Paracrine and endocrine actions of bone——the functions of secretory proteins from osteoblasts, osteocytes, and osteoclasts

The skeleton is a dynamic organ that is constantly remodeled. Proteins secreted from bone cells, namely osteoblasts, osteocytes,and osteoclasts exert regulation on osteoblastogenesis, osteclastogenesis, and angiogenesis in a paracrine manner. Osteoblasts secrete a range of different molecules including RANKL/OPG, M-CSF, SEMA3A, WNT5A, and WNT16 that regulate osteoclastogenesis. Osteoblasts also produce VEGFA that stimulates osteoblastogenesis and angiogenesis. Osteocytes produce sclerostin（SOST） that inhibits osteoblast differentiation and promotes osteoclast differentiation. Osteoclasts secrete factors including BMP6, CTHRC1, EFNB2, S1P, WNT10B, SEMA4D, and CT-1 that act on osteoblasts and osteocytes, and thereby influencea A osteogenesis. Osteoclast precursors produce the angiogenic factor PDGF-BB to promote the formation of Type H vessels, which then stimulate osteoblastogenesis. Besides, the evidences over the past decades show that at least three hormones or ＂osteokines＂from bone cells have endocrine functions. FGF23 is produced by osteoblasts and osteocytes and can regulate phosphate metabolism. Osteocalcin（OCN） secreted by osteoblasts regulates systemic glucose and energy metabolism, reproduction, and cognition. Lipocalin-2（LCN2） is secreted by osteoblasts and can influence energy metabolism by suppressing appetite in the brain.We review the recent progresses in the paracrine and endocrine functions of the secretory proteins of osteoblasts, osteocytes, and osteoclasts, revealing connections of the skeleton with other tissues and providing added insights into the pathogenesis of degenerative diseases affecting multiple organs and the drug discovery process.

Regulation of spermatogenesis by paracrine/autocrine testicular factors

Spermatogenesis is a complex process regulated by endocrine and testicular paracrine/autocrine factors. Gonadotropins are involved in the regulation of several testicular paracrine factors, mainly of the IL-1 family and testicular hormones. Testicular cytokines and growth factors (such as IL-1, IL-6, TNF, IFN-γ, LIF and SCF) were shown to affect both the germ cell proliferation and the Leydig and Sertoli cells functions and secretion. Cytokines and growth factors are produced by immune cells and in the interstitial and seminiferous tubular compartments by various testicular cells, including Sertoli, Leydig, peritubular cells, spermatogonia, differentiated spermatogonia and even spermatozoa. Corresponding cytokine and growth factor receptors were demonstrated on some of the testicular cells. These cytokines also control the secretion of the gonadotropins and testosterone in the testis. Under pathological conditions the levels of pro-inflammatory cytokines are increased and negatively affected spermatogenesis. Thus, the expression levels and the mechanisms involved in the regulation of testicular paracrine/autocrine factors should be considered in future therapeutic strategies for male infertility.

Adipose-derived stem cells enhance myogenic differentiation in the mdx mouse model of muscular dystrophy via paracrine signaling

Adipose-derived stem cells have been shown to promote peripheral nerve regeneration through the paracrine secretion of neurotrophic factors. However, it is unclear whether these cells can promote myogenic differentiation in muscular dystrophy. Adipose-derived stem cells （6 × 106） were injected into the gastrocnemius muscle of mdx mice at various sites. Dystrophin expression was found in the muscle fibers. Phosphorylation levels of Akt, mammalian target of rapamycin （mTOR）, eIF-4E binding protein 1 and $6 kinase 1 were increased, and the Akt/mTOR pathway was activated. Simultaneously, myogenin levels were increased, whereas cleaved caspase 3 and vimentin levels were decreased. Necrosis and fibrosis were reduced in the muscle fibers. These findings suggest that adipose-derived stem cells promote the re- generation and survival of muscle cells by inhibiting apoptosis and fibrosis, thereby alleviating muscle damage in muscular dystrophy.

Intravitreal stem cell paracrine properties as a potential neuroprotective therapy for retinal photoreceptor neurodegenerative diseases

Retinal degenerations are the leading causes of irreversible visual loss worldwide. Many pathologies included under this umbrella involve progressive degeneration and ultimate loss of the photoreceptor cells, with age-related macular degeneration and inherited and ischemic retinal diseases the most relevant. These diseases greatly impact patients' daily lives, with accompanying marked social and economic consequences. However, the currently available treatments only delay the onset or slow progression of visual impairment, and there are no cures for these photoreceptor diseases. Therefore, new therapeutic strategies are being investigated, such as gene therapy, optogenetics, cell replacement, or cell-based neuroprotection. Specifically, stem cells can secrete neurotrophic, immunomodulatory, and anti-angiogenic factors that potentially protect and preserve retinal cells from neurodegeneration. Further, neuroprotection can be used in different types of retinal degenerative diseases and at different disease stages, unlike other potential therapies. This review summarizes stem cell-based paracrine neuroprotective strategies for photoreceptor degeneration, which are under study in clinical trials, and the latest preclinical studies. Effective retinal neuroprotection could be the next frontier in photoreceptor diseases, and the development of novel neuroprotective strategies will address the unmet therapeutic needs.

SIMULTANEOUS OVER-EXPRESSION OF INSULIN-LIKE GROWTH FACTOR- Ⅱ (IGF- Ⅱ ) AND IGF- Ⅱ RECEPTOR(IGF- Ⅱ R) GENES IN HUMAN PRIMARY CANCER-IMPLICATION OF AUTOCRINE AND PARACRINE MECHANISM IN AUTONOMOUS GROWTH OF HEPATIC CANCER

This is first report about the simultaneous over-expression of both Insulin-like growth factor (IGF- I ) and its receptor (IGF- I R) at mRNA level in human primary hepatic Cancer (PHC). In 10 PHC samples from China, IGF-I and IGF- I R were both over-expressed, whereas only a background signal was detected in normal liver. In 5 pairs of PHC and its non- tumorous adjacent liver tissues from South Africa, IGF- I and IGF- I R were also over-expressed in PHC. mRNA expression of IGF- I in all 5 cases and IGF- I R in 4 of 5 cases were higher in cancer than non- tumorous adjacent liver tissues. These results strongly implicate that an autocrine and/ or paracrine mechanism might be Involved in formation and progression of PHC.

A paracrine role for white thermogenic adipocytes in innervation: an evidence-based hypothesis

White adipose tissue(WAT) stores energy and also plays an important endocrine role in producing adipokines for communication with the peripheral and central nervous system. WAT consists of the major lipogenic unilocular adipocytes and the minor populations of beige and brite multilocular adipocytes. These multilocular adipocytes express thermogenic genes and have phenotypic similarity with thermogenic brown adipose tissue. According to a current paradigm, multilocular adipocytes have a thermogenic function in WAT. In this mini review, we discuss data revealing heterogeneity among multilocular cell subsets in WAT and their functions beyond thermogenesis. We propose a hypothetical neuroendocrine role for multilocular adipocytes subsets in the formation of adaptive sensory-sympathetic circuits between the central nervous system and adipose tissue, which activate lipolysis and thermogenesis in WAT in high energy demand situations.

Paracrine factors for neurodegenerative disorders:special emphasis on Parkinson's disease

The progressive loss of dopaminergic neurons in the ventral mesencephalon is the main pathological hallmark of Parkinson’s disease（PD）.Drugs currently available only alleviate the principal symptomatic motor-related disturbances and their benefit is counteracted by side effects in the long time.

Activin-Directed Differentiation of Human Embryonic Stem Cells Differentially Modulates Alveolar Epithelial Wound Repair via Paracrine Mechanism

Differentiated embryonic stem cells (ESC) can ameliorate lung inflammation and fibrosis in animal lung injury models;therefore, ESC, or their products, could be candidates for regenerative therapy for incurable lung diseases, such as idiopathic pulmonary fibrosis (IPF). In this study, we have investigated the paracrine effect of differentiated and undifferentiated human ESC on alveolar epithelial cell (AEC) wound repair. hESC line, SHEF-2 cells were differentiated with Activin treatment for 22 days in an embryoid body (EB) suspension culture. Conditioned media (CM) which contain cell secretory factors were collected at different time points of differentiation. CM were then tested onin vitro?wound repair model with human type II AEC line, A549 cells (AEC). Our study demonstrated that CM originated from undifferentiated hESC significantly inhibited AEC wound repair when compared to the control. Whereas, CM originated from Activin-directed hESC differentiated cell population demonstrated a differential reparative effect on AEC wound repair model. CM obtained from Day-11 of differentiation significantly enhanced AEC wound repair in comparison to CM collected from pre- and post-Day-11 of differentiation. Day-11 CM enhanced AEC wound repair through significant stimulation of cell migration and cell proliferation. RT-PCR and immunocytochemistry confirmed that Day-11 CM was originated form a mixed population of endodermal/mesodermal differentiated hESC. This report suggests a putative paracrine-mediated epithelial injury healing mechanism by hESC secreted products, which is valuable in the development of novel stem cell-based therapeutic strategies.

Effects of microenvironment and biological behavior on the paracrine function of stem cells

Mesenchymal stem cells(MSCs),the most well-studied cell type in the field of stem cell therapy,have multi-lineage differentiation and self-renewal potential.MsC-based thera-pies have been used to treat diverse diseases because of their ability to potently repair tissue and locally restore function.An increasing body of evidence demonstrates that paracrine func-tion is central to the effects of MsC-based therapy.Growth factors,cytokines,chemokines,extracellular matrix components,and extracellular vehicles all contribute to the beneficial ef-fects of MSCs on tissue regeneration and repair.The paracrine substances secreted by MSCs change depending on the tissue microenvironment and biological behavior.In this review,we discuss the bioactive substances secreted by MsCs depending on the microenvironment and biological behavior and their regulatory mechanisms,which explain their potential to treat human diseases,to provide new ideas for further research and clinical cell-free therapy.

Heparinized PGA host-guest hydrogel loaded with paracrine products from electrically stimulated adipose-derived mesenchymal stem cells for enhanced wound repair

The microenvironment of the wound bed is essential in the regulation of wound repair.In this regard,strategies that provide a repairing favorable microenvironment may effectively improve healing outcomes.Herein,we attempted to use electrical stimulation(ES)to boost the paracrine function of adipose-derived stem cells from rats(rASCs).By examining the concentrations of two important growth factors,VEGF and PDGF-AA,in the cell culture supernatant,we found that ES,especially 5𝜇A ES,stimulated rASCs to produce more paracrine factors(5𝜇A-PFs).Further studies showed that ES may modulate the paracrine properties of rASCs by upregulating the levels of TRPV2 and TRPV3,thereby inducing intracellular Ca^(2+) influx.To deliver the PFs to the wound to effectively improve the wound microenvironment,we prepared a heparinized PGA host-guest hydrogel(PGA-Hp hydrogel).Moreover,PGA-Hp hydrogel loaded with 5𝜇A-PFs effectively accelerated the repair process of the full-thickness wound model in rats.Our findings revealed the effects of ES on the paracrine properties of rASCs and highlighted the potential application of heparinized PGA host-guest hydrogels loaded with PFs derived from electrically stimulated rASCs in wound repair.

Use of priming strategies to advance the clinical application of mesenchymal stromal/stem cell-based therapy

Mesenchymal stromal/stem cells(MSCs)have garnered significant attention in the field of regenerative medicine due to their remarkable therapeutic potential.MSCs play a pivotal role in maintaining tissue homeostasis and possess diverse functions in tissue repair and recovery in various organs.These cells are charac-terized by easy accessibility,few ethical concerns,and adaptability to in vitro cultures,making them a valuable resource for cell therapy in several clinical conditions.Over the years,it has been shown that the true therapeutic power of MSCs lies not in cell engraftment and replacement but in their ability to produce critical paracrine factors,including cytokines,growth factors,and exosomes(EXOs),which modulate the tissue microenvironment and facilitate repair and regeneration processes.Consequently,MSC-derived products,such as condi-tioned media and EXOs,are now being extensively evaluated for their potential medical applications,offering advantages over the long-term use of whole MSCs.However,the efficacy of MSC-based treatments varies in clinical trials due to both intrinsic differences resulting from the choice of diverse cell sources and non-standardized production methods.To address these concerns and to enhance MSC therapeutic potential,researchers have explored many priming strategies,including exposure to inflammatory molecules,hypoxic conditions,and three-dimensional culture techniques.These approaches have optimized MSC secretion of functional factors,empowering them with enhanced immunomodulatory,angiogenic,and regenerative properties tailored to specific medical conditions.In fact,various priming strategies show promise in the treatment of numerous diseases,from immune-related disorders to acute injuries and cancer.Currently,in order to exploit the full therapeutic potential of MSC therapy,the most important challenge is to optimize the modulation of MSCs to obtain adapted cell therapy for specific clinical disorders.In other words,to unlock the complete potential of MSCs in regenerative medicine,it is crucial to identify the most suitable tissue source and develop in vitro manipulation protocols specific to the type of disease being treated.

Both Juxtacrine and Paracrine Signaling Indispensable in Spermatogonial Stem Cell Cultures

Objective To prove that juxtacrine and paracrine signaling are essential in the culture of spermatogonial stem cells （SSCs） with Sertoli cell feeder layer in vitro. Methods Mice aged 7 d were chosen to harvest testes. A two-step enzyme digestion method was applied in testis suspension. The SSCs and Sertoli cells were separated by adherence distinguishing methods and biologically identified by immunofluorescence and Oil Red 0 staining methods. Flow cytometry was used to analyze purity of SSCs. Three groups were constructed according to different culture conditions. SSC and Sertoli cell co-culture group, SSC conditional culture group and SSC routine culture group. The conditional medium was collected from supernate of culture Sertoli cell in vitro and double-concentrated with DMEM/F12 and fetal bovine serum in a proportion of 4.5 ： 4.5 ： 1. The routine medium was DMEM/F12 containing 10% fetal bovine serum. Adherence rates were measured by Trypan blue staining. Absorbance of SSCs of each group was measured by MTT assay and proliferation curves shown to demonstrate proliferative features of SSCs. Proliferative features and colony formation were observed by inverted microscope. With 24 h difference in adherence rates, proliferations were compared and analyzed.Results The adherence rate of co-culture group was greater than that in the others（P〈0.05）, with insignificant difference in conditional culture group and routine group （P〉0. 05）. SSCs of co-culture group showed stable proliferation immediately following inoculation..4 stable colony formed within 7-10 d and maintained for 30 d. SSCs in conditional culture group and routine group decreased rapidly following transient proliferation. Conclusion The actions of SSCs in Sertoli cell cultures in vitro depended on both juxtacrine and paracrine signaling, Sertoli cell paraerine signaling was unable to promote SSC adherence and proliferation alone.

Paracrine Fibroblast Growth Factor-Based Therapy:An Unexpected Panacea for Metabolic-DysfunctionAssociated Fatty Liver Disease(MAFLD)

Metabolic-dysfunction-associated fatty liver disease(MAFLD)is a group of highly heterogeneous multi-system diseases,which is closely related to metabolic dysfunction and is one of the most important public health problems in the world.Studies have shown that paracrine fibroblast growth factors(FGFs)play an important role in the occurrence and development of MAFLD by regulating glucose and lipid metabolism,inflammation,and fibrosis.This article reviews the latest progress in understanding of the distribution,function,and metabolic regulation of paracrine FGFs,which paves the way for future FGF-based therapies targeting MAFLD.

Different priming strategies improve distinct therapeutic capabilities of mesenchymal stromal/stem cells:Potential implications for their clinical use

Mesenchymal stromal/stem cells(MSCs)have shown significant therapeutic potential,and have therefore been extensively investigated in preclinical studies of regenerative medicine.However,while MSCs have been shown to be safe as a cellular treatment,they have usually been therapeutically ineffective in human diseases.In fact,in many clinical trials it has been shown that MSCs have moderate or poor efficacy.This inefficacy appears to be ascribable primarily to the heterogeneity of MSCs.Recently,specific priming strategies have been used to improve the therapeutic properties of MSCs.In this review,we explore the literature on the principal priming approaches used to enhance the preclinical inefficacy of MSCs.We found that different priming strategies have been used to direct the therapeutic effects of MSCs toward specific pathological processes.Particularly,while hypoxic priming can be used primarily for the treatment of acute diseases,inflammatory cytokines can be used mainly to prime MSCs in order to treat chronic immune-related disorders.The shift in approach from regeneration to inflammation implies,in MSCs,a shift in the production of functional factors that stimulate regenerative or anti-inflammatory pathways.The opportunity to fine-tune the therapeutic properties of MSCs through different priming strategies could conceivably pave the way for optimizing their therapeutic potential.

WJSC 6^(th) Anniversary Special Issues(2):Mesenchymal stem cells Enhancing the efficacy of mesenchymal stem cell therapy

Mesenchymal stem cell(MSC)therapy is entering a challenging phase after completion of many preclinical and clinical trials.Among the major hurdles encountered in MSC therapy are inconsistent stem cell potency,poor cell engraftment and survival,and age/disease-related host tissue impairment.The recognition that MSCs primarily mediate therapeutic benefits through paracrine mechanisms independent of cell differentiation provides a promising framework for enhancing stem cell potency and therapeutic benefits.Several MSC priming approaches are highlighted,which will likely allow us to harness the full potential of adult stem cells for their future routine clinical use.

Stroma-epithelium crosstalk in prostate cancer

The critical role played by stroma-epithelium crosstalk in carcinogenesis and progression of prostate cancer has been increasingly recognized. These interactions are mediated by a variety of paracrine factors secreted by cancer cells and/or stromal cells. In human prostate cancer, reactive stroma is characterized by an increase in myofibroblasts and a corresponding amplification of extracellular matrix production and angiogenesis. Permanent genetic mutations have been reported in stromal cells as well as in tumour cells. Transforming growth factor-J3, vascular endothelial growth factor, platelet-derived growth factor and fibroblast growth factor signalling pathways are involved in the process of angiogenesis, whereas hepatocyte growth factor, insulin-like growth factor-l, epidermal growth factor, CXC12 and Interleukin-6 play active roles in the progression, androgen-independent conversion and distal metastasis of prostate cancer. Some soluble factors have reciprocal interactions with androgens and the androgen receptor （AR）, and can even activate AR in the absence of the androgen ligand. In this article, we review the complex interactions between cancer cells and the surrounding microenvironment, and discuss the potential therapeutic targets in the stromal compartment of prostate cancer.

Human umbilical cord mesenchymal stem cells ameliorate liver fibrosis in vitro and in vivo: From biological characteristics to therapeutic mechanisms

Liver fibrosis is a wound-healing response to chronic injuries, characterized by the excessive accumulation of extracellular matrix or scar tissue within the liver;in addition, its formation is associated with multiple cytokines as well as several cell types and a variety of signaling pathways. When liver fibrosis is not well controlled, it can progress to liver cirrhosis, but it is reversible in principle. Thus far, no efficient therapy is available for treatment of liver fibrosis. Although liver transplantation is the preferred strategy, there are many challenges remaining in this approach, such as shortage of donor organs, immunological rejection, and surgical complications. Hence, there is a great need for an alternative therapeutic strategy. Currently, mesenchymal stem cell (MSC) therapy is considered a promising therapeutic strategy for the treatment of liver fibrosis;advantageously, the characteristics of MSCs are continuous self-renewal, proliferation, multipotent differentiation, and immunomodulatory activities. The human umbilical cord-derived (hUC)-MSCs possess not only the common attributes of MSCs but also more stable biological characteristics, relatively easy accessibility, abundant source, and no ethical issues (e.g., bone marrow being the adult source), making hUC-MSCs a good choice for treatment of liver fibrosis. In this review, we summarize the biological characteristics of hUC-MSCs and their paracrine effects, exerted by secretion of various cytokines, which ultimately promote liver repair through several signaling pathways. Additionally, we discuss the capacity of hUC-MSCs to differentiate into hepatocyte-like cells for compensating the function of existing hepatocytes, which may aid in amelioration of liver fibrosis. Finally, we discuss the current status of the research field and its future prospects.

Optimizing stem cells for cardiac repair:Current status and new frontiers in regenerative cardiology

Cell therapy has the potential to improve healing of ischemic heart, repopulate injured myocardium and restore cardiac function. The tremendous hope and potential of stem cell therapy is well understood, yet recent trials involving cell therapy for cardiovascular diseases have yielded mixed results with inconsistent data thereby readdressing controversies and unresolved questions regarding stem cell efficacy for ischemic cardiac disease treatment. These controversies are believed to arise by the lack of uniformity of the clinical trial methodologies, uncertainty regarding the underlying reparative mechanisms of stem cells, questions concerning the most appropriate cell population to use, the proper delivery method and timing in relation to the moment of infarction, as well as the poor stem cell survival and engraftment especially in a diseased microenvironment which is collectively acknowledged as a major hindrance to any form of cell therapy. Indeed, the microenvironment of the failing heart exhibits pathological hypoxic, oxidative and inflammatory stressors impairing the survival of transplanted cells. Therefore, in order to observe any significant therapeutic benefit there is a need to increase resilience of stem cells to death in the transplant microenvironment while preserving or better yet improving their reparative functionality. Although stem cell differentiation into cardiomyocytes has been observed in some instance, the prevailing reparative benefits are afforded through paracrine mechanisms that promote angiogenesis, cell survival, transdifferentiate host cells and modulate immune responses. Therefore, to maximize their reparative functionality, ex vivo manipulation of stem cells through physical, genetic and pharmacological means have shown promise to enable cells to thrive in the postischemic transplant microenvironment. In the present work, we will overview the current status of stem cell therapy for ischemic heart disease, discuss the most recurring cell populations employed, the mechanisms by which stem cells deliver a therapeutic benefit andstrategies that have been used to optimize and increase survival and functionality of stem cells including ex vivo preconditioning with drugs and a novel "pharmacooptimizer" as well as genetic modifications.

Bridging long gap peripheral nerve injury using skeletal muscle-derived multipotent stem cells

Long gap peripheral nerve injuries usually reulting in life-changing problems for patients. Skeletal muscle derived-multipotent stem cells （Sk-MSCs） can differentiate into Schwann and perineurial/endoneurial cells, vascular relating pericytes, and endothelial and smooth muscle cells in the damaged peripheral nerve niche. Application of the Sk-MSCs in the bridging conduit for repairing long nerve gap injury resulted favorable axonal regeneration, which showing superior effects than gold standard therapy--healthy nerve autograft. This means that it does not need to sacrifice of healthy nerves or loss of related functions for repairing peripheral nerve injury.

Potential advantages of acute kidney injury management by mesenchymal stem cells

Mesenchymal stem cells are currently considered as a promising tool for therapeutic application in acute kidney injury(AKI) management. AKI is characterized by acute tubular injury with rapid loss of renal function. After AKI, inflammation, oxidative stress and excessive deposition of extracellular matrix are the molecular events that ultimately cause the end-stage renal disease. Despite numerous improvement of supportive therapy, the mortality and morbidity among patients remain high. Therefore, exploring novel therapeutic options to treat AKI is mandatory. Numerous evidence in animal models has demonstrated the capability of mesenchymal stem cells(MSCs) to restore kidney function after induced kidney injury. After infusion, MSCs engraft in the injured tissue and release soluble factors and microvesicles that promote cell survival and tissue repairing. Indeed, the main mechanism of action of MSCs in tissue regeneration is the paracrine/endocrine secretion of bioactive molecules. MSCs can be isolated from several tissues, including bone marrow, adipose tissue, and blood cord; pre-treatment procedures to improve MSCs homing and their paracrine function have been also described. This review will focus on the application of cell therapy in AKI and it will summarize preclinical studies in animal models and clinical trials currently ongoing about the use of mesenchymal stem cells after AKI.