Isolation and characterization of a variant of B16-mouse melanoma resistant to MSH growth inhibition

Cell-substrate adhesion and metastatic potential of cultured mesothelioma cells induced by asbestos

Cell-substrate adhesion was quantified for two cultured mesothelioma cell lines (epitheliomatous and sarcomatous) on glass, fibronectin and laminin substrates. Interference reflection microscopy (IRM) was used to image the adhesion patterns of cells and a grey level analysis was employed to quantify adhesion. Sarcomatous cells demonstrated marked adhesion to glass and fibronectin-coated substrates but not to laminin-coated substrate, with the greatest adhesion occurring on the fibronectin-coated surface. This adhesion was accompanied by cytoplasmic spreading. By contrast, epitheliomatous cells showed little tendency to adhere to any of the substrates and only showed significant spreading when in contact with the laminin substrate (P < 0.01). A bioassay was used to determine the metastatic potential of each of the cell lines. Via the intravenous route, the sarcomatous cells killed the host rats in 24.7 +/- 1.5 (S.D.) days compared to 27.3 +/- 0.9 (S.D.) days for the epitheliomatous cells (P < 0.01). After subcutaneous inoculation of tumour cells, the sarcomatous cells killed the host rats in 54.7 +/- 0.7 (S.D.) days compared to 48.5 +/- 0.5 (S.D.) days for the epitheliomatous cells (P < 0.01). We conclude that the results of the metastasis bioassays were consistent with the predicted behavior of these cell lines based on their ability to adhere to substrates in the in vitro adhesion assays.

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.

A positive association between agonist-induced cyclic AMP production in vitro and metastatic potential in murine B16 melanoma and hamster fibrosarcoma

A positive association between agonist-stimulated cyclic AMP production in vitro and both experimentally induced (B16 melanoma) and spontaneous (fibrosarcoma) metastases were found. Five B16 melanoma cell lines producing varying degrees of lung colonization following intravenous injection and three hamster fibrosarcoma cell lines producing a varying number of metastases in lungs and regional lymph nodes after removal of the primary tumour were studied. Agonist-stimulated (forskolin and melanocyte-stimulating hormone), but not basal cyclic AMP accumulation, increased with increasing metastatic potential. This relationship did not extend to other intracellular signalling systems as determined by investigation of basal or foetal-calf stimulated phosphatidylinositol hydrolysis for either tumour type. Intracellular free calcium was also similar in B16 melanoma cell lines of varying metastatic potential.

alpha-MSH-induced changes in protein phosphorylation of Cloudman S91 mouse melanoma cells

The role of protein phosphorylation in MSH-induced melanogenesis was investigated with an in vivo phosphorylation assay using intact cultured Cloudman S91 mouse melanoma cells preincubated with [32P]orthophosphate. Exposure of the cells to alpha-MSH increased the extent of labelling of two protein bands on SDS gel electrophoresis with estimated molecular weights of 43 and 34 kDa, respectively. The 32P incorporation was concentration-dependent and reached a maximal value at 10(-8) M alpha-MSH for the 43 kDa band (156% of controls) and at 10(-5) M alpha-MSH for the 34 kDa band (250% of controls). The corresponding ED50s were 5 X 10(-10) M (43 kDa) and 3 X 10(-8) M (34 kDa). The 32P incorporation into the 34 kDa band reached a maximum after a 5 min exposure to alpha-MSH whereas 43 kDa phosphorylation was maximal after a 30-60 min incubation with hormone. The effect was completely reversible after removal of the hormone and specific for melanotropic peptides. Dibutyryl cAMP (10(-3) M) and forskolin (10(-4) M) together with isobutylmethylxanthine (10(-4) M) mimicked the effect of alpha-MSH, pointing to an involvement of adenylate cyclase activation in the phosphorylation of both the 34 kDa and the 43 kDa protein. Preliminary observations showed that the 34 kDa protein is membrane-bound whereas the 43 kDa protein is of mitochondrial or melanosomal origin.

alpha-Melanotropin-induced changes in protein phosphorylation in melanophores

To investigate a possible role of protein phosphorylation in the mechanism of action of alpha-MSH, excised tail-fins of Xenopus tadpoles were incubated with or without alpha-MSH. After homogenization, in vitro endogenous protein phosphorylation was assayed using [gamma-32P]ATP. alpha-MSH treatment of intact tail-fins, producing full pigment dispersion, resulted in a 5-fold increase in 32P-incorporation into a 53 kDa protein band. This increase in 53 kDa phosphorylation was completely reversible. The increase was not found in homogenates from the melanophore-free part of the alpha-MSH-treated tail-fins. Phosphorylation of the 53 kDa protein could be detected in homogenates of alpha-MSH-treated primary cultured melanophores. Incubation of tail-fins with ACTH1-24, an alpha-MSH-like peptide producing full pigment dispersion, also induced an increase in 53 kDa phosphorylation. A structurally related peptide (ACTH15-24) and an unrelated peptide (LH-RH), neither of which induced pigment dispersion, were ineffective in stimulating 53 kDa phosphorylation. Injection of white adapted tadpoles with 1 micrograms of alpha-MSH or adaptation of tadpoles to a black background also resulted in a significant increase in 53 kDa phosphorylation. alpha-MSH added to the homogenates did not affect 53 kDa phosphorylation, indicating that alpha-MSH acts through a receptor-mediated mechanism. The increase in 53 kDa phosphorylation measured in vitro (post hoc), most likely reflects an alpha-MSH-induced decrease in 53 kDa phosphorylation in vivo. Our results strongly suggest that a decrease in 53 kDa phosphorylation is involved in the mechanism of action of alpha-MSH on melanophores.

Characterization of alpha-MSH-induced changes in the phosphorylation of a 53 kDa protein in Xenopus melanophores

alpha-Melanotropin has been shown to induce specific changes in the degree of phosphorylation of a 53 kDa melanophore protein, concomitant with pigment dispersion. To further characterize the alpha-MSH-induced changes in 53 kDa phosphorylation in melanophores from the ventral tail-fin of Xenopus tadpoles, we investigated the concentration and time dependency of the effect. A significant increase in 53 kDa phosphorylation was detectable at 5 X 10(-8) M alpha-MSH. The maximal increase in 53 kDa phosphorylation was found after an incubation time of 10-15 min, whereas pigment dispersion was optimal after 60 min. The phosphorylated 53 kDa band showed clear cross-reactivity with monoclonal anti-beta-tubulin, and migrates as a single protein after two-dimensional (2D) separation. On a 2D-separation system the 53 kDa protein (IEP 5.1) migrated in the acidic tail of purified beta-tubulin. Our data strongly indicate that the 53 kDa protein is a beta-tubulin-like protein. We suggest that the degree of 53 kDa phosphorylation may be an important factor in the regulation of microtubule function in melanophores.

Experimental metastasis correlates with cyclic AMP accumulation in B16 melanoma clones

Metastasis is a complex process whereby tumour cells from a primary neoplastic growth disseminate throughout the body and establish secondary tumour foci in distant organs. Biochemical traits associated with, or essential for, the expression of the metastatic phenotype have not yet been identified. In the course of examining stimulation of the B16 murine melanoma adenylate cyclase by melanocyte-stimulating hormone (MSH) and by the diterpene forskolin, we noted that tumour cell clones isolated from common parent cell populations differed widely in their responses to these agonists. We report here that the accumulation of cyclic AMP induced by MSH or forskolin shows a strong positive correlation with the ability of B16 melanoma clones to form pulmonary tumour colonies when injected intravenously (i.v.) into syngeneic mice ('experimental metastasis'). In parallel in vitro analyses of cyclic AMP metabolism and in vivo assays of experimental metastasis using replicate cell preparations, highly metastatic tumour cell clones consistently show greater than a 30-fold increase in cellular cyclic AMP when exposed to MSH or forskolin. By contrast, clones with limited metastatic abilities respond to the same agonists with only a two- to threefold increase in cellular cyclic AMP. These data suggest that cyclic AMP metabolism is linked with biochemical pathways that are responsible for the formation of experimental metastasis by the B16 melanoma.