Contributed equally
Cycloastragenol (CAG) is a triterpenoid saponin compound and a hydrolysis product of the main active ingredient in
Bunge is a plant used in traditional Chinese medicine (TCM), with known ‘Qi tonifying’ adaptogenic effects, as documented in the Chinese Materia Medica.
The Geron Corporation in cooperation with The Hong Kong University of Science and Technology screened natural compounds from
Telomeres are located at the ends of linear chromosomes capped by nucleoprotein structures that consist of tandem repeats of hexameric sequences (TTAGGG in vertebrates) bound by a dedicated set of proteins. Telomeres shorten with every mitotic event. Telomerase is a ribonucleoprotein complex that lengthen telomeres and fundamentally consists of catalytic reverse transcriptase enzymes (TERT) and a telomerase RNA component (TERC). A key function of telomeres is to protect chromosomal ends from fusion and degradation by capping chromosomal ends. Cells recognize critically short telomeres as DNA damage; therefore, the shortest telomere, rather than average telomere length, is critical for cell viability and chromosomal stability (
Several studies have shown that CAG activates telomerase both
The above study led to another question: How does CAG activate telomerase? Telomerase activity in human embryonic kidney HEK293 fibroblasts increases upon treatment with CAG and is not meditated via common secondary messenger pathways, including Ca2+, inositol trisphosphate (IP3), cyclic adenosine monophosphate (cAMP), and protein kinase B (Akt). However, CAG induced the phosphorylation of extracellular signal-regulated kinase (ERK) in many cell lines, including HEK293 and HEK-neo keratinocytes, as well as cells from lung, brain, mammary, endothelial, and hematopoietic origins. Further, proto-oncogene tyrosine-protein kinase (c-Src), ERK kinase (MEK), and epidermal growth factor receptors are involved in CAG-induced ERK phosphorylation. CAG may activate telomerase and other cellular effects by activating the Src/MEK/ERK pathway (
A prevailing view is that the immune system gradually ages during the process of senescence. Weak immune systems lead to an increased susceptibility to infections, which accelerates the process of senescence. The immune system consists of immune organs, immune cells, and active immune substances and is associated with senescence (
An
Within the past decade, numerous diseases or maladies have been associated with mutations in telomerase and/or are known to accelerate telomere loss, including AIDS, congenital dyskeratosis, aplastic anemia, and idiopathic pulmonary fibrosis (IPF). CAG, a telomerase activator, has the possibility to treat these diseases or maladies; to this end, several studies have been conducted. CAG dose-dependently hindered bleomycin-induced fibrosis in mTERT heterozygous mice (TRET+/−mice). However, CAG did not alter the inflammatory response to bleomycin-induced fibrosis, suggesting that the underlying mechanisms are not related to the anti-inflammatory properties of CAG. Prevention against fibrosis and suppression of senescence in cells are dependent on telomerase activation. Further, data suggest that the mechanisms associated with the protective effects of CAG against bleomycin-induced lung fibrosis involve specific types of lung cells rather than all cell types in the lung (
Telomere length is proportional to cell proliferation at the cellular level; that is, telomerase activation and elongation of telomeres can increase the proliferative ability of cells. The present study indicates that this hypothesis is correct. CAG can improve skin fitness, wound healing, and fur recovery in elderly mice and is a more remarkable agent for wound healing in both
In addition to its wound healing properties, CAG promotes recovery from brain injuries, as shown in both cell and animal experiments. Neural stem cells are important for recovery from brain injuries such as hypoxic-ischemic brain injury. CAG increases proliferation and improves survival in neural stem cells and counters the effects of oxygen-glucose deprivation injury
Lipids store energy; however, when lipids accumulate too much, it is harmful for health. CAG could ameliorate various biomarkers associated with lipid metabolism. CAG has been shown to reduce cytoplasmic lipid droplets in 3T3-L1 adipocytes at a low dose. At high doses, CAG prevents differentiation of 3T3-L1 preadipocytes. Further, it was observed that CAG dose-dependently stimulates calcium influx in 3T3-L1 preadipocytes. Since elevated intracellular calcium plays a vital role in suppressing adipocyte differentiation, CAG may mediate the balance in lipid metabolism by activating calcium influx in 3T3-L1 preadipocytes (
Farnesoid X receptor (FXR) is a potential drug target for the treatment of non-alcoholic fatty liver disease (NAFLD), and CAG is a direct activator of FXR. Animal studies have shown that CAG reduces high-fat diet-induced lipid accumulation in the liver, accompanied by lowered blood glucose levels, serum triglyceride levels, and hepatic bile acids. CAG also ameliorates hepatic steatosis in methionine- and choline-deficient l-amino acid diet (MCD)-induced non-alcoholic steatohepatitis (NASH) mice. These results indicate that CAG ameliorates NAFLD via enhancement of the FXR signaling pathway (
The oxidative stress hypothesis is a generally accepted mechanism of senescence. Reactive oxygen species (ROS), including superoxide (O2−), hydrogen peroxide (H2O2), and the hydroxyl radical (-OH) damage cell functions and are a primary cause of disease and cell senescence. In a d-galactose-induced senescence mouse model, CAG treatment up-regulated total antioxidant capacity (T-AOC) and superoxide dismutase (T-SOD) activity, increased hydroxyproline (HYP) stores, and down-regulated malondialdehyde (MDA) in the skin, heart, and liver. Further, CAG enhanced the antioxidant capacity in a non-dose-dependent manner (
Inflammation is a very common and important basic pathological process. Body surface trauma, infections, common organ diseases, and frequently-occurring diseases (such as pneumonia, hepatitis, and nephritis) are all related to inflammation. ROS generation was suppressed in human umbilical vein EA.hy926 cells exposed to CAG in the setting of endoplasmic reticulum (ER) stress. Further, CAG attenuates phosphorylation of inositol-requiring enzyme 1 (IRE1)-α, suppresses thioredoxin-interacting protein/NLR family pyrin domain containing 3 (TXNIP/NLRP3) inflammasome activation, inhibits mitochondrial cell death, and reduces apoptosis in endothelial cells. Further, CAG enhances 5′AMP-activated protein kinase (AMPK) phosphorylation, an effect that is diminished by AMPK inhibitors, indicating a potential role for AMPK in the anti-inflammatory properties of CAG (
We discussed the role of CAG in the normal immune system above; however, CAG also plays a role in the activated immune system. The effects of CAG in a concanavalin (Con) A pan-activated lymphocyte model were reported by Sun
The Patton protocol-1 provided 37 subjects with a comprehensive dietary supplement pack containing CAG for 12 months. This clinical trial concluded that CAG lengthens critically short telomeres and remodels the relative proportion of circulating leukocytes in cytomegalovirus-positive [CMV(+)] subjects toward a more ‘youthful’ profile, as seen in CMV(−) subjects (
A random clinical trial conducted in 117 relatively healthy CMV (+) subjects 53–87 years old verified that TA-65 can significantly lengthen telomeres (
CAG activates human telomerase and has various positive functions. It is still unknown whether these different functions are related to each other and to CAG dosages and whether CAG functions differently in different groups of people.
Critically short telomeres will lead to cell senescence and apoptosis, and numerous diseases or maladies are related to short telomeres. Telomerase activators could attenuate this process. However, telomeres are elongated via the telomerase enzyme in more than 80% of tumors. As such, activated telomerase is a hallmark of cancer (
According to current research results, CAG is relatively safe within a certain dose range and has no serious adverse reactions; CAG was determined to be generally recognized as safe (GRAS) by an independent expert panel of the Food and Drug Administration (FDA) on the 19th of November, 2014. CAG has been used as a medical food (
CAG is efficiently absorbed through the intestinal epithelium by passive diffusion, as shown by a study that investigated the intestinal absorption and metabolism of CAG using an
In the process of searching documents, we found that CAG (CAS number: 78574-94-4) is noted by diverse names in different documents. CAG is named (2aR,3R,4S,5aS,5bS,7S,7aR,9S,11aR,12aS)-3-((2R,5S)-5- (2-hydroxypropan-2-yl)-2-methyltetrahydrofuran-2-yl)-2a,5a,8,8-tetramethylhexadecahydrocyclopenta[a]cyclopropa[e]phenanthrene-4,7,9-triol, other synonyms include CAG, CA, astramembrangenin, cyclogalegigenin, GRN510, and TA-65. TA-65 was named by the T.A Science Corporation as a nutrition and health care product. A document reported that GRN510 is a novel and proprietary chemical entity derived from GRN665/TAT2. However, GRN510 is considered equivalent to CAG in PubChem (an open chemistry database); therefore, we considered GRN510 as equivalent to CAG in this article. For simplicity, we termed cycloastragenol as CAG in this report.
CAG has multiple pharmacological effects, including activation of telomerase, improved lipid metabolism, anti-oxidation, and anti-inflammatory properties. Clinical trials have proven components involved in CAG functions. No serious adverse effects are associated with CAG within a certain dose range. However, the carcinogenic potential of CAG is the main factor preventing the use of CAG clinically. Clarification of the quantitative effects and efficacy associated with CAG in different individuals is needed.
While there is a large body of literature regarding CAG, there are still significant gaps in knowledge.
CAG stimulates calcium influx or inhibits calcium influx depending on the cell type and pathological state. Further, CAG has different pharmacological effects at diverse concentrations in 3T3-L1 preadipocytes. The underlying mechanisms regarding these effects have not yet been reported.
CAG has extensive pharmacological effects; however, the detailed underlying mechanisms for most of these effects remain unclear. A summary of CAG-associated effects and the corresponding mechanisms are show in
CAG could increase BMD and improve glucose metabolism in naturally aged rats. There is no literature describing whether CAG has anti-osteoporosis or anti-diabetic effects in corresponding animal models.
In this review, the current state of CAG research is detailed and elucidated, and the efficacy, pharmacokinetics, and adverse reactions of CAG are summarized. According to the present research results, CAG has extensive pharmacological effects, including telomerase activation, telomere elongation, anti-inflammation, anti-oxidation, anti-viral, anti-pulmonary fibrosis, anti-ischemic and hypoxic injury, and anti-lipid accumulation properties. Studies also suggest that CAG improves liver metabolic homeostasis, promotes wound healing, promotes feather growth, and improves certain health-span indicators both in humans and animals. However, more attention and further studies are needed to evaluate whether CAG has potential adverse reactions, and studies examining the detailed underlying mechanisms associated with CAG are required. In addition, further multicenter clinical studies are required for specific diseases such as steatohepatitis, AIDS, pulmonary fibroses, wound healing etc.
We anticipate that researchers will conduct more studies on CAG to evaluate its efficacy and adverse reactions, as well as understand how to use CAG rationally to enhance its effects and reduce adverse reactions so that CAG can be used for the clinical treatment of corresponding diseases. We believe that development of these clinical uses will benefit people, which is the purpose of this article.
Not applicable.
The present study was supported by grants from National Natural Science Foundation of China (grant nos. 81102450 and 81673814), the Open Fund Project of Key Laboratory of Guangdong Province (grant no. 4CX16010G), the Science and Technology Plan of Guangdong Province (grant nos. 2016ZC0178 and 2016A020215148) and Open Foundation of Guangdong Key Laboratory for Research and Development of Natural Drugs (grant no. TRYW201603).
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
LZ performed the literature review and mapping. YL and YYg made suggestions and edited the manuscript. YYu wrote the manuscript.
Not applicable.
Not applicable.
The authors declare that they have no competing interests.
cycloastragenol
traditional Chinese medicine
catalytic reverse transcriptase enzymes
telomerase RNA component
inositol trisphosphate
cyclic adenosine monophosphate
protein kinase B
proto-oncogene tyrosine-protein kinase
extracellular signal-regulated kinase
ERK kinase
Janus kinase 2
signal transducer and activator of transcription 5b
human immunodeficiency virus
idiopathic pulmonary fibrosis
acquired immunodeficiency syndrome
astragaloside IV
cyclocephaloside I
cyclocanthoside E
Farnesoid X receptor
non-alcoholic fatty liver disease
methionine- and choline-deficient L-amino acid diet
non-alcoholic steatohepatitis
reactive oxygen species
total antioxidant capacity
total superoxide dismutase
hydroxyproline
malondialdehyde
endoplasmic reticulum
inositol-requiring enzyme 1
thioredoxin-interacting protein
NLR family pyrin domain containing 3
5′AMP-activated protein kinase
bone mineral density
no-observed-adverse-effect level
UDP-glucuronosyltransferase
generally recognized as safe
A schematic for cycloastragenol synthesis.
Pharmacological effects of CAG. CAG, cycloastragenol.
Hypothetical mechanisms associated with CAG. Arrow heads represent stimulatory modifications. Solid lines are direct stimulatory modifications based on the literature. Dashed lines indicate stimulatory modifications; grey dashed lines indicate possible stimulatory modifications. CAG may activate telomerase through pathway 1, 2, 3 and 4, and then exert various effects. Through pathway 5, CAG directly stimulates the FXR to improve hepatitis. Through pathway 6, CAG indirectly stimulates AMPK to improve inflammation. CAG, cycloastragenol; FXR, farnesoid X receptor; AMPK, 5′adenosine monophosphate-activated protein kinase; EGFR, epidermal growth factor receptor; ERK, extracellular signal-regulated kinase; MEK, mitogen-activated protein kinase kinase; JAK, Janus kinase; STAT, signal transducer and activator of transcription; TERT, telomerase reverse transcriptase.
CAG pharmacological effects and mechanisms.
Author, year | Object of study | Effect of CAG | Mechanism | Tool or method of studying mechanism | (Refs.) |
---|---|---|---|---|---|
Fauce |
CD8(+) T lymphocytes from HIV-infected human donors | Enhanced antiviral functions | Increase telomerase activity | Telomerase template antagonist-GRN163L | ( |
Fauce |
CD8(+) T lymphocytes from HIV-infected human donors | Increase telomerase activity | ERK/MAPK pathway | MAKP inhibitor, ERK inhibitor | ( |
Molgora |
Healthy human CD4 and CD8 T cells | Increase telomerase activity | MAPK pathway | MAKP inhibitor(PD98059) | ( |
Zhao |
Endothelial cell | Ameliorated endothelial inflammation and reduced cell apoptosis | AMPK pathway | AMPK inhibitor | ( |
Sun |
Activated lymphocytes | Anti-inflammation | Inhibited Ca2+ overload | Flow cytometry | ( |
Bernardes de Jesus |
MEF Terc+/− | Lengthened telomeres, reduced critically short telomeres and DNA damage | Increase telomerase activity | Gene knockout | ( |
Yung |
Human embryonic kidney HEK293 fibroblasts | Increased telomerase activity | Src/MEK/ERK pathway | Selective inhibitors and dominant negative mutants | ( |
Le Saux |
TRET+/− mice | Inhibited fibrosis and prevented senescent cell accumulation | Increased telomerase activity | Telomerase inhibitor-GRN163L | ( |
Wang |
3T3-L1 preadipocytes | Reduced cytoplasmic lipid droplets | Stimulated calcium influx | Calcium mobilization assay | ( |
Gu |
High-fat diet mice | Improved hepatic steatosis | Activated farnesoid X receptor signaling | PCR, WB, Molecular docking, gene knockout | ( |
Bernardes de Jesus |
2-year old mice | Improved hepatic lipid accumulation | Increased telomerase activity via c-Myc and c-Jun | PCR, IHC | ( |
Ip |
Human neonatal keratinocytes | Improved wound healing | Increased telomerase activity | RQ-TRAP assay | ( |
Meng |
Hypoxic-ischemic brain injury | Countered hypoxic-ischemic brain injury | Increased telomerase activity | Telomerase inhibitor | ( |
Reichert |
Zebra finches | Improved flight feather renewal capacity | Increased telomere length | qPCR | ( |
Cao |
D-galactose-induced senescent mouse model | Enhanced antioxidant capacity | CAG anti-oxidant | Corresponding detection method | ( |
Salvador |
Relatively healthy cytomegalovirus-positive subjects | Lengthened telomeres | – | Oral CAG | ( |
Dow and Harley, 2016 | Patients | Improved macular function | – | Oral CAG | ( |
Ip |
PC12 cells and primary neurons and Bcl2 expression | Induced telomerase activity and CREB activation followed by tert | CAG function related to CREB activation | Blockade of CREB expression via RNA | ( |
HIV, human immunodeficiency virus; ERK, extracellular-signal-related kinase; MAPK, mitogen-activated protein kinase; AMPK, 5′adenosine monophosphate-activated protein kinase; PCR, polymerase chain reaction; WB, western blot; IHC, immunohistochemistry; TRAP, telomeric repeat amplification protocol; CREB, cAMP response element-binding protein; CAG, cycloastragenol.