Regulation of cyclic adenosine 3':5'-monophosphate-binding protein in N-18 mouse neuroblastoma cells.

The regulation of cyclic adenosine 3':5'-monophosphate (cAMP)-binding protein in N-18 neuroblastoma cells in tissue culture was studied by the covalent incorporation of 8-azido-cyclic adenosine 3':5'-[32P]monophosphate, together with the techniques of sodium dodecyl sulfate:polyacrylamide gel electrophoresis and autoradiography. Greater than 95% of the total cAMP binding activity of N-18 neuroblastoma cells was identified as being regulatory subunits of the type I (RI) and type II (RII) species, with RI being the predominant form of the two (RI:RII = 3:1). The specific activity of RI but not of RII increased 3-fold when cells were grown in medium containing 1% rather than 10% fetal calf serum. Under the same conditions, the specific activity of acetylcholinesterase increased 3- to 5-fold. The increase in RI was inversely related to the serum concentration in the medium and was specific for cells at the stationary phase of growth. An increase in intracellular cAMP, concomitant with the increase in RI, was also observed. Morphological examination of stationary-phase neuroblastoma cells maintained in medium containing 1% fetal calf serum suggested the presence of a high proportion of highly-differentiated cells. It is proposed that the regulatory control of RI cAMP-binding protein by serum may involve modulation of intracellular cAMP and that the expression RI may be used as a biochemical index of differentiation in mouse neuroblastoma cells.