Activation of cyclic AMP‐dependent protein kinase and stimulation of protein phosphorylation in response to adenosine in C‐1300 murine neuroblastoma

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Abstract
DEAE‐cellulose chromatography of the 20,000g supernatant fraction of homogenates of C‐1300 murine neuroblastoma (clone N2a) yields one major and two minor peaks of cyclic AMP‐dependent protein kinase activity. Assessment of the endogenous activation state of the enzyme(s) reveals that the enzyme is fully activated by the treatment of whole cells with adenosine (10 μM) in the presence of the phosphodiesterase inhibitor Ro 20 1724 (0.7 mM). This treatment produces a large elevation in the cyclic AMP content of the cells. The treatment of whole cells with adenosine alone (1–100 μM) or Ro 20 1724 alone (0.1–0.7 mM) produces minimal elevations in cyclic AMP but nevertheless causes significant activations of cyclic AMP‐dependent protein kinase. The autophosphorylation of whole homogenates of treated and untreated cells was studied using [γ‐32P] ATP, sodium dodecyl sulfate‐polyacrylamide gel electrophoresis and autoradiography. Treatments which activate cyclic AMP‐dependent protein kinase selectively stimulate the incorporation of 32P into several proteins. This stimulation is most prominent in the 15,000‐dalton protein band. The addition of cyclic AMP to phosphorylation reactions containing homogenate of untreated cells stimulates the phosphorylation of the same protein bands. These results indicate that adenosine may have regulatory functions through its effect on the cyclic AMP: cyclic AMP‐dependent protein kinase system.