We have compared the properties of cyclic
adenosine 3':5'-monophosphate (
cAMP)-dependent protein kinases I and II in
hormone-dependent/cAMP-sensitive (DMBA
tumor) and
hormone-independent/cAMP-resistant (DMBA 1
tumor) rat mammary
carcinomas. cAMP-resistance was not due to less total
kinase in the
hormone-independent
tumor, grossly altered distribution between soluble and particulate forms of the
kinase (80% soluble in either
tumor), alteration in the relative proportion of
isozymes I and II of the
protein kinase (the soluble and the particulate fraction from both
tumors contained about 50% of either
isozyme), or a decreased sensitivity towards cAMP (both
isozymes had affinities for cAMP and its derivatives that corresponded closely with those of
isozymes from normal tissues). Furthermore, the sensitivity of the
enzymes towards thermal denaturation was identical for samples from the two
tumor types. Subtle differences did, however, exist between the regulatory moieties [regulatory subunit of
cAMP-dependent protein kinase II (RII)] of
isozyme II from the two
tumors: autophosphorylated RII from the
hormone-independent
tumor migrated as a doublet corresponding to Mrs 54,000 and 52,000 on
sodium dodecyl sulfate-
polyacrylamide gels, against Mrs 53,000 and 52,000 for RII from the
hormone-dependent
tumor; RII from the two
tumors showed different elution profiles upon
DEAE-cellulose chromatography; a considerable proportion of the soluble RII in the
hormone-independent
tumor formed supramolecular aggregates as judged by size-exclusion chromatography. No such microheterogeneity was noted for
isozyme I. This study thus shows that the lack of cAMP-responsiveness of one
tumor is related either to a defect distal to the
cAMP-dependent protein kinases or to the appearance of the new subtype of RII in the resistant
tumor. If the latter explanation is correct, it means that the part of the RII molecule responsible for interaction with other
proteins rather than that responsible for cAMP-binding and control of
protein kinase activity modulates the growth-inhibiting response to cAMP.