The role of adenosine 3?, 5?-cyclic monophosphate in the density-dependent regulation of growth and tyrosinase activity of b-16 melanoma cells

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Abstract
Incubation of cultured B-16 melanoma cells with 1-methyl-3-isobutyl xanthine (MIX) produced a sustained rise in intracellular adenosine 3′,5′-cyclic monophosphate (cAMP) which preceded an increase in the specific activity of tyrosinase (EC 1.10.3.1). Cultures of two clones of melanoma cells, one having a mean population doubling time twice that of the other, showed density-dependent inhibition of growth. The tyrosinase activity of each line increased progressively during logarithmic growth, reaching maximal values shortly after the cultures achieved confluence. Intracellular cAMP levels fell during logarithmic growth, being minimal in confluent cultures. The stimulatory effects of MIX and confluence on tyrosinase activity were additive. Cells plated at high density had a lower tyrosinase activity than cells allowed to achieve a similar density by successive division from sparsely planted cultures although the intracellular cAMP levels of such cultures were not different. We support the observations of other investigators that agents which increase intracellular cAMP concentrations can both inhibit cell division and stimulate tyrosinase activity. There are, however, mechanisms for increasing tyrosinase activity and inhibiting cell division which are expressed as B-16 melanoma cells approach confluence and which are not mediated by an increase in intracellular cAMP concentrations.