COX-2-dependent and potentially cardioprotective effects of negative inotropic substances released after ischemia

Disruption of Ephx2 in cardiomyocytes but not endothelial cells improves functional recovery after ischemia-reperfusion in isolated mouse hearts

Cytochromes P450 metabolize arachidonic acid to epoxyeicosatrienoic acids (EETs) which have numerous effects. After cardiac ischemia, EET-induced coronary vasodilation increases delivery of oxygen/nutrients to the myocardium, and EET-induced signaling protects cardiomyocytes against postischemic mitochondrial damage. Soluble epoxide hydrolase 2 (EPHX2) diminishes the benefits of EETs through hydrolysis to less active dihydroxyeicosatrienoic acids. EPHX2 inhibition or genetic disruption improves recovery of cardiac function after ischemia. Immunohistochemical staining revealed EPHX2 expression in cardiomyocytes and some endothelial cells but little expression in cardiac smooth muscle cells or fibroblasts. To determine specific roles of EPHX2 in cardiac cell types, we generated mice with cell-specific disruption of Ephx2 in endothelial cells (Ephx2fx/fx/Tek-cre) or cardiomyocytes (Ephx2fx/fx/Myh6-cre) to compare to global Ephx2-deficient mice (global Ephx2-/-) and WT (Ephx2fx/fx) mice in expression, EET hydrolase activity, and heart function studies. Most cardiac EPHX2 expression and activity is in cardiomyocytes with substantially less activity in endothelial cells. Ephx2fx/fx/Tek-cre hearts have similar EPHX2 expression, hydrolase activity, and postischemic cardiac function as control Ephx2fx/fx hearts. However, Ephx2fx/fx/Myh6-cre hearts were similar to global Ephx2-/- hearts with significantly diminished EPHX2 expression, decreased hydrolase activity, and enhanced postischemic cardiac function compared to Ephx2fx/fx hearts. During reperfusion, Ephx2fx/fx/Myh6-cre hearts displayed increased ERK activation compared to Ephx2fx/fx hearts, which could be reversed by EEZE treatment. EPHX2 did not regulate coronary vasodilation in this model. We conclude that EPHX2 is primarily expressed in cardiomyocytes where it regulates EET hydrolysis and postischemic cardiac function, whereas endothelial EPHX2 does not play a significant role in these processes.

Negative inotropic mechanisms of β-cardiotoxin in cardiomyocytes by depression of myofilament ATPase activity without activation of the classical β-adrenergic pathway

Beta-cardiotoxin (β-CTX) from the king cobra venom (Ophiophagus hannah) was previously proposed as a novel β-adrenergic blocker. However, the involvement of β-adrenergic signaling by this compound has never been elucidated. The objectives of this study were to investigate the underlying mechanisms of β-CTX as a β-blocker and its association with the β-adrenergic pathway. The effects of β-CTX on isolated cardiac myocyte functions, calcium homeostasis, the phosphorylation level of targeted proteins, and the myofibrillar ATPase activity were studied. Healthy Sprague Dawley rats were used for cardiomyocytes isolation. Like propranolol, β-CTX attenuated the cardiomyocyte inotropy and calcium transient alterations as induced by isoproterenol stimulation. In contrast, these effects were not observed in forskolin-treated cells. Interestingly, cardiomyocytes treated with β-CTX showed no changes in phosphorylation level at any PKA-targeted sites in the myofilaments as demonstrated in Western blot analysis. The skinned fibers study revealed no change in myofilament kinetics by β-CTX. However, this protein exhibited the direct inhibition of myofibrillar ATPase activity with calcium de-sensitization of the enzyme. In summary, the negative inotropic mechanism of β-CTX was discovered. β-CTX exhibits an atypical β-blocker mechanism. These properties of β-CTX may benefit in developing a novel agent aid to treat hypertrophic cardiomyopathy.

The Role of Arachidonic Acid Metabolism in Myocardial Ischemia-Reperfusion Injury

Patients with myocardial ischemic diseases or who are undergoing one of various heart treatments, such as open heart surgery, coronary artery bypass grafting, percutaneous coronary artery intervention or drug thrombolysis, face myocardial ischemia-reperfusion injury (MIRI). However, no effective treatment is currently available for MIRI. To improve the prognosis of people with cardiovascular disease, it is important to research the mechanism of MIRI. Arachidonic acid (AA) is one of the focuses of current research. The various metabolic pathways of AA are closely related to the development of cardiovascular disease, and the roles of various metabolites in ischemia-reperfusion injury have gradually been confirmed. AA is mainly metabolized in the cyclooxygenase (COX) pathway, lipoxygenase (LOX) pathway, and cytochrome P450 monooxygenase (CYP) pathway. This paper summarizes the progress of research on these three major AA metabolic pathways with respect to MIRI.

Basal autophagy is pivotal for Hodgkin and Reed-Sternberg cells' survival and growth revealing a new strategy for Hodgkin lymphoma treatment

As current classical Hodgkin lymphoma (cHL) treatment strategies have pronounced side-effects, specific inhibition of signaling pathways may offer novel strategies in cHL therapy. Basal autophagy, a regulated catabolic pathway to degrade cell's own components, is in cancer linked with both, tumor suppression or promotion. The finding that basal autophagy enhances tumor cell survival would thus lead to immediately testable strategies for novel therapies. Thus, we studied its contribution in cHL.We found constitutive activation of autophagy in cHL cell lines and primary tissue. The expression of key autophagy-relevant proteins (e.g. Beclin-1, ULK1) and LC3 processing was increased in cHL cells, even in lymphoma cases. Consistently, cHL cells exhibited elevated numbers of autophagic vacuoles and intact autophagic flux. Autophagy inhibition with chloroquine or inactivation of ATG5 induced apoptosis and reduced proliferation of cHL cells. Chloroquine-mediated inhibition of basal autophagy significantly impaired HL growth in-vivo in NOD SCID γc-/- (NSG) mice. We found that basal autophagy plays a pivotal role in sustaining mitochondrial function.We conclude that cHL cells require basal autophagy for growth, survival and sustained metabolism making them sensitive to autophagy inhibition. This suggests basal autophagy as useful target for new strategies in cHL treatment.

Preserved heart function and maintained response to cardiac stresses in a genetic model of cardiomyocyte-targeted deficiency of cyclooxygenase-2

Cyclooxygenase-1 and -2 are rate-limiting enzymes in the formation of a wide array of bioactive lipid mediators collectively known as prostanoids (prostaglandins, prostacyclins, and thromboxanes). Evidence from clinical trials shows that selective inhibition of the second isoenzyme (cyclooxygenase-2, or Cox-2) is associated with increased risk for serious cardiovascular events and findings from animal-based studies have suggested protective roles of Cox-2 for the heart. To further characterize the function of Cox-2 in the heart, mice with loxP sites flanking exons 4 and 5 of Cox-2 were rendered knockout specifically in cardiac myocytes (Cox-2 CKO mice) via cre-mediated recombination. Baseline cardiac performance of CKO mice remained unchanged and closely resembled that of control mice. Furthermore, myocardial infarct size induced after in vivo ischemia/reperfusion (I/R) injury was comparable between CKO and control mice. In addition, cardiac hypertrophy and function four weeks after transverse aortic constriction (TAC) was found to be similar between the two groups. Assessment of Cox-2 expression in purified adult cardiac cells isolated after I/R and TAC suggests that the dominant source of Cox-2 is found in the non-myocyte fraction. In conclusion, our animal-based analyses together with the cell-based observations portray a limited role of cardiomyocyte-produced Cox-2 at baseline and in the context of ischemic or hemodynamic challenge.

Prostaglandin receptors mediate effects of substances released from ischaemic rat hearts on non-ischaemic cardiomyocytes

BACKGROUND

After ischaemia and during reperfusion, rat hearts release cardiodepressive substances that are putatively cyclooxygenase-2-dependent. The present study analyses the mechanisms by which these substances mediate their effect downstream of cyclooxygenase-2.

MATERIALS AND METHODS

After 10 min of global stop-flow ischaemia, isolated rat hearts were reperfused and post-ischaemic coronary effluent was collected over a period of 30 s. Non-ischaemic effluent collected before ischaemia was used as a control. We investigated the effect of the effluents on cell shortening and Ca(++)-metabolism, by application of fluorescence microscopy of field-stimulated adult rat cardiomyocytes incubated with fura-2. Cells were pre-incubated with inhibitors of protein kinase A and C and with antagonists of protein kinase A-dependent prostaglandin receptors. We examined the expression of prostaglandin receptors in cardiomyocytes by Western blotting.

RESULTS

In contrast to non-ischaemic effluent, post-ischaemic effluent induced reduction of Ca(++) transient and cell shortening in the cardiomyocytes. In contrast to protein kinase C inhibitor Myr-PKC [19-27], the protein kinase A inhibitor Rp-cAMPS completely blocked the effect of post-ischaemic effluent. Furthermore, we determined a cyclic adenosine monophosphate increase in cardiomyocytes that were pre-incubated with post-ischaemic effluent. The antagonist of prostaglandin E-receptor EP2 AH6809 and the antagonist of receptor subtype EP4 AH23848 attenuated the effect of post-ischaemic effluent in contrast to other antagonists of prostaglandin D and I receptors, which did not influence the effect. In lysates of adherend cardiomyocytes, expression of prostaglandin D, E and I receptors was detected by Western blotting.

CONCLUSIONS

The effect of post-ischaemic effluent is mediated by the protein kinase A-dependent prostaglandin-receptor subtypes EP2 and EP4 downstream of cyclooxygenase-2.