Resumen de Declined Expression of Histone Deacetylase 6 Contributes to Periodontal Ligament Stem Cell Aging
Qian Li, Yushi Ma, Yunyan Zhu, Ting Zhang, Yanheng Zhou
Background: Identification of regulators for aging-associated stem cell (SC) dysfunctions is a critical topic in SC biology and SC-based therapies. Periodontal ligament stem cell (PDLSC), a kind of dental mesenchymal SC with dental regeneration potential, ages with functional deterioration in both in vivo and ex vivo expansion. However, little is known about regulators for PDLSC aging.
Methods: Expression changes of a potential regulator for PDLSC aging, histone deacetylase 6 (HDAC6), were evaluated within various models. Senescence-associated phenotypic and functional alternations of PDLSC in loss-of-function models for HDAC6 were examined using HDAC6-specific pharmacologic inhibitors or RNA interference-based knockdown. Involvement of p27Kip1 in HDAC6-associated aging was demonstrated by its acetylation and stability changes along with overexpression or functional inhibition of HDAC6.
Results: Expression of HDAC6 decreased significantly in replicative senescence and induced SC aging models. Loss-of-function experiments suggested that pharmacologic inhibition of deacetylase activity of HDAC6 accelerated PDLSC senescence and impaired its SC activities, which showed reduced osteogenic differentiation and diminished migration capacities. Examination of markers for proliferative exhaustion of SCs revealed that protein level of p27Kip1 was specifically elevated after HDAC6 inhibition. HDAC6 physically interacted with p27Kip1 and could deacetylate p27Kip1. Importantly, acetylation of p27Kip1 was negatively regulated by HDAC6, which correlated with alteration of p27Kip1 protein levels.
Conclusion: Data suggest that HDAC6 plays an important role in PDLSC aging, which is dependent, at least partially, on regulation of p27Kip1 acetylation.
Accumulating damages to various biologic macromolecules leads to universal decline of tissue homeostasis and regeneration during aging, a hallmark of stem cell (SC) exhaustion.1 Similarly, many ex vivo expanded SCs with the potential for clinical therapies experience age-dependent functional deterioration along with repopulation.2 As a prototypical adult SC with capacities for differentiation into a broad class of tissues, mesenchymal SCs (MSCs) are developing into a promising therapeutic agent for tissue regeneration. However, application of MSCs is not exempted from such vulnerabilities to senescence, which are believed to be set by both intrinsic factors and environmental niche.3 Clinical treatment with MSCs from older donors was reported to be less effective than their younger counterparts.4 These senescent MSCs feature a panel of characteristics, including: 1) morphologically enlarged cell bodies; 2) slow or halted cell proliferation; 3) elevated senescence-associated (SA) β-galactosidase (SA-β-gal) activity; 4) reduced migration capacity; and 5) decreased differentiation potential.5 Despite various models and markers being proposed to explain or validate the aging state of MSCs, activation of tumor suppressors, such as p16INK4a, p53/p21Cip1, and p27Kip1, is emerging as a pivotal event in cellular senescence and SC degeneration.5 In this regard, all these proteins were reported to be barriers for induced pluripotency.6-8 Periodontal ligament (PDL) SC (PDLSC) is a type of dental MSC isolated from PDL.9 Besides its basic function to support collagenous fibers in dense connective tissue of teeth, PDLSC is implicated in the maintenance of periodontal homeostasis and replenishment of impaired cells during healing of dental damages. In line with these tissue regeneration capacities, multipotent PDLSCs are able to differentiate into neurogenic, cardiomyogenic, chondrogenic, and osteogenic lineages.10 Accordingly, cell therapy with PDLSCs is considered to be encouraging for treatment of human periodontitis and other diseases demanding dental regeneration.11-14 However, a general hindrance against clinical application of PDLSCs is degeneration of SC function of PDLSCs in older individuals or younger ones with prematurely developed age-related conditions.15 Indeed, the critical value of PDLSCs for osteoblastic differentiation has been reported to be severely compromised in the senescent state after replicative exhaustion or exposure to stress.16 Histone deacetylase 6 (HDAC6) is a member of Class IIb HDACs, with a unique duplicated deacetylase domain and ubiquitin-binding domain.17 HDAC6 is mainly localized in the cytoplasm, and its primary enzymatic targets include α-tubulin, cortactin, and chaperone heat shock protein HSP90α.18 Hence, HDAC6 is broadly involved in many cellular processes, such as maintenance of cellular shape and polarity, intracellular transport, and directional movement, based on its erasure of acetylation of two key cytoskeletal components: α-tubulin and cortactin.18 However, compared with its well-defined roles in cancer and neurodegenerative diseases, little is known about its effect on SC aging and cellular senescence.
In this study, HDAC6 is reported to be a critical regulator of PDLSC aging. Decreased expression of HDAC6 is evident in various senescence models including human samples. Inhibition of deacetylase activity of HDAC6 is observed to impair SC function of PDLSCs. Linkage between enzymatic activity of HDAC6 and stability of pivotal senescence protein p27Kip1 is demonstrated.
