cAMP is a ubiquitous second messenger that regulates numerous biological processes. Here we show that the innate immune system molecules MyD88 and TRIF regulate the cAMP signaling in trophoblasts. In JEG3 cells, inhibition of MyD88 signaling blocked the cAMP-induced CRE-promoter and CRH-promoter expression; whereas inhibition of TRIF blocked only the CRH promoter activation.
cAMP is produced in response to hormones and nutrients, and via PKA regulates numerous processes (i.e. cardiovascular function [19
], glucose homeostasis [20
], adipocyte metabolism [21
], growth-factor-dependent cell survival [22
], learning and memory [23
], immune function [24
] and exocytotic processes such as gastric acid secretion [25
]). cAMP also regulates reproductive function. Acute steroid biosynthesis is regulated by cAMP induced cholesterol release from lipid droplets and cholesterol transport across the mitochondrial membrane [26
]. The initiation and maintenance of sperm motility depends on cAMP and PKA [27
]. The gonadotropin induction of ovulation and oocyte maturation are associated with increased cAMP levels in the ovarian follicles [28
During pregnancy, cAMP has diverse functions. Through its effects on calcium and potassium channels and myosin light chain kinase, cAMP promotes myometrial relaxation [29
]. Tocolytic beta-mimetics operate through cAMP to inhibit uterine contractility in preterm labor [30
] whereas in the trophoblasts, cAMP induces the release of CRH. CRH then crosses into the fetus to induce dihydroepiandrosterone (DHEA) release. DHEA is converted into estrogen in the placenta, and estrogen then induces the expression of genes that lead to cervical softening and myometrial contractility [31
]. CRH expression has been shown to be higher in women who deliver preterm and increased CRH levels have been proposed to play role in early parturition [32
In the placenta, cAMP induces CRH expression through activation of PKA, CRE and transcription factor AP-1 [1
]. Here we first confirmed that cAMP induces CRH promoter activation through CRE and AP-1 in the JEG3 cell line and showed that innate immune system molecules, MyD88 and TRIF, play a direct role in cAMP-induced CRH promoter activation in JEG3 cells. Our data suggests that IRAK2 does not play a role in MyD88 regulation of cAMP signaling.
Adenylate cyclases (AC) synthesize cAMP from ATP and these enzymes are found in microbes as well as humans. In microbes, cAMP signaling is involved in the pathogenesis and virulence by regulating microbial metabolism, stress resistance, and maturation [reviewed in [33
]]. Our data potentially suggest that the innate immune system molecule, MyD88, may regulate microbial-cAMP signaling and may potentially induce a direct antimicrobial effect. Cirl and colleagues have recently shown that virulent bacteria evolved a mechanism to inhibit the host MyD88 specific signaling to suppress host innate immunity [34
]. We propose that microbial pathogens may potentially inhibit host cell MyD88 signaling to suppress cAMP signaling to regulate host metabolism, immunity, and cardiovascular function.
cAMP regulates gene transcription via PKA. In the basal state, PKA resides in the cytoplasm as an inactive heterotetramer of paired regulatory (R) and catalytic (C) subunits. cAMP liberates the C subunits, which passively diffuse into the nucleus and phosphorylate CREB [35
]. CREB mediates the activation of cAMP-responsive genes by binding as a dimer to a conserved cAMP-response element (CRE) [36
cAMP is known to inhibit immune activation in macrophages since 1970s [37
]. Scaffold proteins called A-kinase anchoring proteins (AKAPs) are known to mediate cAMP inhibition of immune activation via protein kinase A [38
]. AKAPs also form complexes with other signaling molecules for specificity of signaling. For example, AKAP79 binds to PKA, protein kinase C (PKC), and protein phosphatase 2B. AKAP79 basic regions also bind to membrane vesicles containing acidic phospholipids including phosphatidylinositol-4, 5-bisphosphate [PtdIns(4,5)P2] [40
]. MyD88 is recruited to TLR4 by TIRAP, which interacts with phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2
) rich regions of the plasma membrane through its amino-terminal phosphatidylinositol 4,5-bisphosphate (PIP2)-binding domain [41
]. MyD88 may potentially interact with an AKAP to regulate PKA function and cAMP induced signaling (Figure ). Indeed MyD88 has been shown to contain a PKA binding site (personal communication with Dr. Douglas Golenbock, University of Massachusetts Medical School).
Figure 5 Proposed Mechanism of MyD88 Regulation of cAMP signalling. MyD88 interacts with TIRAP and PIP2 in the plasma membrane and associates with AKAP. AKAPs are known to regulate cAMP signaling through PKA. MyD88 may regulate cAMP signaling in a complex involving (more ...)