Adenosine cyclic 3′:5′-monophosphate (cAMP)-binding proteins have been studied in human neuroblastoma, a tumor thought to be responsive to the antitumor effect of cAMP. Cytosol from this tumor contains at least six high-affinity, cAMP-binding proteins with apparent molecular weights ranging from 36,000 to 55,000 by sodium dodecyl sulfate:polyacrylamide gel electrophoresis. The bands representing the cAMP-binding proteins were detected on the gel by photoaffinity labeling with 8-azidoadenosine cyclic 3′:5′-monophosphate-[32P]triethylammonium salt.

However, only two types of cAMP-dependent protein kinases, a minor type I and a major type II, were resolved by DEAE-cellulose column chromatography.

The cAMP-binding proteins were copurified with either type I or II protein kinases. A protein with a molecular weight of 47,000 seems to represent the regulatory subunit of type I, and two proteins with molecular weights of 55,000 and 51,000 appear to be heterogenous regulatory subunits of type II enzyme. These three proteins incorporated 60 to 90% of the total 8-azidoadenosine cyclic 3′:5′-monophosphate-[32P]triethylammonium salt in the majority of tumor tissues examined. Other smaller molecular weight proteins were found to be the proteolytic products of these three proteins. Proteolysis of cAMP-binding proteins increased in vitro during experimental procedures, and data demonstrate that it probably also occurs in vivo. However, it is unknown if the proteolytic activity in tumor cells may affect the normal function of the native regulatory subunits of cAMP-dependent protein kinases.

Our data demonstrate that the relative amounts of proteins with molecular weights of 55,000, 51,000, and 47,000 are variable from one tumor to another. Since these cAMP-binding proteins may differ functionally, it is essential to define their roles in the regulation of growth and differentiation of human neuroblastoma.

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Presented in part at the Annual Meeting of the American Association for Cancer Research, April 5 to 8, 1978, Washington, D. C. (15). Supported by CORE Grant CA 21765 and Program Grant CA 23099 from the National Cancer Institute, by the Ruby Levi Research Fund, and by American Lebanese Syrian Associated Charities.

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