The phosphatase calcineurin PP2BAß mediates part of mineralocorticoid receptor transcriptional activity.

Recently it was shown that the mineralocorticoid receptor (MR) may exert part of its transcriptional activity by mediation of calcineurin (PP2B). Here we investigated the mechanism of interaction of MR with calcineurin and provide a new MR signaling pathway with potential physiological and pathophysiological relevance. MR → calcineurin crosstalk was assessed in a heterologous expression system (human embryonic kidney cells), which provides the opportunity for detailed mechanistic investigation. SiRNA knockdown experiments show that activated MR, but not GR, reduces CREB- and enhances NFaT-mediated transcriptional activation via the catalytic calcineurin subunit PP2BAß but not via PP2BAα. Altered PP2BAß expression, elevated cytosolic Ca(2+), activation of mitogen-activated kinase [p38, extracellular signal-regulated kinase (ERK) 1/2], or protein kinase C do not seem to be involved, whereas inhibition of the chaperone heat-shock protein 90 (HSP90) abrogated the effect of MR. Coimmunoprecipitation indicates the existence of protein complexes harboring MR and PP2BAß independent of MR activation but dependent on HSP90. Activated MR alters the subcellular distribution of PP2BAß, enhancing its nuclear fraction, and reduces mRNA expression of the endogenous inhibitor CAIN (calcineurin inhibitor) but not of RCAN1 (regulator of calcineurin). Overall, transcriptional relevant MR → calcineurin crosstalk occurs via the catalytic subunit PP2BAß, enables glucocorticoid response element-independent genomic signaling of MR, and is of potential pathophysiological relevance. Mechanistically, the crosstalk results from HSP90-mediated cytosolic protein complex formation, altered subcellular distribution, and altered endogenous inhibitor expression.

Suppression of the AvrBs1-specific hypersensitive response by the YopJ effector homolog AvrBsT from Xanthomonas depends on a SNF1-related kinase.

*Pathogenicity of the Gram-negative plant pathogen Xanthomonas campestris pv. vesicatoria (Xcv) depends on a type III secretion system that translocates a cocktail of > 25 type III effector proteins into the plant cell. *In this study, we identified the effector AvrBsT as a suppressor of specific plant defense. AvrBsT belongs to the YopJ/AvrRxv protein family, members of which are predicted to act as proteases and/or acetyltransferases. *AvrBsT suppresses the hypersensitive response (HR) that is elicited by the effector protein AvrBs1 from Xcv in resistant pepper plants. HR suppression occurs inside the plant cell and depends on a conserved predicted catalytic residue of AvrBsT. Yeast two-hybrid based analyses identified plant interaction partners of AvrBs1 and AvrBsT, including a putative regulator of sugar metabolism, SNF1-related kinase 1 (SnRK1), as interactor of AvrBsT. Intriguingly, gene silencing experiments revealed that SnRK1 is required for the induction of the AvrBs1-specific HR. *We therefore speculate that SnRK1 is involved in the AvrBsT-mediated suppression of the AvrBs1-specific HR.

Mineralocorticoid receptor inhibits CREB signaling by calcineurin activation.

We investigated the interaction of MR with cAMP-response element binding protein (CREB) and provide a mechanistic explanation and insights into the cellular relevance. MR --> CREB crosstalk was assessed in vascular smooth muscle cells and heterologous expression systems. Experiments were designed in a way that only one variable changed at a time and the respective vehicles served as controls. MR, but not GR, activation (aldosterone or hydrocortisone, IC(50), approximately 0.3 nM) inhibits CREB transcriptional activity induced by stimulation of beta1/2-adrenoceptors and adenylyl cyclase or addition of membrane-permeable cAMP up to 70% within 2 h after addition. The MR DNA-binding domain is not required for this inhibition. cAMP formation is virtually unchanged, whereas MR exerts a robust inhibition of CREB(S133) phosphorylation via calcineurin/PP2B activation without changes in PP2B-Aalpha or beta expression. In parallel, the PP2B-sensitive NFaT-pathway is activated. The inhibitory crosstalk attenuates CREB-induced glucose-6-phosphate dehydrogenase expression. Overall, transcriptional relevant MR --> CREB crosstalk occurs at the level of CREB phosphorylation by enhanced calcineurin activity, enables GRE-independent genomic signaling of MR, and is of potential pathophysiological relevance.

Interaction between mineralocorticoid receptor and cAMP/CREB signaling.

Besides regulating water and electrolyte homeostasis, the mineralocorticoid receptor (MR) elicits pathophysiological effects in the renocardiovascular system. Although the MR's closest relative, the glucocorticoid receptor (GR), acts as a transcription factor at the same hormone-response-element (HRE), activated glucocorticoid receptor mediates very different effects. One explanation for this discrepancy is that the MR interacts with additional signaling pathways in the cytosol. In the literature, there are several indications for an interaction between aldosterone/MR and the cAMP/CREB signaling. Here we summarize the current knowledge of the cross-talk between the two signaling pathways, including some unpublished observations of our own that indicate that MR/CREB signaling is mediated by calcineurin and has potentially pathophysiological consequences.