Adipose tissue represents an alternative source of MSC
Adipose tissue represents an alternative source of MSC-like cells, avoiding the problems of pain, morbidity and low cell number associated with bone marrow harvest . In plastic and reconstructive surgery, adipose tissue-derived stromal histamine dihydrochloride (ADSCs) may be used to increase in situ survival of the autologous adipose-tissue graft [18–23]. ADSC have also been utilized as cellular delivery vehicles in bone reconstruction .
The use of adjuvant MSC-like cells in the treatment of osteosarcoma may be an important therapeutic issue for patients with lung metastasis associated with poor outcome (30% survival rate at 5 years) . However, the impact of unmodified MSCs on tumor progression remains unpredictable . For instance, it has been observed that rat and human MSCs can promote tumor growth and metastasis in osteosarcoma models [27–30].
Facing a unique clinical case of osteosarcoma recurrence following autologous adipose-tissue transfer , we started to investigate the interactions between osteosarcoma and adipose tissue by using pre-clinical experiments [30,64]. In the present report, we compared the interactions of MNNG-HOS cells-induced osteosarcoma with human ADSCs/MSCs and with human pre-osteoclasts. It is established that osteoclasts are involved in osteosarcoma progression and are believed to either enhance or suppress metastases [31–33]. In this study, pre-osteoclasts did not increase the tumor size and the lung metastasis. In contrast, ADSCs and MSCs increased the size of MNNG-HOS-induced tumors, but the metastasis process and rate of osteolysis were not exacerbated. Paracrine effects of ADSCs were investigated on osteosarcoma cells after culture in monolayer or oncospheres in order to observe the effects on proliferative or quiescent cell stages. The addition of 50% ADSC-conditioned medium significantly increased the in vitro proliferation of two osteosarcoma cell lines (MNNG-HOS and Saos-2), whereas it did not decrease the proportion of cells in G0 phase. These results suggest that ADSCs/MSCs may be safe in reconstructive surgery after bone tumor resection and not involved in the risk of local recurrence. However, ADSCs/MSCs do not appear to be good candidates for tumor-targeted cell therapy in osteosarcoma, given their enhancing effects on tumor progression.
Materials and methods
Discussion We have previously shown that murine MSC-like cells increased tumor growth when they were co-injected with osteosarcoma cells in mice. In this study, we demonstrate that osteosarcoma growth is also supported by the co-injection of human MSC-like cells, either derived from adipose tissue or bone marrow, but not by osteoclast precursors. In tumors induced by the co-injection of MNNG-HOS cells and ADSCs, ADSCs were not identified at the end of in vivo experiments. Soluble factors produced by ADSC increased by up to 2-fold the in vitro proliferation of two different osteosarcoma cell lines (MMNG-HOS and Saos-2). These results indicate that ADSCs/MSCs may modulate the early MNNG-HOS tumor development, at least partially through paracrine effects as described for cancer-associated adipocytes and breast cancer  or adipose tissue and osteosarcoma . It has been suggested that MSCs improve angiogenesis after an ischemic lesion [50,51] or facilitate successful fat injection during surgery for breast augmentation or reconstruction . By stimulating neovascularization, MSC-like cells may contribute to sarcoma growth [25,44]. VEGF was identified in ADSC/MSC-conditioned medium but in concentrations similar to MMNG-HOS/Saos-2 cell-conditioned medium. In samples of MNNG-HOS-induced tumors which have otherwise overgrown due to MSC-like cells, we could not observe any change in VEGFA expression nor in tumor neovascularization (data not shown). Bian et al.  have reported that human MSCs promoted Saos-2 osteosarcoma growth but through Interleukin 6 (IL-6) secretion. In the present study, IL-6 and IL-8 were indeed detected in the ADSC-conditioned medium. Nonetheless, they were not identified as the molecules that could mediate enhanced proliferation of MMNG-HOS and Saos-2 cells. Similarly, basic fibroblast growth factor (FGF-2) did not enhance the proliferation of osteosarcoma cells in vitro (data not shown), despite the fact that a role for FGF-2 in tumor progression was suggested, especially in an original mouse model of osteosarcoma . Other soluble factors that were not investigated in this study included metalloproteinase 11 which can favor tumor progression, collagen VI involved in the breast cancer-adipocyte interaction  and lysophosphatidic acids implicated in development of bone metastases . All these factors may be implicated, possibly acting in combination, in osteosarcoma cell proliferation.