Interferons modulate the expression of hormone receptors on the surface of murine melanoma cells

Adrenal insufficiency in HIV/AIDS: a review

INTRODUCTION

Adrenal insufficiency (AI) is one of the most common potentially life-threatening endocrine complications in people living with human immunodeficiency virus (PLHIV) infection and acquired immunodeficiency syndrome (AIDS).

AREAS COVERED

In this review, the authors explore the definitions of relative AI, primary AI, secondary AI and peripheral glucocorticoid resistance in PLHIV. It also focuses on the pathophysiology, etiology, diagnosis and management of this endocrinopathy in PLHIV. A literature review was conducted through Medline and Google Scholar search on the subject.

EXPERT OPINION

Physicians need to be aware of the endocrinological implications of HIV infection and its treatment, especially CYP3A4 enzyme inhibitors. A high index of clinical suspicion is needed in the detection of AI, especially in PLHIV, as it may present insidiously with nonspecific signs and symptoms and may be potentially life threatening if left untreated. Patients with overt primary and secondary AI require glucocorticoid replacement therapy. Overt primary AI also necessitates mineralocorticoid replacement. On the other hand, the management of relative AI remains controversial. In order to reduce the risk of adrenal crisis during periods of stress, the short-term use of glucocorticoids may be necessary in relative AI.

Efficient Induction of Cytotoxic T Cells by Viral Vector Vaccination Requires STING-Dependent DC Functions

Modified Vaccinia virus Ankara (MVA) is an attenuated strain of vaccinia virus and currently under investigation as a promising vaccine vector against infectious diseases and cancer. MVA acquired mutations in host range and immunomodulatory genes, rendering the virus deficient for replication in most mammalian cells. MVA has a high safety profile and induces robust immune responses. However, the role of innate immune triggers for the induction of cytotoxic T cell responses after vaccination is incompletely understood. Stimulator of interferon genes (STING) is an adaptor protein which integrates signaling downstream of several DNA sensors and therefore mediates the induction of type I interferons and other cytokines or chemokines in response to various dsDNA viruses. Since the type I interferon response was entirely STING-dependent during MVA infection, we studied the effect of STING on primary and secondary cytotoxic T cell responses and memory T cell formation after MVA vaccination in STING KO mice. Moreover, we analyzed the impact of STING on the maturation of bone marrow-derived dendritic cells (BMDCs) and their functionality as antigen presenting cells for cytotoxic T cells during MVA infection in vitro. Our results show that STING has an impact on the antigen processing and presentation capacity of conventionel DCs and played a crucial role for DC maturation and type I interferon production. Importantly, STING was required for the induction of efficient cytotoxic T cell responses in vivo, since we observed significantly decreased short-lived effector and effector memory T cell responses after priming in STING KO mice. These findings indicate that STING probably integrates innate immune signaling downstream of different DNA sensors in DCs and shapes the cytotoxic T cell response via the DC maturation phenotype which strongly depends on type I interferons in this infection model. Understanding the detailed functions of innate immune triggers during MVA infection will contribute to the optimized design of MVA-based vaccines.

Melanin production through novel processing of proopiomelanocortin in the extracellular compartment of the auricular skin of C57BL/6 mice after UV-irradiation

The production of melanin is regulated by α-melanocyte-stimulating hormone (α-MSH), which is produced from proopiomelanocortin (POMC). Keratinocytes release POMC along with lower levels of α-MSH and ACTH. To clarify the mechanism of melanogenesis after ultraviolet (UV)-irradiation, this study focused on the expression of POMC and POMC-derived peptides after UV-irradiation. Western blot analysis and immunoassays indicated that both POMC and α-MSH-like immunoreactivity (α-MSH-LI) increased after UV-irradiation. However, other POMC-derived products were very low. In hypophysectomized mice, α-MSH-LI increased to the same level as in control mice after UV-irradiation. Structural analysis revealed that the major α-MSH-LI product was ACTH(1–8). Furthermore, ACTH(1–8) competed with [¹²⁵I]-α-MSH for receptor binding and increased melanin production via a melanocortin-1 receptor. These results suggested that melanin was produced through ACTH(1–8) after UV-irradiation. Trypsin-like enzymatic activity, which is responsible for POMC activation, increased after UV-irradiation and was identified as tryptase. In mast cell-deficient mice, which do not produce tryptase, α-MSH-LI levels were unchanged after UV-irradiation. The present study demonstrates the production of ACTH(1–8) from POMC by tryptase, which is a novel peptide-processing mechanism in the extracellular compartment of the skin.

New hope for cancer treatment: exploring the distinction between normal adult stem cells and cancer stem cells

For decades, intensive studies have attempted to identify the mechanisms underlying malignant tumor growth. Despite significant progress, most therapeutic approaches fail to eliminate all tumor cells. The remaining tumor cells often result in recurrence and metastasis. Recently, the idea of a cancer stem cell was proposed to explain of the origin of cancer cells. According to this hypothesis, a small fraction of tumor cells have the capacity for self-renewal, with unlimited slow proliferation potential. They are often resistant to chemotherapy and radiation and thus are responsible for continuously supplying new cancer cells, which themselves may have a limited life span. In recent years, accumulating experimental evidence supports this hypothesis and provides new possibilities to conquer cancer. This review will focus on the distinction between normal adult stem cells and cancer stem cells and identifies possible key targets for effective therapies of cancer.

A tyrosine phosphatase is associated with the somatostatin receptor

Regulation of tyrosine phosphorylation is thought to be an essential step in signal transduction mechanisms that mediate cellular responses. In pancreatic tumour cells we demonstrated that somatostatin analogues inhibited cell proliferation and stimulated a membrane protein tyrosine phosphatase (PTP) activity at concentrations at which they bind to the somatostatin receptor. To elucidate the role of PTP in the signal transduction pathway activated by somatostatin receptors we first studied the interaction of PTP with the somatostatin receptor at the membrane. We purified somatostatin receptors by immunoaffinity from pancreatic membranes that strongly expressed the type 2 somatostatin receptor sstr2. We identified the receptor as an 87 kDa protein. We demonstrated that a PTP activity co-purified with somatostatin receptors. The PTP was identified as a 66 kDa protein immunoreactive to antibodies against SHPTP1. These antibodies immunoprecipitated somatostatin receptors either occupied or unoccupied by ligand indicating that SHPTP1 is associated with somatostatin receptors. We then expressed sstr2A in monkey kidney COS-7 cells and mouse NIH/3T3 fibroblasts and demonstrated that somatostatin analogues (RC 160, octreotide and BIM 23014) which exhibited high affinity for sstr2 stimulated a PTP activity and inhibited cell proliferation in proportion to their affinities for sstr2. Under the same conditions these analogues have no effect on the growth of cells expressing sstr1. All these results suggest that a PTP related to SHPTP1 is associated with somatostatin receptors and may be involved in the negative growth signal promoted by sstr2.

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.

Hyperpigmentation during interferon-alpha therapy for chronic hepatitis C virus infection

Many types of skin disorders concomitantly occur with hepatitis C virus infection. These skin lesions may be induced or worsened during antiviral therapy with interferon-alpha (IFN). To our knowledge, hyperpigmentation of the skin--and especially of the tongue--has not been reported so far. We describe two dark-skinned patients who developed hyperpigmented skin and tongue lesions during combination therapy with IFN and ribavirin. Immunohistochemical analysis of tongue biopsies confirmed the suspicion of melanin deposits in these areas of hyperpigmentation. We hypothesize that during interferon therapy, melanocytes may produce more melanin pigment in the presence of alpha-melanocyte stimulating hormone and sufficient amounts of tyrosine, leading to melanin deposits and clinical hyperpigmentation.

Malignant melanoma

CONTEXT

The rapidly developing fields of melanoma research are revolutionizing the current concepts on melanoma etiology and pathogenesis and are introducing newer diagnostic techniques and potential therapeutic approaches.

OBJECTIVES

To present the most current concepts on the etiology and pathogenesis of melanoma and to introduce the recent diagnostic techniques and the potential therapeutic approaches.

METHODS

Data sources were reports on melanoma published in the English language literature and observations made using specimens available at Harvard University, Johns Hopkins Medical Center, Albany Medical College, Loyola University Medical Center, and University of Tennessee Health Science Center.

RESULTS

Studies on melanoma containing chromosomal or genetic evaluation were selected for further analysis. Current clinical and pathologic categories with the reported genetic abnormalities were related to the latest information on pigment biology. The data extracted were used to develop a conceptual framework on the pathogenesis of melanoma; the generated model was then evaluated and used to suggest potential therapeutic approaches.

CONCLUSIONS

(1) Melanoma is not genetically homogeneous, and the existing differences between the pathologic categories, particularly in areas such as type of growth phase (radial vs vertical growth), total vertical dimension, ulceration of primary tumor, and metastatic process, have profound prognostic and therapeutic implications. (2) Chromosomal aberrations and gene mutations are found in sporadic and familial melanomas; among the most important are those affecting the 9p21, which contains the p16 locus, a site known to be critical for normal progression of the cell cycle. Aberrant p16 expression is associated with more aggressive behavior. (3) Melanoma cells possess a remarkable repertoire of biosynthetic capacities represented by the production of hormones, growth factors, and their receptors that may sustain and accelerate tumor development and progression. For example, expression of the tumoral products alpha-melanocyte-stimulating hormone and adrenocorticotropic hormone is regulated in vitro by ultraviolet light, a known carcinogen. (4) Melanomas differ from other tumors in their intrinsic capability to express melanogenic enzymes with the corresponding structural proteins to actually synthesize melanin. Melanogenesis-related proteins are rapidly entering the clinical arena, being used not only as diagnostic markers, but also as potential targets for melanoma therapy.

Corticotropin releasing hormone and proopiomelanocortin involvement in the cutaneous response to stress

The skin is a known target organ for the proopiomelanocortin (POMC)-derived neuropeptides alpha-melanocyte stimulating hormone (alpha-MSH), beta-endorphin, and ACTH and also a source of these peptides. Skin expression levels of the POMC gene and POMC/corticotropin releasing hormone (CRH) peptides are not static but are determined by such factors as the physiological changes associated with hair cycle (highest in anagen phase), ultraviolet radiation (UVR) exposure, immune cytokine release, or the presence of cutaneous pathology. Among the cytokines, the proinflammatory interleukin-1 produces important upregulation of cutaneous levels of POMC mRNA, POMC peptides, and MSH receptors; UVR also stimulates expression of all the components of the CRH/POMC system including expression of the corresponding receptors. Molecular characterization of the cutaneous POMC gene shows mRNA forms similar to those found in the pituitary, which are expressed together with shorter variants. The receptors for POMC peptides expressed in the skin are functional and include MC1, MC5 and mu-opiate, although most predominant are those of the MC1 class recognizing MSH and ACTH. Receptors for CRH are also present in the skin. Because expression of, for example, the MC1 receptor is stimulated in a similar dose-dependent manner by UVR, cytokines, MSH peptides or melanin precursors, actions of the ligand peptides represent a stochastic (predictable) nonspecific response to environmental/endogenous stresses. The powerful effects of POMC peptides and probably CRH on the skin pigmentary, immune, and adnexal systems are consistent with stress-neutralizing activity addressed at maintaining skin integrity to restrict disruptions of internal homeostasis. Hence, cutaneous expression of the CRH/POMC system is highly organized, encoding mediators and receptors similar to the hypothalamic-pituitary-adrenal (HPA) axis. This CRH/POMC skin system appears to generate a function analogous to the HPA axis, that in the skin is expressed as a highly localized response which neutralizes noxious stimuli and attendant immune reactions.

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.

UV light and MSH receptors

Ultraviolet B (UVB) radiation in the skin induces pigmentation that protects cells from further UVB damage and reduces photocarcinogenesis. Although the mechanisms are not well understood, our laboratory has shown that UVB radiation causes increased MSH receptor activity by redistributing MSH receptors from internal pools to the external surface, with a resultant increase in cellular responsiveness to MSH. By this means, UVB and MSH act synergistically to increase melanin content in the skin of mice and guinea pigs. In humans, MSH causes increased skin pigmentation, predominantly in sun-exposed areas. We have shown recently that UVB irradiation and exposure to MSH or to dbcAMP, stimulates production of mRNAs for both alpha MSH receptors and POMC in human melanocytes and keratinocytes. This indicates that at least one action of UVB on the pigmentary system is mediated through increased MSH receptor production, as well as through the production of the signal peptides, MSH and ACTH, that can further activate MSH receptors. The results add support to the hypothesis that the effects of UVB on cutaneous melanogenesis are mediated through a series of coordinated events in which MSH receptors and POMC-derived peptides play a central role.

Modulation of melanocyte-stimulating hormone receptor expression on normal human melanocytes: evidence for a regulatory role of ultraviolet B, interleukin-1alpha, interleukin-1beta, endothelin-1 and tumour necrosis factor-alpha

Melanocyte-stimulating hormone (MSH) receptor binding activity and melanocortin-1 receptor (MC1-R) gene expression on normal human melanocytes have been studied as responses to the effects of ultraviolet B (UVB), interleukin-1 (IL-1), endothelin-1 (ET-1) and tumour necrosis factor-alpha (TNF-alpha), which are known as UV sensitive regulators of melanocytic function. MSH receptor (MSH-R) binding activity was upregulated by UVB, IL-1alpha, -1beta and ET-1, but was downregulated by TNF-alpha. Northern blot analysis showed that MC1-R mRNA expression was induced 24 h after UVB irradiation in a dose-dependent manner, and that 24-h treatment with ET-1 also induced an expression of MC1-R mRNA, whereas TNF-alpha downregulated the expression. In addition, IL-1alpha and -1beta have a small but real inductive effect on MC1-R mRNA expression. Taken together, our results suggest a model in which higher MC1-R mRNA expression is accompanied by upregulation of MSH-R binding activity, and enhanced by UVB or cytokines sensitive to UVB. Such a regulatory system would enable normal human melanocytes to respond to MSH more efficiently and induce an increase of melanization of the skin through the MSH/MSH-R system after UVB radiation.

Glucocorticoid resistance and the immune function in the immunodeficiency syndrome

Glucocorticoids, the final product of HPA axis, and their receptors (GRs) on mononuclear cells are crucial mediators in the endocrine-immune interaction. An alteration in GRs involving a lower receptor affinity (Kd) for glucocorticoids has been found in a group of advanced AIDS patients, who developed Addisonian symptoms (weakness, weight loss, hypotension, hyponatremia, and intense mucocutaneous melanosis) in spite of hypercortisolism and normal or slightly elevated values of ACTH (AIDS-GR). In these patients, data for the suppression test showed decreased cortisol and ACTH suppression in response to exogenous dexamethasone. The inhibitory effect of dexamethasone on radiolabeled-thymidine incorporation in mononuclear cells from these patients was also reduced. Monocytes of AIDS-GR patients had a receptor Kd of 10.5 +/- 4.2 nmol/l that was higher than that of other AIDS patients (AIDS-C) (2.9 +/- 0.8 nmol/l) and normal subjects (2.0 +/- 0.8 nmol/l: p < 0.01). Correlations were found between plasmatic IFN-alpha and receptor Kd on monocytes of AIDS-GR (r = 0.77). Poly (i)-poly (c)-induced IFN-alpha production by monocytes was inhibited by glucocorticoids in the AIDS-C group and controls (approx. 80% in both groups): The effect was reversed by the receptor antagonist RU-486. By contrast, glucocorticoid did not inhibit IFN-alpha production in AIDS-GR group. In conclusion, levels of plasmatic IFN-alpha, a cytokine with antiviral properties, may be increased several times, and dexamethasone fails to inhibit monocytes' IFN-alpha production only in AIDS with cortisol resistance, a disturbance that confirms an important immunoregulatory role of glucocorticoids in HIV disease.

Quantification of tyrosinase, TRP-1, and Trp-2 transcripts in human melanocytes by reverse transcriptase-competitive multiplex PCR--regulation by steroid hormones

We have introduced a reverse transcriptase polymerase chain reaction based method to measure mRNA levels of the melanogenesis enzymes tyrosinase, tyrosinase-related-protein 1 (TRP-1), and tyrosinase-related-protein 2 (TRP-2). Expression was determined by reverse transcriptase-competitive multiplex polymerase chain reaction of (i) melanogenesis enzyme transcripts and the "housekeeping" gene glyceraldehyde-3-phosphate dehydrogenase, and (ii) two internal standards consisting of mutated melanogenesis enzyme cDNA and mutated gene glyceraldehyde-3-phosphate dehydrogenase cDNA. This was investigated on in vitro cultured melanocytes in the presence of three different steroids; one glucocorticoid (betamethasone-17-valerate) and two sex steroids (diethylstilbestrol and estradiol). All three steroids lead to an increase of about 1.5-2.5-fold of tyrosinase transcripts. The amount of TRP-1 transcripts was likewise enhanced, but only moderately (approximately 1.5-fold). In contrast, TRP-2 transcripts were reduced by approximately 40% in number after betamethasone-17-valerate treatment, whereas the two sex steroids, diethylstilbestrol and estradiol, caused an upregulation of about 20-fold of the initial TRP-2 transcript level. We therefore suggest that hyperpigmentation during pregnancy or under contraceptive treatment is mediated by a direct induction of melanogenesis via sex steroids.

Sorting and secretion of a melanosome membrane protein, gp75/TRP1

The melanosome is an organelle specialized for melanin synthesis that is derived from the endocytic pathway. Several melanosome membrane proteins have been identified, forming a family of proteins known as tyrosinase-related proteins. Two members of this family, tyrosinase and gp75, are well-characterized melanocyte differentiation antigens. Our previous studies have shown that gp75, the mouse brown locus protein, is sorted to melanosomes along the endocytic pathway, directed by a hexapeptide sorting signal located in the cytoplasmic tail. In this study, we report the unexpected finding that a portion of gp75 is secreted. Substantial levels of secretory gp75 were detected in melanocytic cells. Cell surface expression of gp75 was also detected, representing 2% of cellular gp75. Characterization of secretory gp75 cells showed that it is: (i) a truncated form that lacks the transmembrane region, the cytoplasmic tail where the endosomal sorting signal is located, and a small portion of the lumenal domain; (ii) more extensively glycosylated than endocytic/melanosomal gp75, containing trans-Golgi processed sugar residues; and (iii) generated post-translationally in an acid sensitive compartment after processing in the trans-Golgi, and secreted rapidly after generation. Thus, these endocytic/melanosomal membrane proteins can be processed to abundant secretory forms, probably in an endocytic compartment through a potentially novel secretory pathway.

Melanocortin receptors: identification and characterization by melanotropic peptide agonists and antagonists

Hormones are chemical messengers released from cells to act on and control the activity of other cells. Hormonal ligands initiate their actions by interacting with receptive substances (Langley, 1906) of the target cells. These receptors are proteins that are either integral components of the cell membrane or are localized cytoplasmically within cells. Ligand-receptor interaction results in either the stimulation or inhibition of cellular activity. Since most hormones bind rather specifically to receptors possessed by their target cells, labeling of hormonal ligands can be utilized to identify and localize cells within an animal. In this report we discuss what is presently known about melanocortin receptors (MCRs) as studied by the use of labeled melanotropic peptide ligands.

Isotretinoin and recombinant interferon alfa-2a therapy of metastatic malignant melanoma

Twenty-five patients with metastatic malignant melanoma were treated with isotretinoin (13-cis-retinoic acid) orally at 1 mg/kg daily and recombinant interferon alfa-2a (INF-alpha) subcutaneously at 3 million units daily for 16-48 weeks. Therapy was well tolerated; fatigue and hyperlipidemia were the most frequent dose-limiting toxicity and necessitated dose reductions in 14 patients. Two patients achieved a complete response, and 3 responded partially for a total response rate of 20% (95% confidence interval: 4-36%). Responses occurred primarily in patients with limited tumor burden and disease confined to the skin and lymph nodes. Significant elevations in peripheral blood 2'-5'-oligoadenylate synthetase activity and natural killer activity were observed with therapy. The magnitude of these changes, however, was not predictive of response. Biopsy specimens of two responding lesions showed extensive necrosis of tumor. One specimen showed large aggregates of melanophages in association with tumor. The combination of isotretinoin and IFN-alpha is an active, easily administered regimen with acceptable toxicity for metastatic malignant melanoma.

The melanotropic peptide, [Nle4,D-Phe7] alpha-MSH, stimulates human melanoma tyrosinase activity and inhibits cell proliferation

Seventeen human melanoma cell (HMC) lines, both melanotic and amelanotic, were incubated in the continuous presence of a potent melanotropic peptide hormone analog, [Nle4,D-Phe7] alpha-MSH, for 72 hr with daily changes of medium. Only one cell line (HD, melanotic) consistently responded to the hormone analog by increased tyrosinase activity. Three (one melanotic, two amelanotic) of the HMC lines also failed to respond to the peptide by either increased or decreased enzyme activity when incubated continuously in the presence of the peptide for longer periods of time (6,15,27,43 days). The HD cell line, however, again responded with increasingly enhanced basal enzyme activity the longer the cells were incubated in the presence of the melanotropin. One amelanotic cell line (C8161) responded with enhanced enzyme activity when grown to confluency in the continuous presence of the peptide. Basal tyrosinase activity of the C8161 cell line may have increased as cell density in the flasks increased. These results suggest that under conditions of increased cell number, phenotypic expression of tyrosinase activity in so called "amelanotic" (tyrosinase-negative) cells is increased and can be enhanced further by stimulation with a melanotropic peptide. Under conditions of increased cell number, the presence of [Nle4,D-Phe7] alpha-MSH caused morphological differentiation (shape change); the cells became enlarged and very dendritic. The number of cells in monolayer (surface of the flask) and in the medium were drastically reduced in both melanotic and "amelanotic" cell lines incubated with [Nle4,D-Phe7] alpha-MSH. The data support other published reports that melanotropic peptides inhibit human melanoma cell growth (proliferation) in vitro, most likely through a cytostatic mechanism. [Nle4,D-Phe7] alpha-MSH also exhibited a prolonged (residual) inhibitory action on HD cell proliferation. In other words, inhibition of cell growth (proliferation) of the HMCs was evident even several days after removal of the melanotropic peptide from the incubation medium.

Modulation of tumor cell gene expression and phenotype by the organ-specific metastatic environment

The mechanistic basis of a metastatic cell's ability to proliferate in the parenchyma of certain organs and develop organ-specific metastases is under intense investigation. Signals from paracrine or autocrine pathways, alone or in combination, may regulate tumor cell proliferation with the eventual outcome dependent on the net balance of stimulatory and inhibitory factors. This article summarizes recent reports from our laboratory and others demonstrating that the organ microenvironment can profoundly influence the pattern of gene expression and the biological phenotype of metastatic tumor cells, including induction of melanocyte stimulating hormone receptor and production of melanin, regulation of terminal differentiation and apoptosis, resistance to chemotherapy, and regulation of growth at the organ-specific metastatic site. These recent data from both murine and human tumor models support the concept that the microenvironment of different organs can influence the pattern of gene expression and hence the phenotype of tumor cells at different steps of the metastatic process. These findings have obvious implications for the therapy of neoplasms in general and metastases in particular.

Melanocortins and their implication in melanoma

The melanocortins (MCs), that is, the melanocyte-stimulating hormones (MSHs) and ACTH, are a group of related peptides containing the typical melanotropin core sequence, His-Phe-Arg-Trp, and are derived from a common precursor, pro-opiomelanocortin. They are pleiotropic molecules that occur in the pituitary, some brain regions, and also in several peripheral tissues, and they exert a variety of physiologic functions. Their effect on melanogenesis in the skin is well established, but their role in melanocyte and melanoma cell proliferation and metastasis is less clear. The recent cloning of five types of MC receptors (MC1-5), new studies on the regulation of these receptors, the discovery of a naturally occurring MSH antagonist, the agouti protein, and the finding that melanocytes and melanoma cells exclusively express MC1 receptors have laid the basis for the future development of specific MC ligands, which may become useful for melanoma diagnosis and eventually therapy.

The expression of tyrosinase, tyrosinase-related proteins 1 and 2 (TRP1 and TRP2), the silver protein, and a melanogenic inhibitor in human melanoma cells of differing melanogenic activities

The expression of various melanogenic proteins, including tyrosinase, the tyrosinase-related proteins 1 (TRP1) and 2 (TRP2/DOPAchrome tautomerase), and the silver protein in human melanocytes was studied in six different human melanoma cell lines and compared to a mouse derived melanoma cell line. Analysis of the expression of tyrosinase, TRP1, TRP2, and the silver protein using flow cytometry revealed that in general there was a positive correlation between melanin formation and the expression of those melanogenic enzymes. Although several of the melanoma cell lines possessed significant activities of TRP2, the levels of DOPAchrome tautomerase in extracts of human cells were relatively low compared to those in murine melanocytes. Melanins derived from melanotic murine JB/MS cells, from melanotic human Ihara cells and HM-IY cells, from sepia melanin, and from C57BL/6 mouse hair were chemically analyzed. JB/MS cells, as well as Ihara cells and HM-TY cells, possessed significant amounts of 5,6-dihydroxyindole-2-carboxylic acid (DHICA) derived melanins, this being dependent on the activity of TRP2. Kinetic HPLC assays showed that 5,6-dihydroxyindole (DHI) produced during melanogenesis was metabolized quickly to melanin in pigmented KHm-1/4 cells, whereas DHI was stable in amelanotic human SK-MEL-24 cells. A melanogenic inhibitor that has been purified from SK-MEL-24 cells that suppressed oxidation of DHI in the presence or absence of tyrosinase, but had no effect on DHICA oxidation. The sum of these results suggests that the expression of melanogenic enzymes as well as the activity of a melanogenic inhibitor are critical to the production of melanin synthesis in humans.

The syndrome of acquired glucocorticoid resistance in HIV infection

A certain number of HIV-infected patients (about 17% in our series) manifest symptoms of cortisol resistance--weakness, weight loss, hypertension, chronic fatigue and intense mucocutaneous melanosis--symptoms which are also typical of Addison's disease. The diagnosis of cortisol resistance is determined through the increased plasma and urinary cortisol values and limited increases in ACTH values. Compared with patients with primary glucocorticoid resistance, AIDS patients have no symptoms of mineral-corticoid or androgen excess, only of glucocorticoid deficiency at target tissues. Mononuclear leukocytes from these patients show receptor changes which consist of an increased receptor number and decreased receptor affinity for glucocorticoids. They also show defective glucocorticoid-induced inhibition of [3H]thymidine incorporation. Glucocorticoid-resistant AIDS patients have a characteristic persistent increase in interferon-alpha production. The inverse correlation between plasma values of interferon-alpha and the receptor affinity for glucocorticoids clearly suggests that interferon production is regulated by the glucocorticoid receptor: the smaller the glucocorticoid effect on lymphocyte cells is, the greater interferon production is. Owing to the antiviral effect of interferon-alpha, it is possible that glucocorticoid-resistant AIDS patients have greater defences against viral infection than other AIDS patients. As interferon-alpha is melanogenetic, its increased production may also explain the intense skin pigmentation found in patients with the glucocorticoid-resistance syndrome.

Bidirectional regulation of smooth muscle cell proliferation by IFN-gamma

We studied the effects of interferon gamma (IFN-gamma), a T-cell lymphokine, on the proliferation and chemotaxis of vascular smooth muscle cells (SMC). Recombinant human IFN-gamma dose-dependently inhibited the proliferation of SMC cultured in the presence of 20% fetal calf-serum. It also inhibited PDGF-induced chemotaxis of SMC. Similar concentrations of IFN-gamma induced DNA-synthesis of SMC cultured in mitogen-depleted medium for 5 days. The inhibition and the stimulation of SMC proliferation were accompanied by concomitant decrease and increase in the number of PDGF receptors. Our study indicated that IFN-gamma is a bidirectional regulator of SMC proliferation.

Repeated UVB irradiations do not have the same potential to promote stimulation of melanogenesis in cultured normal human melanocytes

The major stimulus for human melanin production is ultraviolet (UV) radiation. Little is known about the mechanisms underlying this response and the eventual enzyme regulation resulting from this activation. We treated normal human melanocytes in culture with daily UVB radiations. Cumulative increases in UVB doses resulted in proportional increases in tyrosinase activity over the first few days whereas an intermittent pattern of tyrosinase activation was observed after the fifth day of irradiation. This intermittent pattern consisted of latency periods where no melanogenic response was elicited despite exposure to UVB. Tyrosinase activity in cellular extracts increased shortly after an effective irradiation, peaked at 3 hours and thereafter decreased to below basal levels. Increased tyrosinase activity was associated with increased amounts of both the newly synthesized and mature forms of the enzyme. Decreased tyrosinase activity following an activation period was correlated with decreases in both the expression of tyrosinase mRNA and the amount of the newly synthesized form of the enzyme present in the melanocytes 24 hours after six irradiations. This particular pattern of stimulation of tyrosinase was not observed in S-91 murine melanoma cells after repeated UVB irradiations. Taken together these results may suggest a photo-protective mechanism developed by irradiated normal human melanocytes.

MSH receptors in immortalized human epidermal keratinocytes: a potential mechanism for coordinate regulation of the epidermal-melanin unit

Receptors for melanotropin (MSH) were found to be expressed by immortalized primary human epidermal keratinocytes (RHEK-1). Using 125I-beta MSH as a probe, the MSH receptors from mouse melanoma cells and human keratinocytes were found to be remarkably similar. In each cell line, there were high and low affinity receptors, with the high affinity classes showing positive cooperativity. Competition of 125I-beta MSH for binding with non-radioactive MSH revealed similar profiles. Cross-linking studies, followed by gel electrophoresis and autoradiography, showed almost identical gel migration patterns. Both cell types expressed internal as well as plasma membrane binding sites. MSH receptors on both cell types were up-regulated by ultraviolet light and by MSH itself. Although the function of MSH receptors expressed by the immortalized keratinocytes is unknown, the results are consistent with recent reports that proliferation of epidermal keratinocytes is stimulated by MSH and that proopiomelanocortin genes are expressed in the epidermis. These results support a model in which keratinocytes and melanocytes, interacting in an "epidermal-melanin unit," each respond to UV light signals with increased MSH receptor activity.

Enhancement of progesterone receptor levels by interferons in AE-7 endometrial cancer cells

BACKGROUND

Interferon (IFN) has been reported to increase hormone receptor expression in breast cancer cells and to sensitize them to antiproliferative hormones. Endometrial cancer cells with high progesterone receptor (PR) level respond better to progesterone therapy than cells with either low or absent PR level. The effect of four different interferons (alpha and beta, both natural [n] and recombinant [r]) on cell proliferation and steroid receptor levels was investigated in the PR positive AE-7 human endometrial cancer cell line over a period of 12 days.

METHODS

Cells were exposed to 10,100 and 1000 IU/ml of each IFN either for 3 days or continuously for 12 days. Hormone receptors were determined by the monoclonal enzyme immunoassay. Chemosensitivity was evaluated with the adenosine triphosphate-cell viability assay.

RESULTS

AE-7 has a low level of estrogen receptors, which was not significantly affected by IFN exposure. The four IFN showed significantly enhanced PR levels over 12 days in both the 3-day and continuous-exposure experiments. No significant difference of PR enhancement was observed between 3 days and continuous exposure to IFN. This increase of receptors did not appear to be dose related. IFN enhanced PR level to a maximum level of about two times control cells. IFN did not produce significant cytotoxicity. Antiproliferative activity was observed with nIFN beta and rIFN beta at 1000 IU/ml dose in continuous-exposure experiments, which showed survival values of 79% and 69% respectively, compared with control at day 12.

CONCLUSIONS

These preliminary data on PR expression modulation support other studies, which have shown that IFN modulate hormone receptor expression and, therefore, may play a role in the treatment of endometrial cancer.

Interferon-alpha in malignant and viral diseases. A review

In the 35 years since the discovery of interferon, significant biological activity has been described for interferon-alpha (IFN alpha) in various cancers, particularly haematological malignancies such as hairy cell leukaemia and chronic myelogenous leukaemia. Except for localised therapy in bladder and ovarian cancer, activity against most solid tumours has been disappointing. Other notable exceptions include Kaposi's sarcoma, renal cell carcinoma and malignant melanoma, tumours known to be susceptible to immunological attack. More recently, broad spectrum antiviral activity has been demonstrated for both recombinant and naturally occurring IFN alpha. Hepatitis C is responsive to IFN alpha in about 40% of patients, but long term remissions are rare. In contrast, long term suppression of hepatitis B is common following IFN alpha therapy. Both diseases respond in a dose proportional fashion, with daily doses of 5 million units (MU) significantly more effective than lower doses. The mechanism of action in viral diseases involves the expression of unique antiviral proteins such as endonuclease and 2'-5'-oligoadenylate synthetase which enhance the destruction of viral RNA. General cellular protein synthesis is also inhibited, including cytochrome P450 enzymes. This forms the basis for potential drug interactions, with IFN alpha slowing the clearance of highly metabolised drugs such as theophylline. As an antitumour agent, the mechanism of action of IFN alpha is unclear, particularly in haematological cancers. In melanoma and renal cell carcinoma, antitumour effects may be mediated by augmented immune responses including activation of natural killer lymphocytes and enhanced expression of cell surface antigens (e.g. MHC I and II). Conversely, antibody formation to recombinant IFN alpha may result in a loss of activity. This has been observed in both renal cell cancer and hepatitis B and C. The elimination half-life of IFN alpha is short, 4 to 5 hours, but biological activity extends for 2 to 3 days after administration, which facilitates daily or thrice weekly administration. Clearance of IFN alpha is mediated by catabolism in the renal tubules; no intact drug is excreted in the urine. It is probable that the antiviral indications of IFN alpha will expand as the agent is more clearly recognised as a primary endogenous defence against various viral conditions.

Pigment production in murine melanoma cells is regulated by tyrosinase, tyrosinase-related protein 1 (TRP1), DOPAchrome tautomerase (TRP2), and a melanogenic inhibitor

Using antibodies that recognize either tyrosinase, tyrosinase-related protein-1 (TRP1), or tyrosinase-related protein-2 (TRP2, DOPAchrome tautomerase), the quantities of those melanogenic enzymes were analyzed in five melanoma cell lines that possess various degrees of melanin production. All cells except JB/MS-W increased melanin production four to 30 times after 4 d of melanocyte-stimulating hormone (MSH) treatment. Melanin production by JB/MS-W cells was always under background, with or without MSH treatment. There was a positive correlation between quantities and synthetic rates of those melanogenic enzymes and their melanin formation or DOPAchrome tautomerase activities. The activity of a heat-resistant melanogenic inhibitory factor was also analyzed. The results showed, surprisingly, that pigmented cells showed higher levels of melanogenic inhibitors activity. Tyrosinase activity was increased dramatically whereas the level of melanogenic inhibitor was remarkably decreased following MSH treatment. Interestingly, melanogenic inhibitor derived from JB/MS-W cells suppressed not only tyrosinase but also DOPAchrome tautomerase, another enzyme functional in melanin production. These results clearly suggest that melanin production is regulated by a subtle balance between the activities of these enzymes and other factors such as the melanogenic inhibitor.

Steroid receptor enhancement by natural interferon-beta in advanced breast cancer

In the current study we investigated the effect of two different doses of natural interferon-beta (IFN-beta) on steroid hormone receptors in 45 patients with advanced breast cancer. IFN-beta seems to regulate the receptor mechanisms, inducing in cutaneous metastases an increase of oestrogen and progesterone receptors. Moreover, using IFN-beta and tamoxifen as a combined therapy in 23 receptor-positive patients, no negative interference of the two drugs was observed and no relevant side-effects due to the treatment were noticed. The modulation of steroid receptor content by IFN-beta in advanced breast cancer might represent an interesting way to ameliorate the clinical responsiveness to anti-oestrogens.

Mitogenic activity of interferon gamma on growth-arrested human vascular smooth muscle cells

Interferon gamma (IFN-gamma) is a multifunctional lymphokine secreted by activated T lymphocytes, which are found in atherosclerotic lesions. IFN-gamma has been reported to suppress the proliferation of vascular smooth muscle cells (SMCs). However, as we report in this paper, IFN-gamma is mitogenic for vascular SMCs under certain circumstances. Recombinant human IFN-gamma (1-100 units/ml), in a dose-dependent fashion, stimulated cell multiplication and [3H]thymidine and 5-bromo-2'-deoxyuridine incorporation into DNA by cultured arterial SMCs that had been growth arrested by culturing in 1% plasma-derived serum for 5 days. IFN-gamma also accentuated the mitogenic activity of platelet-derived growth factor (PDGF)-BB. A time-course study revealed that there was a time lag of 4-6 hours between the G1-->S transition of quiescent SMCs stimulated by IFN-gamma and that of SMCs stimulated by PDGF-BB. A synergistic effect of IFN-gamma on the mitogenicity of PDGF became apparent after a similar time lag, suggesting that the IFN-gamma-related mitogenicity is mediated by a substance(s) secreted by IFN-gamma-treated SMCs. In fact, conditioned medium of IFN-gamma-treated SMCs was mitogenic for SMCs. Mitogenic activity in the conditioned medium was also detected by an assay using Swiss 3T3 cells, which originate from mice and, therefore, are not responsive to human IFN-gamma. The production of the mitogenic factor was blocked by anti-IFN-gamma antibody. Mitogenicity of the conditioned medium was not eliminated by addition of neutralizing antibody against PDGF, indicating that any autocrine growth factor(s) secreted by IFN-gamma-treated SMCs was not PDGF.(ABSTRACT TRUNCATED AT 250 WORDS)

Up-regulation of MSH receptors by MSH in Cloudman melanoma cells

MSH can up-regulate MSH binding capacity of cultured Cloudman melanoma cells in a dose- and time-dependent fashion. Binding is mediated through proteins exhibiting an apparent molecular weight of 50-53kDa, consistent with previous studies implicating them as the principal MSH receptors on Cloudman cells. Pre-incubation of cells with MSH stimulates expression of the receptor proteins both on the plasma membrane surface as well as in internal sites associated with coated vesicles. The effects of MSH are additive with those of UV light, suggesting that UV and MSH might stimulate receptor expression through separate mechanisms.

Treatment of reticulate acropigmentation of Kitamura with azelaic acid. An immunohistochemical and electron microscopic study

No successful therapy has been reported for reticulate acropigmentation of Kitamura, which is an autosomal dominant dermatosis. We treated a patient with 20% azelaic acid ointment. Within several weeks the pigmentation was remarkably decreased and no side effects were observed. Histologic examination revealed an increased number of dopa-positive melanocytes. These cells reacted strongly to staining with antityrosinase antibody or antityrosinase-related protein antibody. Electron microscopic findings showed many melanosomes within melanocytes, keratinocytes, and melanophages. These findings suggest that the hyperpigmentation of reticulate acropigmentation of Kitamura is the result of an excess amount of melanin production caused by activation of melanocytes in the basal layer.

Effect of recombinant human interferon-alpha 2b on receptors for steroid hormones and epidermal growth factor in patients with endometrial cancer

Interferons (IFNs) may modulate oestrogen (ER), progesterone (PR) and epidermal growth factor (EGFR) receptor expression in vitro. ER, PR and EGFR levels in tumour specimens taken from 13 patients with endometrial adenocarcinomas before and after 5 days' intramuscular treatment with 5 x 10(6) U per recombinant human leucocyte interferon-alpha 2b (rh IFN-alpha 2b). After treatment, ER (P less than 0.01) and PR (P less than 0.05) levels were significantly increased with a simultaneous reduction of EGFR content (P less than 0.05). Since the expression of ER and PR characterises more differentiated hormono-sensitive tumours, while EGFR are preferentially expressed in less differentiated tumours, the increase of steroid hormone receptor levels with the reduction of EGFR expression suggests that rh IFN-alpha 2b may induce endometrial cancer cell differentiation. Moreover, the decrease of EGFR levels may explain the antiproliferative effect of IFNs.

Recombinant interferon-gamma (rIFN-gamma) in dermatology

This paper gives a short review on the function, pharmacokinetics, and therapeutic application of recombinant interferon-gamma (rIFN-gamma) in dermatology. Simultaneously, our own experiences are presented for 57 patients (phase II study) suffering from genital warts (21 patients), psoriatic arthritis (10 patients), psoriasis vulgaris (three patients), malignant melanoma (six patients), bowenoid papulosis (four patients), Behcet's disease (four patients), basal cell carcinoma (six patients), as well as herpes simplex recidivans, epidermodysplasia verruciformis, and mycosis fungoides (one patient each). We conclude that there might be an indication for treatment with rIFN-gamma in genital warts, bowenoid papulosis, Behcet's disease, and microbial infections, such as leprosy and cutaneous leishmaniasis. Even though there are reports of a limited beneficial effect of rIFN-gamma on arthritis and skin lesions in psoriasis, we failed to observe any in 10 patients. The main side effects in our low-dose study (50-100 micrograms/d) were mild fever (78%), fatigue (78%), and myalgia (65%). Laboratory tests revealed an increase in the serum triglyceride level, in particular, in psoriatic patients.

Differentiation and the tumorigenic and metastatic phenotype of murine melanoma cells

Using B16 F10 murine melanoma cells and sublines generated from the JB/MS melanoma which exhibit various degrees of melanogenesis, the relationships among differentiation, tumorigenicity, and metastatic potential were examined. The effect of melanocyte-stimulating hormone (MSH), which specifically stimulates differentiation of melanocytes, was also studied. All melanoma lines tested were capable of growing as experimental pulmonary metastases but, surprisingly, the undifferentiated and amelanotic JB/MS-w cells failed to grow as primary subcutaneous tumors. JB/MS-w cells, which had few surface MSH receptors, did not respond to MSH with an increase in melanin production, unlike the other cell lines. Although in vitro treatment with MSH did not change the rates of growth of primary tumors by these cell lines, such treatment decreased the number of pulmonary metastases from B16 F10, JB/MS cells, JB/MS-b1 cells and JB/MS-w cells. Conversely, MSH treatment significantly increased the rates of pulmonary metastases from JB/MS-p cells. The expression of surface melanoma antigens, urokinase-type plasminogen activity and susceptibility to natural killer cells were examined. MSH did not significantly alter surface melanoma antigen expression, but increased the natural killer cell susceptibility of B16 F10, JB/MS and JB/MS-b1 cells, cells which possess abundant surface MSH receptors. There was an inverse correlation between differentiation (pigmentation) and proliferation in vitro, and the more pigmented melanoma cells (B16 F10, JB/MS and JB/MS-b1) expressed relatively lower levels of class-I MHC, relatively higher levels of class-II MHC and the highest metastatic capacity. These results demonstrate that MSH possesses the capacity to regulate not only melanogenesis, but also other factors critical to the metastatic growth of the cells.

Melanotropin receptors demonstrated in situ in human melanoma

Although some cultured human melanoma cell lines are responsive to melanotropins (melanocyte-stimulating hormones [MSH]), the prevalence and tissue distribution of MSH receptors in melanoma are unknown. We report here the use of an in situ binding technique to demonstrate specific MSH receptors in surgical specimens of human melanoma. The distribution and binding properties of specific MSH binding sites were determined by autoradiography and image analysis after incubation of frozen tumor tissue sections with a biologically active, radiolabeled analogue of alpha-MSH, [125I]iodo-Nle4, D-Phe7-alpha-MSH ([125I]NDP-MSH). In melanoma specimens from 11 patients, 3 showed high levels of specific binding, 5 showed low levels, and in 3 patients specific binding of [125I]NDP-MSH was not detectable. Specific MSH binding sites were present in melanoma cells, but not in adjacent connective or inflammatory tissues. Melanotropins, including alpha-MSH, NDP-MSH, and ACTH, inhibited [125I]NDP-MSH binding in a concentration-dependent manner, whereas unrelated peptides (somatostatin and substance P) did not. The apparent affinity of alpha-MSH for this binding site was in the nanomolar range (EC50 = 2 X 10(-9) M for inhibition of [125I]NDP-MSH binding in situ), similar to that recently described for the murine melanoma receptor. In one patient, analysis of multiple intratumor samples and tumors excised on three separate occasions revealed high levels of specific MSH binding in all samples. These results suggest that endogenous melanotropins may modulate the activities of human melanoma cells in vivo.

Characterization of functional melanotropin receptors in lacrimal glands of the rat

The specific melanotropin (MSH) binding sites of rat lacrimal glands were characterized with respect to anatomic distribution, peptide specificity and selectivity, and coupling to a biological response. Tissue distribution of MSH binding sites was determined by autoradiography following in situ binding of a radiolabeled, biologically active preparation of a superpotent alpha-MSH analog, [125I]-[Nle4,D-Phe7]-alpha-MSH ([125I]-NDP-MSH). Intense, specific (i.e., alpha-MSH-displaceable) [125I]-NDP-MSH binding was observed throughout lacrimal acinar tissue, but not in ducts or stroma. In freshly isolated lacrimal acinar cells, specific binding of [125I]-NDP-MSH was maximal within 30 min and rapidly reversible, with a dissociation half-time of about 15 min. A number of melanotropins [alpha-MSH, [N,O-diacetyl-Ser1]-alpha-MSH, [des-acetyl-Ser1]-alpha-MSH, beta-MSH, ACTH(1-24) and ACTH(1-39)] were recognized by these binding sites, as assessed by their inhibition of [125I]-NDP-MSH binding; NDP-MSH was the most potent (IC50 = 1.3 x 10(-9) M). In contrast, other peptides, including ACTH(4-10) and the nonmelanotropic peptides VIP, substance P, somatostatin, and ACTH(18-39) (CLIP), had no effects on tracer binding. In isolated lacrimal acinar cells, alpha-MSH and NDP-MSH stimulated intracellular cyclic AMP accumulation. We conclude that lacrimal acinar cells express functional receptors recognizing melanotropins, suggesting that the lacrimal gland may be a target for physiological regulation by endogenous melanotropins.

Regulation of mammalian melanogenesis by tyrosinase inhibition

Melanocyte stimulating hormone (MSH) specifically induces differentiation of mammalian melanocytes. To further define the biochemical events elicited by this stimulus, we have cloned murine melanoma cells which are either highly responsive or nonresponsive to MSH, and have examined their ultrastructural appearance, their melanogenic activities, and also their expression of tyrosinase. We have found that the basal levels of melanogenic activity in pigmented and nonpigmented cells correlate with expression of surface MSH receptors rather than with production of tyrosinase. Nonpigmented cells produce a potent, highly stable inhibitor of melanogenesis; this inhibitor acts directly on tyrosinase to dramatically and abruptly suppress melanin production. This posttranslational control of tyrosinase activity may represent a critical regulatory point in mammalian pigmentation.