Mesenchymal stem cells (MSCs) have the ability to home to growing tumors, and for this reason, many researchers have examined the possibility of using MSCs to treat various types of cancers. However, there is a genuine safety concern with using MSCs in cancer patients, because in laboratory animals, MSCs can form blood vessels that help tumors grow and spread. Consider the following publications:
1. Klopp AH, Gupta A, Spaeth E, Andreeff M, Marini F 3rd (2011) Concise review: Dissecting a discrepancy in the literature: do mesenchymal stem cells support or suppress tumor growth? Stem Cells 29: 11–19.
2. Kidd S, Spaeth E, Klopp A, Andreeff M, Hall B, et al. (2008) The (in) auspicious role of mesenchymal stromal cells in cancer: be it friend or foe. Cytotherapy 10: 657–667.
3. Coffelt SB, Marini FC, Watson K, Zwezdaryk KJ, Dembinski JL, et al. (2009) The pro-inflammatory peptide LL-37 promotes ovarian tumor progression through recruitment of multipotent mesenchymal stromal cells. Proc Natl Acad Sci U S A 106: 3806–3811.
4. Karnoub AE, Dash AB, Vo AP, Sullivan A, Brooks MW, et al. (2007) Mesenchymal stem cells within tumour stroma promote breast cancer metastasis. Nature 449: 557–563.
Because MSCs are multipotental (that is they differentiate into several different adult tissues), they can serve the tumor as a source of blood vessels that augment tumor metastasis and growth. However, several pre-clinical studies with genetically engineered MSCs that deliver chemotherapuetic agents to tumors have proven quite successful (see Waterman RS, Betancourt AM (2012) The role of mesenchymal stem cells in the tumor microenvironment: InTech). So what are we to believe? After MSCs good or bad as tumor treatments?
In 2010, Aline M. Betancourt and colleagues at Tulane University, New Orleans, Louisiana defined two distinct MSC subypes in a MSC population. They referred to these subtypes as MSC1 and MSC2. When challenged with molecules normally found in invading microorganisms, MSC1 populations tend to promote the immune response, where as MSC2 populations tend to suppress the immune response. This simple priming experiment provided a way to distinguish between the two MSC subtypes, but it also gave stem cell scientists a reason why experiments with MSCs tend to give conflicting results in different laboratories – because the two labs were probably working with populations that consisted of different MSC subtypes. See Waterman RS, Tomchuck SL, Henkle SL, Betancourt AM (2010) A New Mesenchymal Stem Cell (MSC) Paradigm: Polarization into a Pro-Inflammatory MSC1 or an Immunosuppressive MSC2 Phenotype. PLoS ONE 5(4): e10088. doi:10.1371/journal.pone.0010088.
With this in mind, Betancourt and co-workers examined the ability of the distinct MSC subtypes to interact with cancers. When grown in culture with several different types of tumor-causing cell lines, they discovered that MSC1 do not support tumor growth but MSC2 robustly support tumors growth. MSC2 also increased the ability of the tumors to invade other tissues and migrate in culture whereas MSC1 supported neither tumor invasion nor tumor migration.
Other features were different as well. For example, MSC1 recruited a completely different cadre of white blood cells to the tumor when compared to MSC2. Also, the molecules deposited in the vicinity of the tumor by MSC1 and MSC2 differed greatly. By providing a bed of molecules upon which tumors cell like to move and grow, MSC2s promoted tumor cell activity, but the materials laid down by MSC1 were not nearly as attractive to the tumor cells.
These show why MSCs can promote the growth of particular tumors in some experiments but not others. Furthermore, it shows that there is a relatively simple test to separate these two MSC subtypes. All further pre-clinical experiments with MSCs, should account for these distinct MSC subtypes and determine if one MSC subtype is a better candidate for an anticancer treatment regime than the other.
See Waterman RS, Henkle SL, Betancourt AM (2012) Mesenchymal Stem Cell 1 (MSC1)-Based Therapy Attenuates Tumor Growth Whereas MSC2-Treatment Promotes Tumor Growth and Metastasis. PLoS ONE 7(9): e45590. doi:10.1371/journal.pone.0045590.