My apologies to my readers. I have been in Seattle, Washington for the past week at the Free Methodist National Bible Quizzing Tournament for this week, and I have not had a chance to blog at all. Nevertheless, I have time now. In case you are interested, my team made it to the Senior Teen Veteran Division by winning their subdivision, and then during the double elimination portion of the tournament, they were eliminated in the third round. My quizzers quizzed gallantly, but they happened to quiz the first place and second place teams at the beginning and both teams were quizzing particularly well. They did not go down without a fight, but they were simply out-jumped by the extremely talented quizzers from Winona Lake, IN and Rainer Avenue, Seattle, WA. Oh well; someone has to lose.
I have a paper in my hot little (well not so little) hands that is from C.J. Bruns’ lab at the University of Munich in Munich, Germany. In this paper, Bruns and his German and American collaborators review the use of mesenchymal stem cells (MSCs) as vehicles to specifically deliver toxic genes to tumor cells. These experiments are still preliminary, but with the proper refinements, they might lead to clinical trials for cancers that are difficult to treat with more traditional methods.
MSCs are found in many different locations throughout the body. They are most easily isolated from bone marrow and fat, and they can also be cultured and grown to larger numbers in the laboratory for limited periods of time. When isolated from bone marrow, MSCs appear as a subpopulation of cells that adhere to the plastic tissue culture dishes.
Because of the ease of their isolation and manipulation, researchers have used MSCs to introduce genes into tumor cells. There are several advantages that make MSCs a very attractive cell for such a venture. First, MCSs do not activate the immune system when they are introduced into another body. Secondly, they seem to home to tumors and provide them with a kind of scaffold upon which the tumor cells grow. Thus MSCs and tumors cells form a kind of natural partnership. This means that introduced tumor cells will readily integrate into a growing tumor. Imaging studies that implanted labeled MSCs have borne this out. For example, when implanted into mice with melanomas that have spread to the lung, the transplanted MSCs, after eight days, surrounded the lung tumors (Gao, et al., Oncogene 2010 29(19):2784-94). A similar experiment with tumors in the pancreas also showed similar results (MSCs surrounded the tumors) after three days (Beckermann et al., Br J Cancer 2008 99(4):622-31). Third, given the tendency of MSCs to home to tissue damage and heal it, introduced MSCs and endogenous MSCs tend to view the growing and invading tumor and one big wound that constantly required healing. Thus the interaction between the tumor and the MSCs gets even cozier. For all these reasons, MSCs are very good vehicles for tumor-targeted gene therapy.
One of the first experiments that utilized MSCs for tumor-targeted gene therapy (TTGT) was described by Studeny and colleagues (Studeny M, et al., Cancer Res. 2002 Jul 1;62(13):3603-8). In this paper, MSCs were engineered to express a protein called interferon-beta in order to treat melanomas in mice. Those mice that received intravenous injections of the engineered MSCs showed reduced tumor growth and increased times of survival. Interferon beta (INF-B) is a member of a large group of secreted proteins called interferons. There are three main classes of interferons and the type of receptor bound by the interferon determines which class it belongs to. Type 1 interferons (INFs) are used to treat patients with blood-based tumors (leukemias) or solid tumors. Type 1 INFs prevent tumor growth, staunch the tendencies of tumors to induce the growth of new blood vessels into the tumor mass, and also induce cell death within the cells of the tumor. Clinically, those patients who suffer from recurrent melanomas receive treatments with recombinant IFN-α2b. Thus, getting cells that express INF-B into the tumor could kill of the tumor cells and shut the tumor down.
Since Studeny’s pioneering work, several different studies have used MSCs from bone marrow (Studeny et al., J Natl Cancer Inst 2004 96(21): 1593-603; Loebinger, et al., Cancer Res 2009 69(10):4134-42; Nakamizo, et al., Cancer Res 2005 65(8): 3307-18; Hakkarainen, et al., Hum Gene Ther 2007 18(7):627-41), fat (Grisendi et al., Cancer Res 2010 70(9) 3718-29; Zolochevska et al., Stem Cells Dev 2012 21(7):1112-23), and umbilical cord (Kim, et al., Stem Cells 2010 28(12):2217-28) have been used in experiments like it. Also, several different types of genes other than INF-B have been engineered into MSCs and used to shrink tumors in laboratory animals. These engineered MSCs have also been used to treat melanomas (Studeny M, et al., Cancer Res. 2002 Jul 1;62(13):3603-8; Studeny et al., J Natl Cancer Inst 2004 96(21): 1593-603), breast cancers (Eliopoulos et al., Cancer Res 2008 68(12): 4810-8), lung cancers (Loebinger, et al., Cancer Res 2009 69(10):4134-42; Xim et al., Mol Med 2009 15(9-10):321-7), cervical (Grisendi et al., Cancer Res 2010 70(9) 3718-29) and prostate cancers (Zolochevska et al., Stem Cells Dev 2012 21(7):1112-23), soft tumors (Xiang, et al., Cytotherapy 2009 11(5):516-26), and various types of brain tumors (Gu, et al., Cancer Lett 2010 291(2): 256-62; Miletic, et al., Mol Ther 2007 15(7): Amano, et al., Int J Oncol 35(6):1265-70). The genes with which the MSCs have been engineered include TRAIL (TNF-related apoptosis-inducing ligand), which encodes a protein that causes cells to die (Studeny et al., J Natl Cancer Inst 2004 96(21): 1593-603; Grisendi et al., Cancer Res 2010 70(9) 3718-29; Kim, et al., Stem Cells 2010 28(12):2217-28), PEDF (Pigment epithelium-derived factor), a protein that prevents the growth of blood vessels (Zolochevska et al., Stem Cells Dev 2012 21(7):1112-2), IL-12, a gene that encodes a protein that makes tumors recognizable by the immune system (Eliopoulos et al., Cancer Res 2008 68(12): 4810-8), HSK-Tk, a viral gene that makes tumors susceptible to anti-viral drugs (Gu, et al., Cancer Lett 2010 291(2): 256-62; Uchibori, et al., J Gene Med 2009 11(5):373-81; Miletic, et al., Mol Ther 2007 15(7): Amano, et al., Int J Oncol 35(6):1265-70), and iNOS, a gene that encodes a protein that makes nitric oxide; a toxic molecule (Xiang, et al., Cytotherapy 2009 11(5):516-26). All of these strategies have had some successes in treating artificially induced tumors in laboratory animals.
The main focus of this paper is the use of genetically engineered MSCs to treat tumors of the digestive system. When it comes to tumors found outside the digestive system, the data seem to suggest that MSC-based gene therapies are rather successful. However, when it comes to tumors in the digestive system, the data are unclear, since the experiments show conflicting results. There are some indications that cultured rodent MSCs have the ability to form tumorous growths in extended culture (Miura, et al., Stem Cells 2006 24(4):1095-103; Li, et al., Cancer Res 2007 67(22):10889-98; Qin, et al., Cloning Stem Cells 2009 11(3):445-52). Secondly, implantation of extensively cultured rodent MSCs into the bodies of living rodents leads to the formation of soft tumors (Tolar, et al., Stem Cells 2007 25(2):371-9). Therefore, some risks might accompany MSC-based treatments. In contrast to these concerns, which largely stem from experiments in rodents, the thousands of patients who have had MSC treatments have not experienced cancers as a result of them (Le Blanc, et al., Lancet 2008 371(9624):1579-86).
Animal studies of MSC transplantations into laboratory rodents afflicted with digestive tumors have shown that MSCs stimulate tumor growth, and, in other experiments, inhibit tumor growth. The biology of tumors in the digestive tract is more complicated than other tumors, and, therefore, the results of experiments with MSCs vary from laboratory to laboratory. For example, Qiao and others gave mice with defective immune systems injections of liver cancer cells plus human MSCs that had been engineered to grow continuously in culture. The injected MSCs were able to inhibit tumor growth (Qiao, et al., Cell Res 2008 18(4):333-40). However, work from Bruns’ own lab showed that infused MSCs promoted the growth of pancreatic cancers (Zischek, et al., Ann Surg 2009 250(5):747-53; Conrad, et al., Ann Surg 2011 253(3):566-71) and liver cancers (Niess, et al., Ann Surg 2011 254(5):767-74). Li and his co-workers also showed that human MSC infusions could inhibit the invasion and metastasis of liver cancers in culture. Invasion assays in cultured usually consist of tumor cells grown on a layer of cells and the ability of the tumor cells to penetrate this layer of cells and grow on either side of them. Li and others showed that co-culturing MSCs with liver cancer cells prevented the liver cells from penetrating and invading the cell layer. However, when the same cells were infused into laboratory animals, the MSCs enhanced tumor grow (Li, et al., Cancer Sci 2010 101(12):2546-53). Li and his colleagues found very similar results with cancers of the esophagus (Tian, et al., J Cell Physiol. 2011 226(7):1860-7).
With respect to genetically engineered MSCs, the results are not as equivocal.
A scientist named You and his fellow scientists transplanted stomach cancer cells into mice with hMSCs that had been engineered with a suicide gene called cytosine deaminase (CD). These mice were then given a drug called 5-fluorouracil (5-FU), which kills cells that express CD, and this resulted in a pronounced inhibition of tumor growth (You MH, et al., 2009 J Gastroenterol Hepatol 24:1393–1400). In another experiment by Kidd and others showed that hMSCs engineered with IFN-B or without IFN-B were both found to suppress tumor growth of pancreatic cancers (Kidd S, et al., 2010 Cytotherapy 12:615–625). Additionally, Bruns’ lab also showed that infusion of MSCs in animals with pancreatic cancer strongly promoted tumor growth and increased the tendency of the tumor to spread. However, if the MSCs were engineered to express HSV-Tk, the MSCs substantially inhibited pancreatic tumor growth and prevented the spread of the tumor after the mice we treated with ganciclovir, a drug that kills cells that express HSV-Tk (Zischek C, et al., 2009 Ann Surg 250:747–753).
From all these data, it seems that there is reason to be optimistic that such a treatment strategy might work in humans. However, given the ability of MSCs to stimulate the growth of tumors in animal models is reason for concern, However, with the proper controls and safety regulations in place, anti-cancer treatments with genetically engineered MSCs could be one of the clinical trials we will see in the near future.