Aug 16, 2022, 15:34 PM by Nathan Wick, DO, Tolga Suvar, MD, and Thomas Buchheit, MD; Regenerative SIG

Over the last decade, regenerative medicine and cellular therapies such as stem cells have emerged as highly regarded topics of research and development in the treatment of pain and functional limitations. Mesenchymal stem cells (MSCs) are derived from multiple sources and remain differentiated as connective tissue cells. Other stem cell therapies such as human pluripotent stem cells (hPSCs) are undifferentiated cells that can either be isolated from human embryos or induced by exposing somatic cells to specific transcription factors—referred to as induced pluripotent stem cells (iPSCs). The majority of stem cells used for degenerative musculoskeletal conditions are adult differentiated MSCs.

The properties of stem cells are known to have self-renewal, immunomodulatory, anti-inflammatory, signaling, and differentiating properties. The self-renewal capacity of MSCs is characterized by the specific tissue or organ of origin. Common derivates of multipotent adult MSCs are bone marrow, umbilical cord tissue, and adipose-derived stem cells. Umbilical cord tissue can be sourced from Wharton’s jelly, cord lining, or the perivascular region of the umbilical cord; MSCs, regardless of source, are low in immunogenicity.

During the preparation process, autologous, uncultured MSCs are received from the designated origins by needle aspiration and often spun down by centrifugal force. The concentrate is then injected into the desired anatomical region. MSCs may also be culture-expanded to increase cell concentrations and to differentiate, if desired, into other potential mesenchymal tissues. However, these cells will not revert to more pluripotent cell types. Unlike MSCs, iPSCs are reprogrammed back into an embryonic-like state through culturing techniques to enable the development of multiple cell lineages. Theoretical cancer risk may come from two distinct mechanisms. First, dysregulated pluripotent stem cells or induced pluripotent stem cells could have malignant potential. Second, differentiated MSCs may cause immune modulation, potentially disrupting surveillance of existing cancer within the body or inducing MSC microenvironments with a propensity for tumorgenicity.

Stem cell engineering allows the ability to induce an embryonic, pluripotent state by transferring transcription factors to somatic cells. Yamanaka factors are transcription factors that propel the removal of epigenetic markers on somatic cells, causing them to revert to a pluripotent state. Induced pluripotency presents a valuable role in therapeutic development due to its capacity for regeneration. However, the induction process can cause in vivo tumorgenicity, and this remains one of the biggest hurdles to achieve sound clinical application. IPSCs can generate a wide variety of cellular subsets, and transplanted iPSCs have been shown to generate benign teratomas and malignant teratocarcinomas. In mice models, undifferentiated iPSCs differentiated while forming teratoma tumors.1 Additionally, somatic tumor development is also a potential outcome of iPSC transplantation. The tumorgenicity of iPSCs highlights the importance of the safety profile of culturing techniques, as avoiding contamination and genetic aberrations is paramount in cellular preparation. 

MSCs are gaining popularity in disease-modifying treatment because of their harvestability, utility of enhancing native growth factors, and promotion of tissue restoration. Unlike pluripotent stem cells, MSCs are multipotent, as they are restricted in their differentiation capabilities, such as chondrogenic or osteogenic cells. Bone marrow is often the isolation source of choice due to its ability to be minimally manipulated in processing and homologous function between the donor and recipient.2

MSC therapy has been frequently utilized as a therapy for the treatment of knee pain due to mild or moderate osteoarthritis, as it is a less invasive option than a total joint replacement.3 Homing has been identified in numerous studies for cultured MSCs but not for non-cultured therapy in the United States.4 Additionally, culture-expanded MSCs demonstrate a capacity for differentiation into chondrocytes and for immunomodulatory properties.5 Cultured MSCs can migrate to the appropriate site of injury, inhibit pro-inflammatory pathways, and promote tissue repair and regeneration. Despite these promising characteristics of MSC therapy, unsolved challenges in the practical applications of stem cell therapy still exist. In particular, the safety profile as it relates to the fate of transplanted cells in the body is a topic of concern.6 Due to the lack of studies evaluating the long-term effects, there have been questions regarding the relationship between cultured MSCs and cancer origin, growth, and development. Recent studies have highlighted potential adverse effects of these types of MSCs, including tumor growth, metastasis, and transformation into cancer cells.7

MSCs have been discovered in multiple tissue types, including bone, cartilage, fat, muscle, and tendon; however, cultured MSCs allow for a wide-multilineage differentiation potential.8 A recent study reported that pulmonary tissue-derived MSCs exhibit high proliferative capacity; however, no malignant transformation was demonstrated.9 Houghton et al., in an animal model, showed cultured MSCs progressed to gastric epithelial cancer in mice experimentally infected with Helicobacter hepaticus.10 These results were hypothesized to be due to a heterogenous MSC population that was used. Furthermore, Rosland et al. showed bone marrow-derived MSCs’ propensity to undergo spontaneous malignant transformation.11 However, the MSCs were later found to be cross contaminated in this study.10 Although the validity and application of these studies could be debated, they do illustrate MSCs’ ability for malignant transformation.

There is evidence that MSCs may be the cellular origin of many types of cancers when exposed to certain proteins in the in-vitro setting.12 Studies have shown that introduction of FLI-1/EWS fusion protein into MSCs may cause transformation into malignant sarcoma cells.12  Additionally, studies demonstrate that expression of FUS/CHOP protein or synovial sarcoma translocated protein (SYT-SSX1) in human MSCs can cause transformation of MSCs into myxoid liposarcomas or sarcoma cells, respectively.12,13 In addition to fusion protein exposure, MSCs can be directed down a cancer cell lineage by stimulation from certain signaling pathways. Specifically, Jung et al. demonstrated that the chemokine chemoreceptor 6 (CXCR6) signaling pathway stimulates MSCs into cancer-associated fibroblasts, which secrete factors that promote metastasis to secondary tumor sites.14 These studies suggest MSCs have a propensity to create a framework for possible manipulation toward transformation into malignant cells within their isolation source.

Rapidly growing cancers have been shown to manifest a similar inflammatory microenvironment as those common in a wound-healing response.7 As a result, systemically administered MSCs may migrate toward tumor sites in a similar way as they do toward sites of tissue degradation. The signaling pathways that cause this phenomenon is a current focused area of research, as it is not entirely understood. The expression of certain growth factors may contribute to the recruitment of MSCs.15 Basic fibroblast growth factor (bFGF), hepatoma-derived growth factor (HDGF), interleukin-6 (IL-6), monocyte chemotactic protein-1 (MCP-1), stroma-cell derived factor (SDF-1), urokinase plasminogen activator (uPA), and vascular endothelial growth factor (VEGF) have been shown to be involved in the migration of MSCs toward tumor xenografts.16-21 There is complex interplay between these aforementioned factors, tumor microenvironments, and signaling pathways in the tropism of MSCs.

In addition to MSCs’ potential to migrate toward tumor cells, they also have been shown to promote cancer development through direct and indirect mechanisms. Within tumor microenvironments, cultured MCSs have illustrated the ability to communicate with tumor cells directly through gap junctions and indirectly through the release of certain proteins. MSC-derived inflammatory modulators chemokine ligand 1 (CXCL1), CXCL2, and CXCL12 have been shown to communicate with and stimulate the growth of cancer cells.22-23 Additionally, MSC-derived chemokines IL-6 and IL-8 have been shown to facilitate malignancy in colon and breast cancer specifically. 24-25 In terms of indirect acceleration of cancer growth, MSCs have illustrated the capability to facilitate angiogenesis via secretion of growth factors including VEGF, keratinocyte growth factor (KGF), insulin-like growth factor 1 (IGF-1), and galectin-1,26-27 which can be a key contributing factor to the growth and maintenance of cancer cells.

Stem cell therapy offers a novel treatment due to its ability to promote both regeneration and an anti-inflammatory immune response. Some studies have illustrated improved pain and disability outcomes along with imaging evidence of reduced structural damage after intraarticular MSC therapy.28 IPSCs hold a pivotal role in stem cell engineering due to their extensive differentiation capacity. MSCs are one of the most utilized stem cells in the regenerative medicine field due to their harvestability and applicability for degenerative joint disease. There is evidence that, in an in vitro environment, iPSCs and MSCs possess the ability for tumorgenicity depending on the microenvironment, culturing conditions, genetic makeup, and stem cell engineering process. By analyzing these components and improving the safety of culturing modalities, therapeutic effectiveness of cellular therapies can likely be accomplished while minimizing risks of tumorgenicity of stem cell therapy.

References

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