directed against antigens localized in early melanosomal vesicles, was investigated in frozen sections of six nail specimens from whites. Both vertical and horizontal sections were assessed to determine the presence of dormant melanocytes. Results showed that the proximal nail matrix melanocytes were clearly identified with MoAbs HMB45 and tyrosinase-related protein-1 (TRP-1). By contrast, melanocytes stained by MoAb against tyrosinase and L-DOPA reaction were evident, especially in the distal matrix. With MoAb TRP-1, the epithelial sheets showed counts of approximately 217+/-84/mm2 in the proximal matrix and of 132+/-34/mm2 in the distal matrix; the nail bed counts were only 45+/-25/mm2. The split epithelial sheets had 103+/-17/mm2 L-DOPA-positive melanocytes in the distal third of the matrix, but only a few of them were detected in the proximal matrix and none were noted in the nail bed. We clearly identified proximal nail melanocytes using MoAb HMB45 and TRP1. The total number of matrix melanocytes can be estimated as approximately 217/mm2. In proximal matrix, the dormant melanocytes compartment was predominant. In the distal matrix, two compartments were identified: a functionally differentiated and a dormant compartment. Contrary to classical opinion, longitudinal melanonychia originates more frequently in the distal matrix, not secondary to the larger melanocyte density but because only the distal matrix contains an active melanin synthesis compartment. Furthermore, the superficial distribution of proximal nail melanocytes in vertical sections showed a histologic feature that may simulate the pagetoid pattern of melanoma in situ.

Reduction of DNA synthesis, pigment synthesis, pigmentation gene mRNA and resistance to UVB in human melanoma cells treated with analogues of a histamine (H2) agonist

Two groups of S-[2-(N,N-dialkylamino)ethyl]isothiourea derivates which depigmented melanoma cells either with inhibition of tyrosinase (group 1, R = methyl, isopropyl) or without inhibition of tyrosinase (group 2, R = benzyl, phenyl) were studied. Treatment of human melanoma cells with non-lethal doses of group 1 drugs led to a reduction in the levels of mRNA for the pigmentation genes tyrosinase, tyrosinase-related protein-1 and Pmel 17. The group 1 drug S-[2-N,N-diisopropylamino)ethyl[isothiourea] (DINOR) (R = isopropyl) produced only moderate inhibition of DNA, RNA and protein synthesis in three cell lines during the first 24 hr of treatment, and there was no correlation between the extent of inhibition and long-term toxicity. A group 2 drug (R = benzyl) rapidly inhibited DNA synthesis in an amelanotic melanoma cell line (MM96E) sensitive to killing by the drug; association of the latter with inhibition of RNA or protein synthesis was less clear. MM96E cells were also sensitive to killing by reactive oxygen species. In pigmented melanoma cells (MM418), incorporation of [125I]thiouracil, a false precursor of melanin, increased during the first 24 hr of treatment with DINOR whereas a group 2 drug (R = phenyl) inhibited incorporation of [125I]thiouracil. Cells depigmented by treatment with drugs from either group suffered the same amount of DNA damage as pigmented cells after UVB irradiation, as judged by inhibition of DNA synthesis, but did not recover as well as pigmented cells, whether or not drug was present during recovery. The results suggested that (1) group 1 agents down-regulated message for several pigmentation genes, possibly at the transcriptional level; (2) the toxicity of group 2 drugs was related to reactive oxygen species; and (3) melanin protected cells from UVB by enhancing cellular recovery.

Inhibition of melanogenesis in human melanoma cells by novel analogues of the partial histamine (H2) agonist nordimaprit

A series of analogues of the weak histamine (H2) agonist S-[2-(N,N-dimethylamine)ethyl] isothiourea (nordimaprit) was produced to investigate the possibility that bulky substituents on the tertiary amine of nordimaprit would enhance potency for depigmentation and killing of melanoma cells. Cell survival studies showed that neither an increase in lipophilicity nor an increase in size of these groups produced selective toxicity, with only the N,N-diisobutyl derivative being more effective than the N,N-diisopropyl derivative in killing the constitutively pigmented human melanoma cell line MM418. The most hydrophobic analogue, the N,N-dibenzyl derivative, was also the most toxic to all cell lines tested. The association of toxicity with lipophilicity was confirmed by the piperidine derivative having greater toxicity than the less hydrophobic morpholine analogue. The ability to decrease tyrosinase activity was lost when lipophilicity and size of the N-terminal groups were increased, but these analogues produced marked depigmentation, even greater than that found with either nordimaprit or the diisopropyl derivative. Surprisingly, an increase in tyrosinase activity was achieved, the most potent agent being the N-ethyl-N-anilino analogue which caused complete depigmentation (0.6% of control) and elevated tyrosinase activity (148%) in MM418 cells after 1 month of treatment. This indicates that nordimaprit and its derivatives possess two different mechanisms of depigmentation, the first being tyrosinase dependent and the second being tyrosinase independent. The latter pathway is yet to be elucidated but appears to require a high degree of hydrophobicity.

Alteration of melanoma melanogenesis by phenotypic modifiers

Human melanoma cells (MM96E) were incubated with a phenotypic modifier (L-ethionine) to compare its effects on phenotypic expression with those induced by sodium butyrate and dimethyl sulfoxide. In contrast to the latter agents, L-ethionine (8mM) failed to arrest the cell cycle at the G1 phase or to inhibit colony formation ability after 48 hr incubation. Tyrosinase activity changed in parallel with 5-S-cysteinyldopa (5-S-CD) content during treatment with sodium butyrate or dimethyl sulfoxide. Tyrosinase was inhibited in L-ethionine-treated cells, probably because of metabolism of L-ethionine to sulfhydryl compounds; this remains to be clarified. Gamma-glutamyl transpeptidase activity changed inversely with tyrosinase activity after sodium butyrate or dimethyl sulfoxide incubation, whereas L-ethionine did not significantly alter the enzyme activity. In addition, only sodium butyrate induced alkaline phosphatase activity. L-ethionine was less effective than sodium butyrate or dimethyl sulfoxide in inhibiting expression of the B8G3 melanosomal antigen, as determined by Western blotting. These results suggest that phenotypic modifiers (differentiation inducers) affect melanoma cells in various ways and that melanogenesis therefore reflects only one aspect of differentiation in pigment cells.