Specific alterations in phosphorylation of cytosol proteins from differentiating neuroblastoma cells grown in culture

THERE have been a number of reports 1–10 which indicate that an experimentally induced elevation in the intracellular level of adenosine-3′,5′-monophosphate (cyclic AMP) in mouse neuroblastoma cells induces and increases the expression of many differentiated functions that are characteristic of the mature neurone. The irreversibility of cyclic AMP-induced “differentiation” 3 was attributed to the increased level of proteins which bind cyclic AMP thereby protecting the newly synthesised cyclic nucleotide from hydrolysis 11,12 . However, the mechanisms by which cyclic AMP initiates the observed morphological 1–3 and biochemical 3–10 alterations remain unclear since the activity of cyclic AMP-dependent protein kinase did not change in differentiated cells 13 . We have investigated the phosphorylation of specific proteins in control and ‘differentiated’ neuroblastoma cells. We report here that the cyclic AMP-dependent phosphorylation of a specific protein in the cytosol of ‘differentiated’ cells increased while the cyclic AMP-independent phosphorylation of another protein in this cell fraction decreased. These selective alterations may represent intermediate steps in a series of events which ultimately lead to the expression of differentiated functions.