Stress-induced regulation of the renal peripheral benzodiazepine receptor: possible role of the renin-angiotensin system

Angiotensin-converting enzyme inhibitor treatment early after myocardial infarction attenuates acute cardiac and neuroinflammation without effect on chronic neuroinflammation

Purpose

Myocardial infarction (MI) triggers a local inflammatory response which orchestrates cardiac repair and contributes to concurrent neuroinflammation. Angiotensin-converting enzyme (ACE) inhibitor therapy not only attenuates cardiac remodeling by interfering with the neurohumoral system, but also influences acute leukocyte mobilization from hematopoietic reservoirs. Here, we seek to dissect the anti-inflammatory and anti-remodeling contributions of ACE inhibitors to the benefit of heart and brain outcomes after MI.

Methods

C57BL/6 mice underwent permanent coronary artery ligation (n = 41) or sham surgery (n = 9). Subgroups received ACE inhibitor enalapril (20 mg/kg, oral) either early (anti-inflammatory strategy; 10 days treatment beginning 3 days prior to surgery; n = 9) or delayed (anti-remodeling; continuous from 7 days post-MI; n = 16), or no therapy (n = 16). Cardiac and neuroinflammation were serially investigated using whole-body macrophage- and microglia-targeted translocator protein (TSPO) PET at 3 days, 7 days, and 8 weeks. In vivo PET signal was validated by autoradiography and histopathology.

Results

Myocardial infarction evoked higher TSPO signal in the infarct region at 3 days and 7 days compared with sham (p < 0.001), with concurrent elevation in brain TSPO signal (+ 18%, p = 0.005). At 8 weeks after MI, remote myocardium TSPO signal was increased, consistent with mitochondrial stress, and corresponding to recurrent neuroinflammation. Early enalapril treatment lowered the acute TSPO signal in the heart and brain by 55% (p < 0.001) and 14% (p = 0.045), respectively. The acute infarct signal predicted late functional outcome (r = 0.418, p = 0.038). Delayed enalapril treatment reduced chronic myocardial TSPO signal, consistent with alleviated mitochondrial stress. Early enalapril therapy tended to lower TSPO signal in the failing myocardium at 8 weeks after MI (p = 0.090) without an effect on chronic neuroinflammation.

Conclusions

Whole-body TSPO PET identifies myocardial macrophage infiltration and neuroinflammation after MI, and altered cardiomyocyte mitochondrial density in chronic heart failure. Improved chronic cardiac outcome by enalapril treatment derives partially from acute anti-inflammatory activity with complementary benefits in later stages. Whereas early ACE inhibitor therapy lowers acute neuroinflammation, chronic alleviation is not achieved by early or delayed ACE inhibitor therapy, suggesting a more complex mechanism underlying recurrent neuroinflammation in ischemic heart failure.

Electronic supplementary material

The online version of this article (10.1007/s00259-020-04736-8) contains supplementary material, which is available to authorized users.

Regulation of translocator protein 18 kDa (TSPO) expression in health and disease states

Translocator protein (TSPO) is an 18 kDa high affinity cholesterol- and drug-binding protein found primarily in the outer mitochondrial membrane. Although TSPO is found in many tissue types, it is expressed at the highest levels under normal conditions in tissues that synthesize steroids. TSPO has been associated with cholesterol import into mitochondria, a key function in steroidogenesis, and directly or indirectly with multiple other cellular functions including apoptosis, cell proliferation, differentiation, anion transport, porphyrin transport, heme synthesis, and regulation of mitochondrial function. Aberrant expression of TSPO has been linked to multiple diseases, including cancer, brain injury, neurodegeneration, and ischemia-reperfusion injury. There has been an effort during the last decade to understand the mechanisms regulating tissue- and disease-specific TSPO expression and to identify pharmacological means to control its expression. This review focuses on the current knowledge regarding the chemicals, hormones, and molecular mechanisms regulating Tspo gene expression under physiological conditions in a tissue- and disease-specific manner. The results described here provide evidence that the PKCepsilon-ERK1/2-AP-1/STAT3 signal transduction pathway is the primary regulator of Tspo gene expression in normal and pathological tissues expressing high levels of TSPO.

Immunohistochemical assessment of the peripheral benzodiazepine receptor in breast cancer and its relationship with survival

PURPOSE

The peripheral benzodiazepine receptor (PBR) expression has been shown dramatically increased in neoplastic tissues and tumor cell lines originated from ovary, liver, colon, breast, or brain relative to untransformed tissues. Its expression has been also associated with tumor progression and aggressiveness. To explore whether PBR expression level could be of prognostic value in invasive breast cancer, we studied a series of 117 patients who underwent surgery for primary breast carcinomas and were followed-up for 8 years.

EXPERIMENTAL DESIGN

Using an immunohistochemical approach, we first compared PBR expression in normal and tumoral tissues, then we studied PBR expression together with clinicopathological variables (histological type, histological grade, lymph node, estrogen and progesterone receptor status), and biological markers such as BclII, Ki-67, and HER2/Neu.

RESULTS

Our results revealed a significant increase of PBR expression in tumoral versus normal breast cells. We found a negative correlation between PBR expression and estrogen receptor status (P = 0.03) as well as a positive correlation between PBR and Ki-67 (P = 0.044). Although the disease-free survival was not affected by PBR in the whole population, high PBR expression level was significantly correlated with a shorter disease-free survival in the lymph node-negative patients, P = 0.038.

CONCLUSIONS

As the axillary lymph node-negative status is generally considered as a good prognosis factor, the high expression of PBR in this patient subgroup may be used to identify a new high risk population, for which a more specific therapy would be beneficial.

Peripheral benzodiazepine receptors reflect trait (early handling) but not state (avoidance learning)

Behavioral animal paradigms and experimental neuroendocrinological and neurochemical studies have shown that early environmental manipulations have profound effects on the late response to stress. The aim of the present study was to investigate the interactive effects of environmental manipulation (early handling) and experimentally induced behavioral differences on the peripheral benzodiazepine receptor (PBR) system, which is known to be involved in the response to stressors. Adult early-handled (EH) and nonhandled (NH; control) Wistar rats were placed in a two-way active avoidance/latent inhibition (LI) paradigm, and PBR densities in the adrenal glands, kidneys, and gonads were assessed. In line with previous studies, overall avoidance learning improved in the EH group, and LI was disrupted in the NH group (primarily in males). PBR densities were up-regulated in EH subjects, and more so in females than males. However, PBR densities did not correlate with any of the behavioral measures. These findings strengthen the hypothesis that differences in PBR densities between EH and NH rats are a reflection of trait rather than state, and they suggest that the PBR system is characterized by a highly stressor-specific response.

Effect of noise exposure on rat cardiac peripheral benzodiazepine receptors

Noise is an environmental physical agent, which is regarded as a stressful stimulus: impairment and modifications in biological functions are reported, after loud noise exposure, at several levels in human and animal organs and apparatuses, as well as in the endocrine, cardiovascular and nervous system. In the present study equilibrium binding parameters of peripheral benzodiazepine receptors (PBRs) labelled by the specific radioligand [3H]PK 11195, were evaluated in cardiac tissue of rats submitted to 6 or 12 h noise exposure and of rats treated "in vivo" with PBR ligands such as PK 11195, Ro54864, diazepam and then noise-exposed. Results revealed a statistically significant decrease in the maximum number of binding sites (Bmax) of [3H]PK 11195 in atrial membranes of 6 or 12 h noise exposed rats, compared with sham-exposed animals, without any change in the dissociation constant (Kd). The "in vivo" PBR ligand pre-treatment counteracted the noise-induced modifications of PBR density. As PBRs are mainly located on mitochondria we also investigated whether noise exposure can affect the [3H]PK 11195 binding parameters in isolated cardiac mitochondrial fractions. Results indicated a significant Bmax value decrease in right atrial mitochondrial fractions of rats 6 or 12 h noise-exposed. Furthermore, as PBR has been suggested to be a supramolecular complex that might coincide with the not-yet-established structure of the mitochondrial permeability transition (MPT)-pore, the status of the MPT-pore in isolated heart mitochondria was investigated in noise- and sham-exposed rats. The loss of absorbance associated with the calcium-induced MPT-pore opening was greater in mitochondria isolated from hearts of 6 h noise- than those of sham-exposed rats. In conclusion, these findings represent a further instance for PBR density decrease in response to a stressful stimulus, like noise; in addition they revealed that "in vivo" administration of PBR ligands significantly prevents this decrease. Finally, our data also suggest the involvement of MPT in the response of an organism to noise stress.

Peripheral benzodiazepine receptors in cerebral cortex, but not in internal organs, are increased following inescapable stress and subsequent avoidance/escape shuttle-box testing

Stress-induced alterations in peripheral benzodiazepine receptor (PBR) density have been reported in humans and in rats. However, the PBR response is highly specific, and its function remains largely unexplained. The aim of the present study was to investigate the relationship between behavior in the two-way active avoidance paradigm (2WAA) and post-test PBR densities in adrenal, testis, kidney, and cerebral cortex. Adult male Wistar rats were tested in the 2WAA either in the naive state (AA) or 24 h following shock preexposure (PE), known to interfere with avoidance/escape response acquisition, and decapitated immediately after testing. Control subjects were decapitated without experimental experience. The stressful characteristic of the experiment was validated by significantly increased post-test corticosterone levels in AA and PE subjects compared with controls, with a trend towards higher corticosterone levels in PE relative to AA rats. Similarly, PE compared with AA subjects tended to show retarded acquisition of the escape/avoidance response. PBR densities in adrenal, kidney, and testis and central benzodiazepine receptors (CBR) in the cerebral cortex remained unaffected by avoidance testing. Cerebral cortex PBR density was significantly increased in PE subjects. These findings suggest that avoidance testing, although stressful to the animals, led to changes confined to cerebral cortex PBR, indicating that the hypothalamic-pituitary-adrenal (HPA) response occurs independently of the PBR response in peripheral organs, and also suggest that the opportunity for coping alters the impact of the stressor on the subject and prevents the expression of PBR response in peripheral organs.

Increased basal activity of the HPA axis and renin-angiotensin system in congenital learned helpless rats exposed to stress early in development

Learned helpless behavior has been successfully bred in rats and designated as a genetic animal model of human depression and/or anxiety. Since congenital learned helpless animals have an impaired stress response in adulthood, we examined the effects of early stressors (at postnatal day 7, 14 or 21) on the hypothalamic-pituitary-adrenal axis and the renin-angiotensin system. The functioning of the hypothalamic-pituitary-adrenal axis was monitored through changes in corticosterone plasma levels in the adult animals after acute exposure to cold stress and maternal deprivation early in development. Renin-angiotensin system functioning was assessed by plasma renin activity. Unstressed congenital learned helpless rats had corticosterone levels that were similar to control animals (congenital non-learned helpless rats not stressed during development), but unstressed plasma renin activity levels of congenital learned helpless rats were lower than congenital non-learned helpless rats. There was a step-wise increase in corticosterone plasma levels in the congenital learned helpless rats with age of acute presentation of either cold stress or maternal deprivation stress (day 7, 49%; day 14, 84%; and day 21, 543% for cold stress). However, these baseline corticosterone levels were significantly lower in congenital learned helpless rats compared to congenital non-learned helpless controls. Similarly, in response to early exposure to cold stress and maternal deprivation, there was an increase in plasma renin activity levels of congenital learned helpless rats with age of presentation to either stressors. However, this increase in plasma renin activity levels was not evident in congenital non-learned helpless controls. Taken together, these results suggest that exposure to stress early in development has long-term effects on both the hypothalamic pituitary-adrenal axis and the renin-angiotensin system, two neuroendocrine indicators of stress responsivity.

Peripheral-type benzodiazepine receptor ligands and serum steroid hormones

The peripheral-type benzodiazepine receptors (PBR) are involved in various cellular functions, including steroidogenesis. The impact of these receptor ligands has been demonstrated mainly in steroidogenic cells. The aim of the present study was to assess in intact female rats the effect of chronic (21 days) administration of the PBR ligands PK 11195 (15 mg/kg) and Ro 5-4864 (5 mg/kg), the mixed ligand diazepam (5 mg/kg), and the central benzodiazepine receptor ligand clonazepam (1 mg/kg) on PBR binding characteristics in steroidogenic (ovary and adrenal) and non-steroidogenic (uterus and kidney) organs, as well as on serum hormonal steroids (estradiol, progesterone, and corticosterone). Selective and mixed PBR ligands up-regulated PBR density in the two steroidogenic organs, while Ro 5-4864 also induced elevation of the receptor density in the non-steroidogenic organs. In contrast to Ro 5-4864, PK 11195 treatment down-regulated renal PBR. Clonazepam elevated adrenal PBR. On the serum hormonal level, Ro 5-4864 suppressed estradiol secretion. The other ligands did not affect hormonal steroid levels. It appears that in female rats, at least at these doses and dosing schedules, there is no correlation between the impact of chronic in vivo exposure to these agents on PBR density and ovarian and adrenal hormone levels.

Environmentally induced changes in peripheral benzodiazepine receptors are stressor and tissue specific

The stress-induced changes in peripheral benzodiazepine receptors (PBR) can be observed in a number of different tissues, depending upon the nature and chronicity of the aversive experience. In addition, virtually all stress procedures that cause rapid changes in PBR simultaneously increase the physical activity or metabolic rate of the subjects. The present study analyzed the contributions of rapid alterations in activity or metabolic rate with and without aversive stimulation and their subsequent impact on PBR. Mechanically induced increases in activity by forced running stress results in a significant reduction in [3H]Ro 5-4864 binding to PBR in olfactory bulb, opposite to the PBR changes in this tissue following forced cold-water swim stress. Pharmacological induction of increased locomotor activity as well as metabolic rate by d-amphetamine causes a significant increase in cardiac PBR binding, again, opposite to the response typically observed following inescapable shock stress. Finally, administration of the anxiogenic beta-carboline, FG-7142, causes increases in both hippocampus and adrenal gland PBR binding reminiscent of acute noise stress exposure. These experiments demonstrate that increased locomotor activity or metabolic rate alone is not a necessary and sufficient condition for previous stress-induced changes in PBR. Conversely, increased metabolic rate coupled with an aversive stimulus appears to be an important factor for inducing stress-like changes in PBR. This data, coupled with previous reports, suggests that rapid alterations in these sites are stressor and tissue dependent. Finally, we propose that the PBR may be involved in many aspects of the stress response including: a) a blowarning system in adrenal gland, b) participation in stress-induced hypertension via renal PBR, and c) a modulator of stress-induced immunosuppression and subsequent recovery of function or recuperation by actions on immune cells.