Mechanisms of Sorafenib Resistance in HCC Culture Relate to the Impaired Membrane Expression of Organic Cation Transporter 1 (OCT1)

Introduction

Sorafenib, an FDA-approved drug for advanced hepatocellular carcinoma (HCC) treatment, encounters resistance in many patients. Deciphering the mechanisms underlying sorafenib resistance is crucial for devising alternative strategies to overcome it.

Aim

This study aimed to investigate sorafenib resistance mechanisms using a diverse panel of HCC cell lines.

Methods

HCC cell lines were subjected to continuous sorafenib treatment, and stable cell lines (Huh 7.5 and Huh 7PX) exhibiting sustained growth in its presence were isolated. The investigation of drug resistance mechanisms involved a comparative analysis of drug-targeted signal transduction pathways (EGFR/RAF/MEK/ERK/Cyclin D), sorafenib uptake, and membrane expression of the drug uptake transporter.

Results

HCC cell lines (Huh 7.5 and Huh 7PX) with a higher IC50 (10μM) displayed a more frequent development of sorafenib resistance compared to those with a lower IC50 (2-4.8μM), indicating a potential impact of IC50 variation on initial treatment response. Our findings reveal that activated overexpression of Raf1 kinases and impaired sorafenib uptake, mediated by reduced membrane expression of organic cation transporter-1 (OCT1), contribute to sorafenib resistance in HCC cultures. Stable expression of the drug transporter OCT1 through cDNA transfection or adenoviral delivery of OCT1 mRNA increased sorafenib uptake and successfully overcame sorafenib resistance. Additionally, consistent with sorafenib resistance in HCC cultures, cirrhotic liver-associated human HCC tumors often exhibited impaired membrane expression of OCT1 and OCT3.

Conclusion

Intrinsic differences among HCC cell clones, affecting sorafenib sensitivity at the expression level of Raf kinases, drug uptake, and OCT1 transporters, were identified. This study underscores the potential of HCC tumor targeted OCT1 expression to enhance sorafenib treatment response.

Human host genetics and susceptibility to ZIKV infection

Managing emerging infectious diseases is a current challenge in the fields of microbiology and epidemiology. Indeed, among other environmental and human-related factors, climate change and global warming favor the emergence of new pathogens. The recent Zika virus (ZIKV) epidemic, of which the large and rapid spread surprised the scientific community, is a reminder of the importance to study viruses currently responsible for sporadic infections. Increasing our knowledge of key factors involved in emerging infections is essential to implement specific monitoring that can be oriented according to the pathogen, targeted population, or at-risk environment. Recent technological developments, such as high-throughput sequencing, genome-wide association studies and CRISPR screenings have allowed the identification of human single nucleotide polymorphisms (SNPs) involved in infectious disease outcome. This review focuses on the human genetic host factors that have been identified and shown to be associated with the pathogenesis of ZIKV infection and candidate SNP targets.

Mayaro Virus Infects Human Brain Cells and Induces a Potent Antiviral Response in Human Astrocytes

Mayaro virus (MAYV) and chikungunya virus (CHIKV) are known for their arthrotropism, but accumulating evidence shows that CHIKV infections are occasionally associated with serious neurological complications. However, little is known about the capacity of MAYV to invade the central nervous system (CNS). We show that human neural progenitors (hNPCs), pericytes and astrocytes are susceptible to MAYV infection, resulting in the production of infectious viral particles. In primary astrocytes, MAYV, and to a lesser extent CHIKV, elicited a strong antiviral response, as demonstrated by an increased expression of several interferon-stimulated genes, including ISG15, MX1 and OAS2. Infection with either virus led to an enhanced expression of inflammatory chemokines, such as CCL5, CXCL10 and CXCL11, whereas MAYV induced higher levels of IL-6, IL-12 and IL-15 in these cells. Moreover, MAYV was more susceptible than CHIKV to the antiviral effects of both type I and type II interferons. Taken together, this study shows that although MAYV and CHIKV are phylogenetically related, they induce different types of antiviral responses in astrocytes. This work is the first to evaluate the potential neurotropism of MAYV and shows that brain cells and particularly astrocytes and hNPCs are permissive to MAYV, which, consequently, could lead to MAYV-induced neuropathology.

Mayaro Virus Infects Human Chondrocytes and Induces the Expression of Arthritis-Related Genes Associated with Joint Degradation

Mayaro virus (MAYV) is an emerging arthritogenic alphavirus belonging to the Togaviridae family. Infection leads to a dengue-like illness accompanied by severe polyarthralgia. However, the molecular and cellular mechanisms of arthritis as a result of MAYV infection remain poorly understood. In the present study, we assess the susceptibility of human chondrocytes (HC), fibroblast-like synoviocytes and osteoblasts that are the major cell types involved in osteoarthritis, to infection with MAYV. We show that these cells are highly permissive to MAYV infection and that viral RNA copy number and viral titers increase over time in infected cells. Knowing that HC are the primary cells in articular cartilage and are essential for maintaining the cartilaginous matrix, gene expression studies were conducted in MAYV-infected primary HC using polymerase chain reaction (PCR) arrays. The infection of the latter cells resulted in an induction in the expression of several matrix metalloproteinases (MMP) including MMP1, MMP7, MMP8, MMP10, MMP13, MMP14 and MMP15 which could be involved in the destruction of articular cartilage. Infected HC were also found to express significantly increased levels of various IFN-stimulated genes and arthritogenic mediators such as TNF-α and IL-6. In conclusion, MAYV-infected primary HC overexpress arthritis-related genes, which may contribute to joint degradation and pathogenesis.

Zika virus differentially infects human neural progenitor cells according to their state of differentiation and dysregulates neurogenesis through the Notch pathway

Zika virus (ZIKV) is a mosquito-borne Flavivirus that causes Zika disease with particular neurological complications, including Guillain-Barré Syndrome and congenital microcephaly. Although ZIKV has been shown to directly infect human neural progenitor cells (hNPCs), thereby decreasing their viability and growth, it is as yet unknown which of the cellular pathways involved in the disruption of neurogenesis are affected following ZIKV infection. By comparing the effect of two ZIKV strains in vitro on hNPCs, the differentiation process of the latter cells was found to lead to a decreased susceptibility to infection and cell death induced by each of the ZIKV strains, which was associated with an earlier and stronger antiviral innate immune response in infected, differentiated hNPCs, as compared to undifferentiated cells. Moreover, ZIKV modulated, both in hNPCs and in vivo in fetal brain in an experimental mouse model, the expression of the Notch pathway which is involved in cellular proliferation, apoptosis and differentiation during neurogenesis. These results show that the differentiation state of hNPCs is a significant factor contributing to the outcome of ZIKV infection and furthermore suggest that ZIKV infection might initiate early activation of the Notch pathway resulting in an abnormal differentiation process, implicated in ZIKV-induced brain injury.

Zika virus infection modulates the metabolomic profile of microglial cells

Zika virus (ZIKV) is an emerging arbovirus of the Flaviviridae family. Although infection with ZIKV generally leads to mild disease, its recent emergence in the Americas has been associated with an increase in the development of the Guillain-Barré syndrome in adults, as well as with neurological complications, in particular congenital microcephaly, in new-borns. To date, little information is available on neuroinflammation induced by ZIKV, notably in microglial cells in the context of their metabolic activity, a series of chemical transformations that are essential for their growth, reproduction, structural maintenance and environmental responses. Therefore, in the present study we investigated the metabolomic profile of ZIKV-infected microglia. Microglial cells were exposed to ZIKV at different time points and were analyzed by a Liquid Chromatography-High Resolution mass spectrometry-based metabolomic approach. The results show that ZIKV infection in microglia leads to modulation of the expression of numerous metabolites, including lysophospholipids, particulary Lysophosphatidylcholine, and phospholipids such as Phosphatidylcholine, Phosphatidylserine, Ceramide and Sphingomyelin, and carboxylicic acids as Undecanedioic and Dodecanedioic acid. Some of these metabolites are involved in neuronal differentiation, regulation of apoptosis, virion architecture and viral replication. ZIKV infection was associated with concomitant secretion of inflammatory mediators linked with central nervous system inflammation such as IL-6, TNF-α, IL-1β, iNOS and NO. It also resulted in the upregulation of the expression of the gene encoding CX3CR1, a chemokine receptor known to regulate functional synapse plasticity and signaling between microglial cells. These findings highlight an important role for microglia and their metabolites in the process of neuroinflammation that occurs during ZIKV pathogenesis.

Zika virus causes supernumerary foci with centriolar proteins and impaired spindle positioning

Zika virus (ZIKV) causes congenital microcephaly. Although ZIKV can impair cell cycle progression and provoke apoptosis, which probably contributes to disease aetiology through depletion of neural progenitor cells, additional cellular mechanisms may be important. Here, we investigated whether ZIKV infection alters centrosome number and spindle positioning, because such defects are thought to be at the root of inherited primary autosomal recessive microcephaly (MCPH). In addition to HeLa cells, in which centrosome number and spindle positioning can be well monitored, we analysed retinal epithelial cells (RPE-1), as well as brain-derived microglial (CHME-5) and neural progenitor (ReN) cells, using immunofluorescence. We established that ZIKV infection leads to supernumerary foci containing centriolar proteins that in some cases drive multipolar spindle assembly, as well as spindle positioning defects in HeLa, RPE-1 and CHME-5 cells, but not in ReN cells. We uncovered similar phenotypes in HeLa cells upon infection with dengue virus (DENV-2), another flavivirus that does not target brain cells and does not cause microcephaly. We conclude that infection with Flaviviridae can increase centrosome numbers and impair spindle positioning, thus potentially contributing to microcephaly in the case of Zika.

African and Asian Zika virus strains differentially induce early antiviral responses in primary human astrocytes

ZIKA virus (ZIKV) is a newly emerging arbovirus. Since its discovery 60years ago in Uganda, it has spread throughout the Pacific, Latin America and the Caribbean, emphasizing the capacity of ZIKV to spread to non-endemic regions worldwide. Although infection with ZIKV often leads to mild disease, its recent emergence in the Americas has coincided with an increase in adults developing Guillain-Barré syndrome and neurological complications in new-borns, such as congenital microcephaly. Many questions remain unanswered regarding the complications caused by different primary isolates of ZIKV. Here, we report the permissiveness of primary human astrocytes for two clinically relevant, Asian and African ZIKV strains and show that both isolates strongly induce antiviral immune responses in these cells albeit with markedly different kinetics. This study describes for the first time the specific antiviral gene expression in infected primary human astrocytes, the major glial cells within the central nervous system.

The effects of mosquito saliva on dengue virus infectivity in humans

Arboviruses such as Dengue, Chikungunya, and Zika viruses represent a major public health problem due to globalization and propagation of susceptible vectors worldwide. Arthropod vector-derived salivary factors have the capacity to modulate human cells function by enhancing or suppressing viral replication and, therefore, modify the establishment of local and systemic viral infection. Here, we discuss how mosquito saliva may interfere with Dengue virus (DENV) infection in humans. Identification of saliva factors that enhance infectivity will allow the production of vector-based vaccines and therapeutics that would interfere with viral transmission by targeting arthropod saliva components. Understanding the role of salivary proteins in DENV transmission will provide tools to control not only Dengue but also other arboviral diseases transmitted by the same vectors.

Cellular Mechanism for Impaired Hepatitis C Virus Clearance by Interferon Associated with IFNL3 Gene Polymorphisms Relates to Intrahepatic Interferon-λ Expression

The single nucleotide polymorphism located within the IFNL3 (also known as IL28B) promoter is one of the host factors associated with hepatitis C virus (HCV) clearance by interferon (IFN)-α therapy; however the mechanism remains unknown. We investigated how IL28B gene polymorphism influences HCV clearance with infected primary human hepatocytes, liver biopsies, and hepatoma cell lines. Our study confirms that the rs12979860-T/T genotype has a strong correlation with ss469415590-ΔG/ΔG single nucleotide polymorphism that produces IFN-λ4 protein. We found that IFN-α and IFN-λ1 antiviral activity against HCV was impaired in IL28B T/T infected hepatocytes compared with C/C genotype. Western blot analysis showed that IL28B TT genotype hepatocytes expressed higher levels of IFN-λ proteins (IL28B, IL-29), preactivated IFN-stimulated gene (ISG) expression, and impaired Stat phosphorylation when stimulated with either IFN-α or IFN-λ1. Furthermore, we showed that silencing IFN-λ1 in T/T cell line reduced basal ISG expression and improved antiviral activity. Likewise, overexpression of IFN-λ (1 to 4) in C/C cells induced basal ISG expression and prevented IFN-α antiviral activity. We showed that IFN-λ4, produced at low level only in T/T cells induced expression of IL28B and IL-29 and prevented IFN-α antiviral activity in HCV cell culture. Our results suggest that IFN-λ4 protein expression associated with the IL28B-T/T variant preactivates the Janus kinase-Stat signaling, leading to impaired HCV clearance by both IFN-α and IFN-λ.

IFN-λ Inhibits MiR-122 Transcription through a Stat3-HNF4α Inflammatory Feedback Loop in an IFN-α Resistant HCV Cell Culture System

BACKGROUND

HCV replication in persistently infected cell culture remains resistant to IFN-α/RBV combination treatment, whereas IFN-λ1 induces viral clearance. The antiviral mechanisms by which IFN-λ1 induces sustained HCV clearance have not been determined.

AIM

To investigate the mechanisms by which IFN-λ clears HCV replication in an HCV cell culture model.

METHODS

IFN-α sensitive (S3-GFP) and resistant (R4-GFP) cells were treated with equivalent concentrations of either IFN-α or IFN-λ. The relative antiviral effects of IFN-α and IFN-λ1 were compared by measuring the HCV replication, quantification of HCV-GFP expression by flow cytometry, and viral RNA levels by real time RT-PCR. Activation of Jak-Stat signaling, interferon stimulated gene (ISG) expression, and miRNA-122 transcription in S3-GFP and R4-GFP cells were examined.

RESULTS

We have shown that IFN-λ1 induces HCV clearance in IFN-α resistant and sensitive replicon cell lines in a dose dependent manner through Jak-Stat signaling, and induces STAT 1 and STAT 2 activation, ISRE-luciferase promoter activation and ISG expression. Stat 3 activation is also involved in IFN-λ1 induced antiviral activity in HCV cell culture. IFN-λ1 induced Stat 3 phosphorylation reduces the expression of hepatocyte nuclear factor 4 alpha (HNF4α) through miR-24 in R4-GFP cells. Reduced expression of HNF4α is associated with decreased expression of miR-122 resulting in an anti-HCV effect. Northern blot analysis confirms that IFN-λ1 reduces miR-122 levels in R4-GFP cells. Our results indicate that IFN-λ1 activates the Stat 3-HNF4α feedback inflammatory loop to inhibit miR-122 transcription in HCV cell culture.

CONCLUSIONS

In addition to the classical Jak-Stat antiviral signaling pathway, IFN-λ1 inhibits HCV replication through the suppression of miRNA-122 transcription via an inflammatory Stat 3-HNF4α feedback loop. Inflammatory feedback circuits activated by IFNs during chronic inflammation expose non-responders to the risk of hepatocellular carcinoma.

Chaperone-Mediated Autophagy Targets IFNAR1 for Lysosomal Degradation in Free Fatty Acid Treated HCV Cell Culture

BACKGROUND

Hepatic steatosis is a risk factor for both liver disease progression and an impaired response to interferon alpha (IFN-α)-based combination therapy in chronic hepatitis C virus (HCV) infection. Previously, we reported that free fatty acid (FFA)-treated HCV cell culture induces hepatocellular steatosis and impairs the expression of interferon alpha receptor-1 (IFNAR1), which is why the antiviral activity of IFN-α against HCV is impaired.

AIM

To investigate the molecular mechanism by which IFNAR1 expression is impaired in HCV cell culture with or without free fatty acid-treatment.

METHOD

HCV-infected Huh 7.5 cells were cultured with or without a mixture of saturated (palmitate) and unsaturated (oleate) long-chain free fatty acids (FFA). Intracytoplasmic fat accumulation in HCV-infected culture was visualized by oil red staining. Clearance of HCV in FFA cell culture treated with type I IFN (IFN-α) and Type III IFN (IFN-λ) was determined by Renilla luciferase activity, and the expression of HCV core was determined by immunostaining. Activation of Jak-Stat signaling in the FFA-treated HCV culture by IFN-α alone and IFN-λ alone was examined by Western blot analysis and confocal microscopy. Lysosomal degradation of IFNAR1 by chaperone-mediated autophagy (CMA) in the FFA-treated HCV cell culture model was investigated.

RESULTS

FFA treatment induced dose-dependent hepatocellular steatosis and lipid droplet accumulation in HCV-infected Huh-7.5 cells. FFA treatment of infected culture increased HCV replication in a concentration-dependent manner. Intracellular lipid accumulation led to reduced Stat phosphorylation and nuclear translocation, causing an impaired IFN-α antiviral response and HCV clearance. Type III IFN (IFN-λ), which binds to a separate receptor, induces Stat phosphorylation, and nuclear translocation as well as antiviral clearance in FFA-treated HCV cell culture. We show here that the HCV-induced autophagy response is increased in FFA-treated cell culture. Pharmacological inhibitors of lysosomal degradation, such as ammonium chloride and bafilomycin, prevented IFNAR1 degradation in FFA-treated HCV cell culture. Activators of chaperone-mediated autophagy, including 6-aminonicotinamide and nutrient starvation, decreased IFNAR1 levels in Huh-7.5 cells. Co-immunoprecipitation, colocalization and siRNA knockdown experiments revealed that IFNAR1 but not IFNLR1 interacts with HSC70 and LAMP2A, which are core components of chaperone-mediated autophagy (CMA).

CONCLUSION

Our study presents evidence indicating that chaperone-mediated autophagy targets IFNAR1 degradation in the lysosome in FFA-treated HCV cell culture. These results provide a mechanism for why HCV induced autophagy response selectively degrades type I but not the type III IFNAR1.

Sequential biogenesis of host cell membrane rearrangements induced by hepatitis C virus infection

Like most positive-strand RNA viruses, hepatitis C virus (HCV) forms a membrane-associated replication complex consisting of replicating RNA, viral and host proteins anchored to altered cell membranes. We used a combination of qualitative and quantitative electron microscopy (EM), immuno-EM, and the 3D reconstruction of serial EM sections to analyze the host cell membrane alterations induced by HCV. Three different types of membrane alteration were observed: vesicles in clusters (ViCs), contiguous vesicles (CVs), and double-membrane vesicles (DMVs). The main ultrastructural change observed early in infection was the formation of a network of CVs surrounding the lipid droplets. Later stages in the infectious cycle were characterized by a large increase in the number of DMVs, which may be derived from the CVs. These DMVs are thought to constitute the membranous structures harboring the viral replication complexes in which viral replication is firmly and permanently established and to protect the virus against double-stranded RNA-triggered host antiviral responses.

Ultrastructural and biochemical analyses of hepatitis C virus-associated host cell membranes

Like most other positive-strand RNA viruses, hepatitis C virus (HCV) induces changes in the host cell's membranes, resulting in a membranous web. The non-structural proteins of the viral replication complex are thought to be associated with these newly synthesized membranes. We studied this phenomenon, using a Huh7.5 cell clone displaying high levels of replication of a subgenomic replicon of the JFH-1 strain. Electron microscopy of ultrathin sections of these cells revealed the presence of numerous double membrane vesicles (DMVs), resembling those observed for other RNA viruses such as poliovirus and coronavirus. Some sections had more discrete multivesicular units consisting of circular concentric membranes organized into clusters surrounded by a wrapping membrane. These structures were highly specific to HCV as they were not detected in naive Huh7.5 cells. Preparations enriched in these structures were separated from other endoplasmic reticulum-derived membranes by cell cytoplasm homogenization and ultracentrifugation on a sucrose gradient. They were found to contain the non-structural NS3 and NS5A HCV proteins, HCV RNA and LC3-II, a specific marker of autophagic membranes. By analogy to other viral models, HCV may induce DMVs by activating the autophagy pathway. This could represent a strategy to conceal the viral RNA and help the virus to evade double-stranded RNA-triggered host antiviral responses. More detailed characterization of these virus-cell interactions may facilitate the development of new treatments active against HCV and other RNA viruses that are dependent on newly synthesized cellular membranes for replication.

beta1-integrin cytoplasmic subdomains involved in dominant negative function

The beta1-integrin cytoplasmic domain consists of a membrane proximal subdomain common to the four known isoforms ("common" region) and a distal subdomain specific for each isoform ("variable" region). To investigate in detail the role of these subdomains in integrin-dependent cellular functions, we used beta1A and beta1B isoforms as well as four mutants lacking the entire cytoplasmic domain (beta1TR), the variable region (beta1COM), or the common region (beta1 deltaCOM-B and beta1 deltaCOM-A). By expressing these constructs in Chinese hamster ovary and beta1 integrin-deficient GD25 cells (Wennerberg et al., J Cell Biol 132, 227-238, 1996), we show that beta1B, beta1COM, beta1 deltaCOM-B, and beta1 deltaCOM-A molecules are unable to support efficient cell adhesion to matrix proteins. On exposure to Mn++ ions, however, beta1B, but none of the mutants, can mediate cell adhesion, indicating specific functional properties of this isoform. Analysis of adhesive functions of transfected cells shows that beta1B interferes in a dominant negative manner with beta1A and beta3/beta5 integrins in cell spreading, focal adhesion formation, focal adhesion kinase tyrosine phosphorylation, and fibronectin matrix assembly. None of the beta1 mutants tested shows this property, indicating that the dominant negative effect depends on the specific combination of common and B subdomains, rather than from the absence of the A subdomain in the beta1B isoform.

Tilt test results in young and elderly patients with syncope of unknown origin

The effects of aging on the results of prolonged drug-free tilt testing were studied in 175 consecutive patients with unexplained syncope divided into 3 groups: 59 patients < 40 years old; 57 patients between 40 and 60 years; and 59 patients > 60 years old. Tilt-induced vaso-vagal syncope occurred respectively in 17 (29%), 20 (35%), and 18 patients (31%) in the 3 age groups. Vasodepressor, mixed, and cardioinhibitory vaso-vagal syncope occurred similarly in the 3 groups; organic heart disease and systemic hypertension were more frequent in elderly patients without affecting the incidence of tilt-induced syncope. Blood pressure and heart rate variations during syncope were similar in the 3 age groups; in the first 20 minutes of tilt testing, before the appearance of the vaso-vagal reflex, elderly patients showed greater reduction in blood pressure and smaller increase in heart rate than younger patients. Our data indicate that increasing age determines a different blood pressure and heart rate behavior during tilt testing, but apparently does not influence the incidence of vaso-vagal syncope in patients with syncope of undetermined etiology. As the proportion of patients with a positive isoproterenol tilt test was reported to decline with age, our results suggest that the reduced incidence of syncope during isoproterenol tilt testing could be the expression of impaired autonomic response among elderly syncope patients.

Plasma active and inactive renin and fetal complications in women with high risk pregnancies

To examine whether the activation of the renin system, which occurs during pregnancy, may be relevant for the development and the outcome of the fetus, we measured active and inactive renin throughout gestation in 29 women having a pregnancy defined as "high risk" because of a clinical history of hypertension, nephropathy, and unexplained abortions. In 23 of these women who delivered full-term infants with normal weight and status, we found that active renin increased progressively from early pregnancy until the end of the second trimester and then declined slightly thereafter. In contrast, in the remaining six women who had fetal complications consisting of either signs of distress requiring cesarean section or growth retardation, the increase in active renin failed to occur. In all women the levels of inactive renin were more elevated throughout gestation than those observed in nonpregnant women, and were higher, although not significantly, in women without fetal complications than in those with fetal complications. Thus, a blunted activation of the renin system during pregnancy is associated with alteration in fetal development and may possibly contribute to it.

Time-course of the changes in blood pressure and in plasma renin activity during the first week after dilation of renal artery stenosis

We measured arterial pressure and plasma renin activity throughout the first week after a technically successful percutaneous transluminal renal angioplasty (PTRA) in 12 patients with hypertension and unilateral renal artery stenosis. Mean arterial pressure fell from 126 +/- 4 to 105 +/- 3 mmHg within 1-2 days of PTRA and stabilized thereafter; in addition, plasma renin activity decreased sharply during the first 2 days after the angioplasty (from 5.2 +/- 2.3 to 1.3 +/- 0.3 ng/ml per h) but continued to decline, reaching 0.8 +/- 0.2 ng/ml per h at the end of the study. When the antihypertensive effect of PTRA was examined in relation to baseline values of plasma renin activity, the patients with low, intermediate and high plasma renin activity showed percentage decreases in mean arterial pressure of, respectively, 6%, 16% and 19% by the sixth day of observation after the angioplasty. No overall correlation was found between the changes in arterial pressure and those in plasma renin activity induced by PTRA. These data suggest that the beneficial effect of PTRA on blood pressure can be estimated within a few days and that the reduction in the activity of the renin system is the principal but not the sole mechanism responsible for it.