More than 1050 clinical trials are registered at FDA.gov that explore multipotent mesenchymal stromal cells (MSCs) for nearly every clinical application imaginable, including neurodegenerative and cardiac disorders, perianal fistulas, graft-versus-host disease, COVID-19, and cancer. Several companies have or are in the process of commercializing MSC-based therapies. However, most of the clinical-stage MSC therapies have been unable to meet primary efficacy end points. The innate therapeutic functions of MSCs administered to humans are not as robust as demonstrated in preclinical studies, and in general, the translation of cell-based therapy is impaired by a myriad of steps that introduce heterogeneity. In this review, we discuss the major clinical challenges with MSC therapies, the details of these challenges, and the potential bioengineering approaches that leverage the unique biology of MSCs to overcome the challenges and achieve more potent and versatile therapies.

Glioblastoma (GBM) is a highly aggressive brain cancer characterized by local invasion and angiogenic recruitment, yet metastatic dissemination is extremely rare. Here, we adapted a microfl uidic device to deplete hematopoietic cells from blood specimens of patients with GBM, uncovering evidence of circulating brain tumor cells (CTC). Staining and scoring criteria for GBM CTCs were fi rst established using orthotopic patient-derived xenografts (PDX), and then applied clinically: CTCs were identifi ed in at least one blood specimen from 13 of 33 patients (39%; 26 of 87 samples). Single GBM CTCs isolated from both patients and mouse PDX models demonstrated enrichment for mesenchymal over neural differentiation markers compared with primary GBMs. Within primary GBMs, RNA in situ hybridization identifi ed a subpopulation of highly migratory mesenchymal tumor cells, and in a rare patient with disseminated GBM, systemic lesions were exclusively mesenchymal. Thus, a mesenchymal subset of GBM cells invades the vasculature and may proliferate outside the brain. SIGNIFICANCE:GBMs are locally invasive within the brain but rarely metastasize to distant organs, exemplifying the debate over "seed" versus "soil. " We demonstrate that GBMs shed CTCs with invasive mesenchymal characteristics into the circulation. Rare metastatic GBM lesions are primarily mesenchymal and show additional mutations absent in the primary tumor.

Astrocyte elevated gene-1 (AEG-1) is a key contributor to hepatocellular carcinoma (HCC) development and progression. To enhance our understanding of the role of AEG-1 in hepatocarcinogenesis, a transgenic mouse with hepatocyte-specific expression of AEG-1 (Alb/AEG1) was developed. Treating Alb/AEG-1, but not Wild type (WT) mice, with N-nitrosodiethylamine (DEN), resulted in multinodular HCC with steatotic features and associated modulation of expression of genes regulating invasion, metastasis, angiogenesis and fatty acid synthesis. Hepatocytes isolated from Alb/AEG-1 mice displayed profound resistance to chemotherapeutics and growth factor deprivation with activation of pro-survival signaling pathways. Alb/AEG-1 hepatocytes also exhibited marked resistance towards senescence, which correlated with abrogation of activation of a DNA damage response. Conditioned media (CM) from Alb/AEG-1 hepatocytes induced marked angiogenesis with elevation in several coagulation factors. Among these factors, AEG-1 facilitated association of Factor XII (FXII) mRNA with polysomes resulting in increased translation. siRNA-mediated knockdown of FXII resulted in profound inhibition of AEG-1-induced angiogenesis. Conclusion We uncover novel aspects of AEG-1 functions, including induction of steatosis, inhibition of senescence and activation of coagulation pathway to augment aggressive hepatocarcinogenesis. The Alb/AEG-1 mouse provides an appropriate model to scrutinize the molecular mechanism of hepatocarcinogenesis and to evaluate the efficacy of novel therapeutic strategies targeting HCC.

Retinoid X Receptor (RXR) regulates key cellular responses such as cell growth and development, and this regulation is frequently perturbed in various malignancies, including Hepatocellular Carcinoma (HCC). However, the molecule(s) that physically govern this deregulation are mostly unknown. Here, we identified RXR as an interacting partner of Astrocyte Elevated Gene-1 (AEG-1)/Metadherin (MTDH), an oncogene upregulated in all cancers. Upon interaction, AEG-1 profoundly inhibited RXR/Retinoic Acid Receptor (RAR)-mediated transcriptional activation. Consequently, AEG-1 markedly protected HCC and acute myeloid leukemia (AML) cells from retinoid- and rexinoid-induced cell death. In non-tumorigenic cells and primary hepatocytes, AEG-1/RXR co-localizes in the nucleus where AEG-1 interferes with recruitment of transcriptional co-activators to RXR preventing transcription of target genes. In tumor cells and AEG-1 transgenic hepatocytes, overexpressed AEG-1 entraps RXR in cytoplasm, precluding its nuclear translocation. Additionally, ERK, activated by AEG-1, phosphorylates RXR which leads to its functional inactivation and attenuation of ligand-dependent transactivation. In nude mice models, combination of all-trans retinoic acid (ATRA) and AEG-1 knockdown synergistically inhibited growth of human HCC xenografts. The present study establishes AEG-1 as a novel homeostatic regulator of RXR and RXR/RAR that might contribute to hepatocarcinogenesis. Targeting AEG-1 could sensitize HCC and AML patients to retinoid- and rexinoid-based therapeutics.

The deregulation of the epidermal growth factor receptor (EGFR) has a significant role in the progression of tumors. Despite the development of a number of EGFR-targeting agents that can arrest tumor growth, their success in the clinic is limited in several tumor types, particularly in the highly malignant glioblastoma multiforme (GBM). In this study, we generated and characterized EGFRspecific nanobodies (ENb) and imageable and proapoptotic ENb immunoconjugates released from stem cells (SC) to ultimately develop a unique EGFR-targeted therapy for GBM. We show that ENbs released from SCs specifically localize to tumors, inhibit EGFR signaling resulting in reduced GBM growth and invasiveness in vitro and in vivo in both established and primary GBM cell lines. We also show that ENb primes GBM cells for proapoptotic tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. Furthermore, SC-delivered immunoconjugates of ENb and TRAIL target a wide spectrum of GBM cell types with varying degrees of TRAIL resistance and significantly reduce GBM growth and invasion in both established and primary invasive GBM in mice. This study demonstrates the efficacy of SC-based EGFR targeted therapy in GBMs and provides a unique approach with clinical implications.

Therapeutically engineered stem cells (SC) are emerging as an effective tumor-targeted approach for different cancer types. However, the assessment of the long-term fate of therapeutic SC post-tumor treatment is critical if such promising therapies are to be translated into clinical practice. In this study, we have developed an efficient stem cell based therapeutic strategy that simultaneously allows killing of tumor cells and assessment and eradication of SC post highly malignant glioblastoma multiforme (GBM) treatment. Mesenchymal stem cells (MSC) engineered to co-express the prodrug converting enzyme, herpes simplex virus thymidine kinase (HSV-TK) and a potent and secretable variant of tumor necrosis factor apoptosis-inducing ligand (S-TRAIL), induced caspase-mediated GBM cell death and showed selective MSC sensitization to the prodrug ganciclovir (GCV). A significant decrease in tumor growth and a subsequent increase in survival were observed when mice bearing highly aggressive GBM were treated with MSC co-expressing S-TRAIL and HSV-TK. Furthermore, the systemic administration of GCV post-tumor treatment selectively eliminated therapeutic MSC expressing HSV-TK in vitro and in vivo, which was monitored in real time by positron emission-computed tomography (PET) imaging utilizing 18F-FHBG, a substrate for HSV-TK. These findings demonstrate the development and validation of a novel therapeutic strategy that has implications in translating stem cell based therapies in cancer patients.

Summary Tumor-propagating cells (TPCs) share self-renewal properties with normal stem cells and drive continued tumor growth. However, mechanisms regulating TPC self-renewal are largely unknown, especially in embryonal rhabdomyosarcoma (ERMS)—a common pediatric cancer of muscle. Here, we used a zebrafish transgenic model of ERMS to identify a role for intracellular NOTCH1 (ICN1) in increasing TPCs by 23-fold. ICN1 expanded TPCs by enabling the de-differentiation of zebrafish ERMS cells into self-renewing myf5+ TPCs, breaking the rigid differentiation hierarchies reported in normal muscle. ICN1 also had conserved roles in regulating human ERMS self-renewal and growth. Mechanistically, ICN1 up-regulated expression of SNAIL1, a transcriptional repressor, to increase TPC number in human ERMS and to block muscle differentiation through suppressing MEF2C, a myogenic differentiation transcription factor. Our data implicate the NOTCH1/SNAI1/MEF2C signaling axis as a major determinant of TPC self-renewal and differentiation in ERMS, raising hope of therapeutically targeting this pathway in the future.

Tumor cells engineered to express therapeutic agents have shown promise to treat cancer. However, their potential to target cell surface receptors specific to the tumor site and their posttreatment fate have not been explored. We created therapeutic tumor cells expressing ligands specific to primary and recurrent tumor sites (receptor self-targeted tumor cells) and extensively characterized two different approaches using (i) therapy-resistant cancer cells, engineered with secretable death receptor-targeting ligands for "off-the-shelf" therapy in primary tumor settings, and (ii) therapy-sensitive cancer cells, which were CRISPR-engineered to knock out therapy-specific cell surface receptors before engineering with receptor self-targeted ligands and reapplied in autologous models of recurrent or metastatic disease. We show that both approaches allow high expression of targeted ligands that induce tumor cell killing and translate into marked survival benefits in mouse models of multiple cancer types. Safe elimination of therapeutic cancer cells after treatment was achieved by co-engineering with a prodrug-converting suicide system, which also allowed for real-time in vivo positron emission tomography imaging of therapeutic tumor cell fate. This study demonstrates self-tumor tropism of engineered cancer cells and their therapeutic potential when engineered with receptor self-targeted molecules, and it establishes a roadmap toward a safe clinical translation for different cancer types in primary, recurrent, and metastatic settings.