Abstract
Terminally differentiated murine osteocytes and adipocytes can be reprogrammed using platelet-derived growth factor–AB and 5-azacytidine into multipotent stem cells with stromal cell characteristics. We have now optimized culture conditions to reprogram human adipocytes into induced multipotent stem (iMS) cells and characterized their molecular and functional properties. Although the basal transcriptomes of adipocyte-derived iMS cells and adipose tissue–derived mesenchymal stem cells were similar, there were changes in histone modifications and CpG methylation at cis-regulatory regions consistent with an epigenetic landscape that was primed for tissue development and differentiation. In a non-specific tissue injury xenograft model, iMS cells contributed directly to muscle, bone, cartilage, and blood vessels, with no evidence of teratogenic potential. In a cardiotoxin muscle injury model, iMS cells contributed specifically to satellite cells and myofibers without ectopic tissue formation. Together, human adipocyte–derived iMS cells regenerate tissues in a context-dependent manner without ectopic or neoplastic growth.
INTRODUCTION
The goal of regenerative medicine is to restore function by reconstituting dysfunctional tissues. Most tissues have a reservoir of tissue-resident stem cells with restricted cell fates suited to the regeneration of the tissue in which they reside (1–4). The innate regenerative capacity of a tissue is broadly related to the basal rate of tissue turnover, the health of resident stem cells, and the hostility of the local environment. Bone marrow transplants and tissue grafts are frequently used in clinical practice but for most tissues, harvesting and expanding stem and progenitor cells are currently not a viable option (5, 6). Given these constraints, research efforts have been focused on converting terminally differentiated cells into pluripotent or lineage-restricted stem cells (7, 8). However, tissues are often a complex mix of diverse cell types that are derived from distinct stem cells. Therefore, multipotent stem cells may have advantages over tissue-specific stem cells. To be of use in regenerative medicine, these cells would need to respond appropriately to regional cues and participate in context-dependent tissue regeneration without forming ectopic tissues or teratomas. Mesenchymal stem cells (MSCs) were thought to have some of these characteristics (9–11), but despite numerous ongoing clinical trials, evidence for their direct contribution to new tissue formation in humans is sparse, either due to the lack of sufficient means to trace cell fate in hosts in vivo or failure of these cells to regenerate tissues (12, 13).
We previously reported a method by which primary terminally differentiated somatic cells could be converted into multipotent stem cells, which we termed as induced multipotent stem (iMS) cells (14). These cells were generated by transiently culturing primary mouse osteocytes in medium supplemented with azacitidine (AZA; 2 days) and platelet-derived growth factor–AB (PDGF-AB; 8 days). Although the precise mechanisms by which these agents promoted cell conversion was unclear, the net effect was reduced DNA methylation at the OCT4 promoter and reexpression of pluripotency factors (OCT4, KLF4, SOX2, c-MYC, SSEA-1, and NANOG) in 2 to 4% of treated osteocytes. iMS cells resembled MSCs with comparable morphology, cell surface phenotype, colony-forming unit fibroblast (CFU-F), long-term growth, clonogenicity, and multilineage in vitro differentiation potential. iMS cells also contributed directly to in vivo tissue regeneration and did so in a context-dependent manner without forming teratomas. In proof-of-principle experiments, we also showed that primary mouse and human adipocytes could be converted into long-term repopulating CFU-Fs by this method using a suitably modified protocol (14).
AZA, one of the agents used in this protocol, is a cytidine nucleoside analog and a DNA hypomethylating agent that is routinely used in clinical practice for patients with higher-risk myelodysplastic syndrome (MDS) and for elderly patients with acute myeloid leukemia (AML) who are intolerant to intensive chemotherapy (15, 16). AZA is incorporated primarily into RNA, disrupting transcription and protein synthesis. However, 10 to 35% of drug is incorporated into DNA resulting in the entrapment and depletion of DNA methyltransferases and suppression of DNA methylation (17). Although the relationship between DNA hypomethylation and therapeutic efficacy in MDS/AML is unclear, AZA is known to induce an interferon response and apoptosis in proliferating cells (18–20). PDGF-AB, the other critical reprogramming agent, is one of five PDGF isoforms (PDGF-AA, PDGF-AB, PDGF-BB, PDGF-CC, and PDGF-DD), which bind to one of two PDGF receptors (PDGFRα and PDGFRβ) (21). PDGF isoforms are potent mitogens for mesenchymal cells, and recombinant human (rh)PDGF-BB is used as an osteoinductive agent in the clinic (22). PDGF-AB binds preferentially to PDGFRα and induces PDGFR-αα homodimers or PDGFR-αβ heterodimers. These are activated by autophosphorylation to create docking sites for a variety of downstream signaling molecules (23). Although we have previously demonstrated induction of CFU-Fs from human adipocytes using PDGF-AB/AZA (14), the molecular changes, which underlie conversion, and the multilineage differentiation potential and in vivo regenerative capacity of the converted cells have not been determined.
Here, we report an optimized PDGF-AB/AZA treatment protocol that was used to convert primary human adipocytes, a tissue source that is easily accessible and requires minimal manipulation, from adult donors aged 27 to 66 years into iMS cells with long-term repopulating capacity and multilineage differentiation potential. We also report the molecular landscape of these human iMS cells along with that of MSCs derived from matched adipose tissues and the comparative in vivo regenerative and teratogenic potential of these cells in mouse xenograft models.
