Unequivocal evidence for endogenous geranylgeranoic acid biosynthesized from mevalonate in mammalian cells

Induction of Hepatoma Cell Pyroptosis by Endogenous Lipid Geranylgeranoic Acid-A Comparison with Palmitic Acid and Retinoic Acid

Research on retinoid-based cancer prevention, spurred by the effects of vitamin A deficiency on gastric cancer and subsequent clinical studies on digestive tract cancer, unveils novel avenues for chemoprevention. Acyclic retinoids like 4,5-didehydrogeranylgeranoic acid (4,5-didehydroGGA) have emerged as potent agents against hepatocellular carcinoma (HCC), distinct from natural retinoids such as all-trans retinoic acid (ATRA). Mechanistic studies reveal GGA's unique induction of pyroptosis, a rapid cell death pathway, in HCC cells. GGA triggers mitochondrial superoxide hyperproduction and ER stress responses through Toll-like receptor 4 (TLR4) signaling and modulates autophagy, ultimately activating pyroptotic cell death in HCC cells. Unlike ATRA-induced apoptosis, GGA and palmitic acid (PA) induce pyroptosis, underscoring their distinct mechanisms. While all three fatty acids evoke mitochondrial dysfunction and ER stress responses, GGA and PA inhibit autophagy, leading to incomplete autophagic responses and pyroptosis, whereas ATRA promotes autophagic flux. In vivo experiments demonstrate GGA's potential as an anti-oncometabolite, inducing cell death selectively in tumor cells and thus suppressing liver cancer development. This review provides a comprehensive overview of the molecular mechanisms underlying GGA's anti-HCC effects and underscores its promising role in cancer prevention, highlighting its importance in HCC prevention.

3β-Hydroxy-12-oleanen-27-oic Acid Exerts an Antiproliferative Effect on Human Colon Carcinoma HCT116 Cells via Targeting FDFT1

3β-hydroxy-12-oleanen-27-oic acid (ATA), a cytotoxic oleanane triterpenoid with C14-COOH isolated from the rhizome of Astilbe chinensis, has been previously proven to possess antitumor activity and may be a promising antitumor agent. However, its molecular mechanisms of antitumor action were still unclear. This study explored the underlying mechanisms of cytotoxicity and potential target of ATA against human colorectal cancer HCT116 cells via integrative analysis of transcriptomics and network pharmacology in combination with in vitro and in vivo experimental validations. ATA significantly inhibited the proliferation of HCT116 cells in a concentration- and time-dependent manner and induced the cell cycle arrest at the G0/G1 phase, apoptosis, autophagy, and ferroptosis. Transcriptomic analysis manifested that ATA regulated mRNA expression of the genes related to cell proliferation, cell cycle, and cell death in HCT116 cells. The integrated analysis of transcriptomics, network pharmacology, and molecular docking revealed that ATA exerted cytotoxic activity via interactions with FDFT1, PPARA, and PPARG. Furthermore, FDFT1 was verified to be an upstream key target mediating the antiproliferative effect of ATA against HCT116 cells. Of note, ATA remarkably suppressed the growth of HCT116 xenografts in nude mice and displayed an apparent attenuation of FDFT1 in tumor tissues accompanied by the alteration of the biomarkers of autophagy, cell cycle, apoptosis, and ferroptosis. These results demonstrate that ATA exerted in vitro and in vivo antiproliferative effects against HCT116 cells through inducing cell apoptosis, autophagy, and ferroptosis via targeting FDFT1.

Geranylgeranoic acid, a bioactive and endogenous fatty acid in mammals: a review

Geranylgeranoic acid (GGA) was first reported in 1983 as one of the mevalonic acid (MVA) metabolites, but its biological significance was not studied for a long time. Our research on the antitumor effects of retinoids led us to GGA, one of the acyclic retinoids that induce cell death in human hepatoma-derived cell lines. We were able to demonstrate the presence of endogenous GGA in various tissues of male rats, including the liver, testis, and cerebrum, by LC-MS/MS. Furthermore, the biosynthesis of GGA from MVA in mammals including humans was confirmed by isotopomer spectral analysis using ¹³C-labeled mevalonolactone and cultured hepatoma cells, and the involvement of hepatic monoamine oxidase B (MAOB) in the biosynthesis of GGA was also demonstrated. The biological activity of GGA was analyzed from the retinoid (differentiation induction) and non-retinoid (cell death induction) aspects, and in particular, the non-retinoid mechanism by which GGA induces cell death in hepatoma cells was found to involve pyroptosis via ER-stress responses initiated by TLR4 signaling. In addition to these effects of GGA, we also describe the in vivo effects of GGA on reproduction. In this review, based mainly on our published papers, we have shown that hepatic MAOB is involved in the biosynthesis of GGA and that GGA induces cell death in human hepatoma-derived cell lines by non-canonical pyroptosis, one of the mechanisms of sterile inflammatory cell death.

Hepatic CYP3A4 Enzyme Compensatively Maintains Endogenous Geranylgeranoic Acid Levels in MAOB-Knockout Human Hepatoma Cells

Geranylgeranoic acid (GGA), developed as a preventive agent against second primary hepatoma, has been reported to be biosynthesized via the mevalonate pathway in human hepatoma-derived cells. Recently, we found that monoamine oxidase B (MAOB) catalyzed the oxidation of geranylgeraniol (GGOH) to produce geranylgeranial (GGal), a direct precursor of endogenous GGA in hepatoma cells, using tranylcypromine, an inhibitor of MAOs, and knockdown by MAOB siRNA. However, endogenous GGA level was unexpectedly unchanged in MAOB-knockout (KO) cells established using the CRISPR-Cas9 system, suggesting that some other latent metabolic pathways maintain endogenous GGA levels in the MAOB-KO cells. Here, we investigated the putative latent enzymes that oxidize GGOH in Hep3B/MAOB-KO cells. First, the broad-specific cytochrome P450 enzyme inhibitors decreased the amount of endogenous GGA in Hep3B/MAOB-KO cells in a dose-dependent manner. Second, among the eight members of cytochrome P450 superfamily that have been suggested to be involved in the oxidation of isoprenols and/or retinol in previous studies, only the CYP3A4 gene significantly upregulated its cellular mRNA level in Hep3B/MAOB-KO cells. Third, a commercially available recombinant human CYP3A4 enzyme was able to oxidize GGOH to GGal, and fourth, the knockdown of CYP3A4 by siRNA significantly reduced the amount of endogenous GGA in Hep3B/MAOB-KO cells. These results indicate that CYP3A4 can act as an alternative oxidase for GGOH when hepatic MAOB is deleted in the human hepatoma-derived cell line Hep3B, and that endogenous GGA levels are maintained by a multitude of enzymes.

Synergistic Effect of Squalene and Hydroxytyrosol on Highly Invasive MDA-MB-231 Breast Cancer Cells

Several studies relate Mediterranean diet and virgin olive oil (VOO) intake with lower risk of several chronic diseases, including breast cancer. Many of them described antitumor properties of isolated minor compounds present in VOO, but beneficial properties of VOO arise from the effects of all its compounds acting together. The aim of the present study was to test the antitumor effects of two minor compounds from VOO (hydroxytyrosol (HT) and squalene (SQ)) on highly metastatic human breast tumor cells (MDA-MB-231) when acting in combination. Both isolated compounds were previously analyzed without showing any antitumoral effect on highly invasive MDA-MB-231 breast cancer cells, but the present results show that HT at 100 µM, combined with different concentrations of SQ, could exert antitumor effects. When they are combined, HT and SQ are able to inhibit cell proliferation, promoting apoptosis and DNA damage in metastatic breast cancer cells. Therefore, our results suggest that the health-promoting properties of VOO may be due, at least in part, to the combined action of these two minor compounds.

Mevalonate Kinase Deficiency and Squalene Synthase Inhibitor (TAK-475): The Balance to Extinguish the Inflammation

Mevalonate Kinase Deficiency (MKD) is a rare inborn disease belonging to the family of periodic fever syndromes. The MKD phenotype is characterized by systemic inflammation involving multiple organs, including the nervous system. Current anti-inflammatory approaches to MKD are only partially effective and do not act specifically on neural inflammation. According to the new emerging pharmacology trends, the repositioning of drugs from the indication for which they were originally intended to another one can make mechanistic-based medications easily available to treat rare diseases. According to this perspective, the squalene synthase inhibitor Lapaquistat (TAK-475), originally developed as a cholesterol-lowering drug, might find a new indication in MKD, by modulating the mevalonate cholesterol pathway, increasing the availability of anti-inflammatory isoprenoid intermediates. Using an in vitro model for MKD, we mimicked the blockade of the cholesterol pathway and evaluated the potential anti-inflammatory effect of Lapaquistat. The results obtained showed anti-inflammatory effects of Lapaquistat in association with a low blockade of the metabolic pathway, while this effect did not remain with a tighter blockade. On these bases, Lapaquistat could be configured as an effective treatment for MKD’s mild forms, in which the residual enzymatic activity is only reduced and not almost completely absent as in the severe forms.

Age-Dependent Decrease in Hepatic Geranylgeranoic Acid Content in C3H/HeN Mice and Its Oral Supplementation Prevents Spontaneous Hepatoma

Geranylgeranoic acid (GGA) has been developed as a preventive agent against second primary hepatoma. Recently, GGA was reported to induce cell death in human hepatoma cells via TLR4-mediated pyroptosis. We have reported that GGA is enzymatically biosynthesized from mevalonic acid in human hepatoma-derived cells and that endogenous GGA is found in most organs of rats. In addition, we found that upregulation of endogenous GGA levels by zaragozic acid A (ZAA) induced cell death in human hepatoma-derived cells. Therefore, we investigated the age-related changes in hepatic GGA and the possibility of suppressing hepatocarcinogenesis by GGA supplementation using male C3H/HeN mice that spontaneously develop hepatoma. We measured endogenous GGA and mRNA of monoamine oxidase (BMAOB), a key enzyme of GGA biosynthesis, in the liver of male C3H/HeN mice aged 6–93 weeks. We also tried suppressing spontaneous hepatocarcinogenesis by a single administration of GGA to C3H/HeN mice. Hepatic GGA content and Maob mRNA expression level age-dependently decreased in male C3H/HeN mice; some of which produced spontaneous hepatoma in 2 years. A single oral administration of GGA at 11 months of age significantly prevented hepatoma in terms of the number and weight of tumors per mouse at 24 months. Oral supplementation with GGA or geranylgeraniol significantly increased endogenous hepatic GGA contents dose-dependently; and ZAA dramatically upregulated hepatic GGA. In this study; we found an age-dependent decrease in hepatic endogenous GGA in male C3H/HeN mice and efficient prevention of spontaneous hepatoma by a single administration of GGA at 11 months of age.

TLR4-mediated pyroptosis in human hepatoma-derived HuH-7 cells induced by a branched-chain polyunsaturated fatty acid, geranylgeranoic acid

A branched-chain polyunsaturated fatty acid, geranylgeranoic acid (GGA; C_20:4), which is an endogenous metabolite derived from the mevalonate pathway in mammals, has been reported to induce cell death in human hepatoma cells. We have previously shown that the lipid-induced unfolded protein response (UPR) is an upstream cellular process for an incomplete autophagic response that might be involved in GGA-induced cell death. Here, we found that Toll-like receptor 4 (TLR4)-mediated pyroptosis in HuH-7 cells occurred by GGA treatment. The TLR4-specific inhibitor VIPER prevented both GGA-induced cell death and UPR. Knockdown of the TLR4 gene attenuated GGA-induced cell death significantly. Upon GGA-induced UPR, caspase (CASP) 4 (CASP4) was activated immediately and gasdermin D (GSDMD) was translocated concomitantly to the plasma membrane after production of the N-terminal fragment of GSDMD. Then, cellular CASP1 activation occurred following a second gradual up-regulation of the intracellular Ca²⁺ concentration, suggesting that GGA activated the inflammasome. Indeed, the mRNA levels of NOD-like receptor family pyrin domain containing 3 (NLRP3) and interleukin-1 β (IL1B) genes were up-regulated dramatically with translocation of cytoplasmic nuclear factor-κB (NF-κB) to nuclei after GGA treatment, indicating that GGA induced priming of the NLRP3 inflammasome through NF-κB activation. GGA-induced up-regulation of CASP1 activity was blocked by either oleic acid, VIPER, MCC950 (a selective inhibitor of the NLRP3 inflammasome), or CASP4-specific inhibitor peptide cotreatment. Pyroptotic cell death was also confirmed morphologically by bleb formation in time-series live cell imaging of GGA-treated cells. Taken together, the present results strongly indicate that GGA causes pyroptotic cell death in human hepatoma-derived HuH-7 via TLR4 signalling.

Targeting Lipid Metabolism in Liver Cancer

Cancer cells are highly dependent on different metabolic pathways for sustaining their survival, growth, and proliferation. Lipid metabolism not only provides the energetic needs of the cells but also provides the raw material for cellular growth and the signaling molecules for many oncogenic pathways. Mainly processed in the liver, lipids play an essential role in the physiology of this organ and in the pathological progression of many diseases such as metabolic syndrome and hepatocellular carcinoma (HCC). The progression of HCC is associated with inflammation and complex metabolic reprogramming, and its prognosis remains poor because of the lack of effective therapies despite many years of dedicated research. Defects in hepatic lipid metabolism induce abnormal gene expression and rewire many cellular pathways involved in oncogenesis and metastasis, implying that interfering with lipid metabolism within the tumor and the surrounding microenvironment may be a novel therapeutic approach for treating liver cancer patients. Therefore, this review focuses on the latest advances in drugs targeting lipid metabolism and leading to promising outcomes in preclinical studies and some ongoing clinical trials.

GGPS1 Mutations Cause Muscular Dystrophy/Hearing Loss/Ovarian Insufficiency Syndrome

OBJECTIVE

A hitherto undescribed phenotype of early onset muscular dystrophy associated with sensorineural hearing loss and primary ovarian insufficiency was initially identified in 2 siblings and in subsequent patients with a similar constellation of findings. The goal of this study was to understand the genetic and molecular etiology of this condition.

METHODS

We applied whole exome sequencing (WES) superimposed on shared haplotype regions to identify the initial biallelic variants in GGPS1 followed by GGPS1 Sanger sequencing or WES in 5 additional families with the same phenotype. Molecular modeling, biochemical analysis, laser membrane injury assay, and the generation of a Y259C knock-in mouse were done.

RESULTS

A total of 11 patients in 6 families carrying 5 different biallelic pathogenic variants in specific domains of GGPS1 were identified. GGPS1 encodes geranylgeranyl diphosphate synthase in the mevalonate/isoprenoid pathway, which catalyzes the synthesis of geranylgeranyl pyrophosphate, the lipid precursor of geranylgeranylated proteins including small guanosine triphosphatases. In addition to proximal weakness, all but one patient presented with congenital sensorineural hearing loss, and all postpubertal females had primary ovarian insufficiency. Muscle histology was dystrophic, with ultrastructural evidence of autophagic material and large mitochondria in the most severe cases. There was delayed membrane healing after laser injury in patient-derived myogenic cells, and a knock-in mouse of one of the mutations (Y259C) resulted in prenatal lethality.

INTERPRETATION

The identification of specific GGPS1 mutations defines the cause of a unique form of muscular dystrophy with hearing loss and ovarian insufficiency and points to a novel pathway for this clinical constellation. ANN NEUROL 2020;88:332-347.

Hepatic monoamine oxidase B is involved in endogenous geranylgeranoic acid synthesis in mammalian liver cells

Geranylgeranoic acid (GGA) originally was identified in some animals and has been developed as an agent for preventing second primary hepatoma. We previously have also identified GGA as an acyclic diterpenoid in some medicinal herbs. Recently, we reported that in human hepatoma-derived HuH-7 cells, GGA is metabolically labeled from ¹³C-mevalonate. Several cell-free experiments have demonstrated that GGA is synthesized through geranylgeranial by oxygen-dependent oxidation of geranylgeraniol (GGOH), but the exact biochemical events giving rise to GGA in hepatoma cells remain unclear. Monoamine oxidase B (MOAB) has been suggested to be involved in GGOH oxidation. Here, using two human hepatoma cell lines, we investigated whether MAOB contributes to GGA biosynthesis. Using either HuH-7 cell lysates or recombinant human MAOB, we found that: 1) the MAO inhibitor tranylcypromine dose-dependently downregulates endogenous GGA levels in HuH-7 cells; and 2) siRNA-mediated MAOB silencing reduces intracellular GGA levels in HuH-7 and Hep3B cells. Unexpectedly, however, CRISPR/Cas9-generated MAOB-KO human hepatoma Hep3B cells had GGA levels similar to those in MAOB-WT cells. A sensitivity of GGA levels to siRNA-mediated MAOB downregulation was recovered when the MAOB-KO cells were transfected with a MAOB-expression plasmid, suggesting that MAOB is the enzyme primarily responsible for GGOH oxidation and that some other latent metabolic pathways may maintain endogenous GGA levels in the MAOB-KO hepatoma cells. Along with the previous findings, these results provide critical insights into the biological roles of human MAOB and provide evidence that hepatic MAOB is involved in endogenous GGA biosynthesis via GGOH oxidation.