PKA tightly bound to human placental mitochondria participates in steroidogenesis and is not modified by cAMP

• The PKA βI and βII isoforms are located in syncytiotrophoblast mitochondria.
• The PKA associated to the mitochondrial membrane fraction has the highest activity.
• Progesterone synthesis is sensitive to H89 but insensitive to PKA activators.
• PKA modulates steroidogenesis and protein phosphorylation independently of cAMP.

IntroductionProtein phosphorylation plays an important role in the modulation of steroidogenesis and it depends on the activation of different signaling cascades. Previous data showed that PKA activity is related to steroidogenesis in mitochondria from syncytiotrophoblast of human placenta (HPM). PKA localization and contribution in progesterone synthesis and protein phosphorylation of HPM was assessed in this work.MethodsPlacental mitochondria and submitochondrial fractions were used. Catalytic and regulatory PKA subunits were identified by Western blot. PKA activity was determined by the incorporation of 32P into proteins in the presence or absence of specific inhibitors. The effect of PKA activators and inhibitors on steroidogenesis and protein phosphorylation in HPM was tested by radioimmunoassay and autoradiography.ResultsThe PKAα catalytic subunit was distributed in all the submitochondrial fractions whereas βII regulatory subunit was the main isoform observed in both the outer and inner membranes of HPM. PKA located in the inner membrane showed the highest activity. Progesterone synthesis and mitochondrial protein phosphorylation are modified by inhibitors of PKA catalytic subunit but are neither sensitive to inhibitors of the regulatory subunit nor to activators of the holoenzyme.DiscussionThe lack of response in the presence of PKA activators and inhibitors of the regulatory subunit suggests that the activation of intramitochondrial PKA cannot be prevented or further activated.ConclusionsThe phosphorylating activity of PKA inside HPM could be an important component of the steroidogenesis transduction cascade, probably exerting its effects by direct phosphorylation of its substrates or by modulating other kinases and phosphatases.