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Mice lacking the RII beta regulatory subunit of protein kinase A exhibit a 50% reduction in white adipose tissue stores compared with wild-type littermates and are resistant to diet-induced obesity. RII beta(-/-) mice also have an increase in resting oxygen consumption along with a 4-fold increase in the brown adipose-specific mitochondrial uncoupling protein 1 (UCP1). In this study, we examined the basis for UCP1 induction and tested the hypothesis that the induced levels of UCP1 in RII beta null mice are essential for the lean phenotype. The induction of UCP1 occurred at the protein but not the mRNA level and correlated with an increase in mitochondria in brown adipose tissue. Mice lacking both RII beta and UCP1 (RII beta(-/-)/Ucp1(-/-)) were created, and the key parameters of metabolism and body composition were studied. We discovered that RII beta(-/-) mice exhibit nocturnal hyperactivity in addition to the increased oxygen consumption at rest. Disruption of UCP1 in RII beta(-/-) mice reduced basal oxygen consumption but did not prevent the nocturnal hyperactivity. The double knockout animals also retained the lean phenotype of the RII beta null mice, demonstrating that induction of UCP1 and increased resting oxygen consumption is not the cause of leanness in the RII beta mutant mice.

The major regulator of lipolysis in white adipocytes and brown adipocytes is cAMP and the actions of cAMP are mediated by protein kinase A (PKA). Multiple subunits of PKA, including RII beta, R1 alpha, C alpha, and C beta 1, are expressed in fat cells but the major holoenzyme assembled under normal conditions contains RII beta and C alpha. Targeted disruption of the RII beta gene in mice revealed that both white and brown adipocytes are capable of compensating by increasing the level of RI alpha. Nevertheless, the mice display a lean phenotype, have an elevated metabolic rate due to activation and induction of uncoupling protein in brown fat, and are resistant to diet-induced obesity and insulin resistance. Although the metabolic disturbances in white and brown fat tissue may explain most of the phenotypic changes, the loss of neuronal expression of RII beta may also contribute to the alterations in energy balance. Specific neuronal defects have been characterized that prevent the normal changes in gene expression seen with drugs that act through the dopaminergic pathway. The RII beta mutant mouse provides an interesting model of obesity resistance and demonstrates that chronic changes in the PKA signaling system can lead to stable alterations in energy storage and utilization.