Application of Adipose Tissue Stem Cells in Regenerative Dentistry

Abstract

INTRODUCTION Regenerative dentistry is a branch of regenerative medicine that involves the understanding of “cell and molecular biology to design dental therapies that aim to restore, repair, rejuvenate and regenerate the dental tissues.”[1] Adipose tissue stem cells (ADSCs) are a type of mesenchymal stem cells (MSCs) which are isolated from the fat tissues enzymatically.[2] They express the characteristic surface markers and have potential to differentiate into multiple lineages of MSCs. They contain 100–500 times more stem cells (SCs) than bone marrow (BM).[2] They can be isolated from the adipose tissues and injected or encapsulated in biomaterials to be implanted into the wounds.[2] They increase the healing rate and can directly differentiate into specific cell lineages.[2] They have been successfully applied for repairing critical-sized bone defects and constructing engineered bone grafts. They are an excellent alternative to BMSCs.[3] In regenerative dentistry, the ADSCs have been evaluated for their ability to regenerate alveolar bone, periodontal ligament (PDL), cementum, and dental pulp in animal models.[456] Some in vivo clinical studies investigated their efficacy in treating bone deficiencies arising due to trauma, tumor resection, or congenital disorders.[7] Although the BMSCs have been successfully applied in these situations, the ADSCs are more lucrative owing to the quantity of tissue and less invasive procedures involved in their harvesting, isolation, and culture.[8] With this background, the present review focusses on the applications of ADSCs in regenerative dentistry. MATERIALS AND METHODS SEARCH STRATEGY The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed to identify the research publications on the applications of ADSCs in regenerative dentistry. The databases searched were Medline (PubMed) and Scopus till 25 January 2021. A combination of keywords like “Adipose” AND “stem” AND “cells” AND “dental” AND “regeneration” AND “pulp” OR “periodontal” OR “alveolar” AND “bone” OR “tooth” were used. They were verified in the titles, abstracts, or keywords during the initial search. It resulted in a total of 182 articles [Figure 1]. The data were screened for duplicates which resulted in 148 articles wherein the titles and abstracts were read.Figure 1: Evidence search on the applications of ADSCs in regenerative dentistry as per the PRISMA guidelinesINCLUSION AND EXCLUSION CRITERIA The eligibility criteria included free full text of original studies in English related to the applications of ADSCs in regenerative dentistry. Any kind of recommendations, expert statements, reviews, technical reports, case reports, and non-original papers were excluded. This resulted in 51 original research articles of which 18 were excluded after reading the full text. Finally, 33 original studies were reviewed.[4567910111213141516171819202122232425262728293031323334353637] DATA EXTRACTION The type of study, aims and objectives, source of ADSCs, methods of isolation, culture and differentiation, cell surface markers, type of tissue regenerated, results, and conclusions were recorded. RISK OF BIAS ASSESSMENT Two independent review authors (S. G. and R. A.) assessed the risk of bias for animal studies according to the SYRCLE tool and for human studies according to the Cochrane guidelines.[3839] The disagreements were resolved by a discussion. RESULTS Among the studies included in the review, three animal studies[914,21] and one human study[30] had low risk of bias, whereas all other studies had unclear risk of bias [Figure 2].Figure 2: Risk of bias summary for animal studiesThe results obtained from different studies may be summarized as follows: a. Types of studies: Of the 33 included studies, 23 involved the animal models[456910111213141516171819202122232425262728] whereas three trials were done on humans[729,30] and seven studies were conducted in vitro[31323334353637] [Table 1]. b. Sources of ADSCs: The animal studies utilized ADSCs obtained from omentum, inguinal fat pad, epididymis, subcutaneous adipose tissue, abdominal fat tissue, fat deposits at the hip region, subcutaneous scapular and interscapular sites and buccal fat pads (BFPs) of Sprague Dawley rats, beagle dogs, micromini pigs, rabbit, porcine, and sheep.[4569101112131415,1819202122242526,3234] Some studies obtained ADSCs from human liposuction aspirates, infrapatellar or BFP, and commercial cell lines.[717,2327,2930,37] c. Isolation and culture of ADSCs obtained from different sources: In most of the studies, ADSCs were isolated via an enzymatic process which involved the final digestion of the harvested fat tissue with the collagenase[45,79,11121314161718,2022232425,2730313233343536] [Figure 3]. An automated system was also used in a study.[29] d. Cell surface markers expressed by ADSCs: The cell surface markers like CD73, CD90, CD105, CD29 and CD44, CD166, CD54 and CD13 were expressed by ADSCs.[67,911,1314,1617,2023,2427282930313233343536] However, they did not express hematopoietic antigens like CD34, CD45, CD146, CD31, CD271, CD117, CD106, CD133, CD144, CD309, and HLA-DR.[67,911,1416,1720,2324,27282930313233343536] The expression of stemness genes like SOX2, OCT4, NANOG, and KLF4 was also reported.[17] e. Differentiation of ADSCs: The following section discusses the differentiation of ADSCs into the tooth supporting dental tissues. Table 1: Studies to evaluate the role of adipose stem cells in regenerative dentistryFigure 3: Isolation, culture, differentiation, and applications of ADSCs in regenerative dentistryDIFFERENTIATION INTO THE BONE The ADSCs differentiated into osteoblast-like cells and regenerated bone around the titanium discs and peri-implantitis defects in animals.[1621] The ADSCs isolated from swine BFP and micromini pigs showed osteodifferentiation with synthetic supports and in periodontal defect models.[2632] Their combination with inorganic bovine bone increased the bone volume in calvarial defects in rats with type 2 diabetes.[14] Furthermore, a dose–response relationship was observed between bone regeneration and quantity of ADSCs.[9] They regenerated alveolar bone in dehiscence-type defects when combined with platelet-rich plasma (PRP) or fibrin (PRF), hydroxyapatite (HA), or β-tricalcium phosphate (β-TCP) granules.[1023] Similar effects were seen when ADSCs were combined with mediators like semaphorin-3A, β-TCP, and bone morphogenetic protein-2 (BMP-2).[1819] The ADSC extracellular vesicles (exo) significantly increased the levels of newly formed tissues.[24] The 3T3L1-exo enhanced preadipocyte osteogenic differentiation in vitro owing to reduced miR-223 expression.[37] Additionally, different types of scaffolds like poly lactic-co-glycolic acid (PLGA), Gelfoam, amniotic membrane, HA/β-TCA, and the bovine bone teeth enhanced the osteogenic potential of ADSCs.[45,79,1417,2829,36] An in vitro comparative evaluation of ADSCs, BMSCs, and dental pulp SCs showed that addition of dentine matrix components (DMCs) to the SCs promoted mineralization and dentinogenic differentiation of ADSCs in cultures.[34] Both ADSCs and periodontal ligament stem cells (PDLSCs) showed similar calcification potential in another in vitro study.[35] The ADSCs isolated from Bichat’s fat pad expressed the stemness features that enabled osteodifferentiation.[31] Moreover, ADSCs isolated from swine BFP and interscapular subcutaneous adipose tissue increased the calcified ECM.[32] DIFFERENTIATION INTO THE PDL AND CEMENTUM ADSCs combined with PLGA scaffolds resulted in increased bone, PDL, and cementum growth in animal models.[4] In combination with fibrin sealant, they reduced inflammatory resorption, increased PDL formation, and reduced ankylosis in avulsed tooth.[20] It was suggested that implantation of ADSCs increased cementum and PDL fibers regeneration due to improved periodontal microenvironment.[22] Another study reported that the ADSCs cultured in dental follicle conditioned medium (DFC-CM) supplemented with DKK-1 promoted cementogenic differentiation.[33] Further, a combination of autologous ADSCs and PRP promoted periodontal tissue regeneration.[12] DIFFERENTIATION INTO THE DENTAL PULP A study suggested that adipose CD31 side population cells yielded the same amount of regenerated pulp tissue as dental pulp derived cells in the pulpectomized root canals of dogs.[6] This tissue had similar qualitative and quantitative patterns of mRNA expression characteristic of dental pulp. TOOTH REGENERATIVE POTENTIAL OF ADSCS We found only one animal study in which in vivo and in vitro comparisons showed that both ADSCs than dental pulp SCs regenerated the tooth with nerves and vascular system similar to a normal tooth.[11] DISCUSSION The results from the included studies indicate that ADSCs have the potential to regenerate the bone, PDL, cementum, and pulp. The bone was the most common tissue regenerated in the included studies. Different scaffolds in combination with ADSCs resulted in greater osteogenic differentiation. The PLGA scaffold had interconnected pores and larger pore size that promoted cell ingrowth.[4] Likewise, Gelfoam seeded with ADSCs created a three-dimensional environment that favored bone formation.[28] The scaffold of human amniotic membrane had inherent growth factors that enhanced bone regeneration.[717] The bovine bone and biphasic HA/β-TCP ADSCs were osteoconductive.[923,29] Furthermore, ADSC-derived exo produced greater osteogenesis due to reduced miR-223 expression.[2437] Besides the scaffold, the DMCs and bioactives like BMP-2, PRF, and semaphoring-A increased the osteogenic differentiation of ADSCs.[1819,2534] The BMP-2 is osteoinductive, and PRF is rich in various growth factors, whereas semaphorin A is osteoprotective protein which promotes osteogenesis.[1819,25] The property of ADSCs to differentiate into the bone may be successfully applied in the management of fenestration around the implants and periodontal disease defects.[1925] Their ability to regenerate cementum was related to the Wnt/β-catenin signaling in DFC-CM microenvironment.[33] The addition of DMC to the ADSCs increased the expression levels of the dentinogenic markers like dentine sialophosphoprotein and dentine matrix protein-1.[34] They also showed tooth and pulp regeneration ability in animal models.[611] Although the present review reinstates the role of ADSCs in regeneration of alveolar bone, PDL, cementum, and pulp, the outcomes cannot be generalized as most of these studies involved animal models with unclear risk of bias. Therefore, further well-designed human trials are recommended to verify the roles of ADSCs in regenerative dentistry. CONCLUSION The ADSCs are promising SCs for regenerative dentistry. Their ability to regenerate the alveolar bone has been thoroughly investigated in the animal models, the human trials, and in vitro studies. However, there is a need to explore their role in regeneration of PDL and cementum. At present, the information is scarce and there is a need to investigate their effects through in vivo studies in humans. FINANCIAL SUPPORT AND SPONSORSHIP Nil. CONFLICTS OF INTEREST There are no conflicts of interest. AUTHORS CONTRIBUTIONS Authors equally contributed to the paper. ETHICAL POLICY AND INSTITUTIONAL REVIEW BOARD STATEMENT Not applicable. PATIENT DECLARATION OF CONSENT Not applicable. DATA AVAILABILITY STATEMENT Not applicable. ACKNOWLEDGEMENTS None.

Topics & Concepts