Does Human Fat Contain Stem Cells?

Aeon Magazine recently published my longform essay on our research with human liposuction samples and our attempts to use fat for regenerative and therapeutic purposes. Many research groups, including our own group, have been able to isolate stem cells from human fat. However, when it came to using this cells for treating cardiovascular disease, the cells behaved in a manner that we had not anticipated.

Undifferentiated mesenchymal stem cells (left) and their fat neighbors (right) – From our PLOS One paper

We were unable to convert them into heart muscle cells or blood vessel endothelial cells, but we found that they could help build large networks of blood vessels by releasing important growth factors. Within a few years of our initial publication, clinical trials with patients with blocked arteries or legs were already being planned, and are currently underway.

We decided to call the cells “adipose stromal cells” because we wanted to emphasize that they were acting as a “stroma” (i.e. supportive environment for blood vessels) and not necessarily as stem cells (i.e. cells that convert from an undifferentiated state into mature cell types). In other contexts, these same cells were indeed able to act like “stem cells”, because they could be converted into bone-forming or cartilage-forming cells, thus showing the enormous versatility and value of the cells that reside within our fat tissues.

The answer to the question “Does Human Fat Contain Stem Cells?” is Yes, but these cells cannot be converted into all desired tissues. Instead, they have important supportive functions that can be used to engineer new blood vessels, which is a critical step in organ engineering.

In addition to describing our scientific work, the essay also mentions the vagaries of research, the frustrations I had as a postdoctoral fellow when my results were not turning out as I had expected, and how some predatory private clinics are already marketing “fat-derived stem cell therapies” to paying customers, even though the clinical results are still rather preliminary.

 

For the readers who want to dig a bit deeper, here are some references and links:

 

1. The original paper by Patricia Zuk and colleagues which described the presence of stem cells in human liposuction fat:

Zuk, P et al (2001) “Multilineage Cells from Human Adipose Tissue: Implications for Cell-Based Therapies”

 

2. Our work on how the cells can help grow blood vessels by releasing proteins:

Rehman, J et al (2004) “Secretion of Angiogenic and Antiapoptotic Factors by Human Adipose Stromal Cells”

 

3. Preliminary findings from ongoing clinical studies in which heart attack patients receive infusions of fat derived cells into their hearts to improve heart function and blood flow to the heart:

Houtgraf, J et al (2012) “First Experience in Humans Using Adipose Tissue–Derived Regenerative Cells in the Treatment of Patients With ST-Segment Elevation Myocardial Infarction”

 

4. Preliminary results from an ongoing trial using the fat-derived cells in patients with severe blockages of leg arteries:

Bura, A et al (2014) “Phase I trial: the use of autologous cultured adipose-derived stroma/stem cells to treat patients with non-revascularizable critical limb ischemia”

 

5. Example of how “cell therapies” (in this case bone marrow cells) are sometimes marketed as “stem cells” but hardly contain any stem cells:

The Largest Cell Therapy Trial in Heart Attack Patients Uses Hardly Any Stem Cells

 

6. The major scientific society devoted to studying the science of fat and its cells as novel therapies is called International Federation for Adipose Therapeutics and Science (IFATS).

I am not kidding, it is I-FATS!

Explore their website if you want to learn about all the exciting new research with fat derived cells.

 

7. Some of our newer work on how bone marrow mesenchymal stem cells turn into fat cells and what role their metabolism plays during this process:

Zhang, Y et al (2013) “Mitochondrial Respiration Regulates Adipogenic Differentiation of Human Mesenchymal Stem Cells”

 

Zuk PA, Zhu M, Mizuno H, Huang J, Futrell JW, Katz AJ, Benhaim P, Lorenz HP, & Hedrick MH (2001). Multilineage cells from human adipose tissue: implications for cell-based therapies. Tissue engineering, 7 (2), 211-28 PMID: 11304456

 

 

 

Rehman J, Traktuev D, Li J, Merfeld-Clauss S, Temm-Grove CJ, Bovenkerk JE, Pell CL, Johnstone BH, Considine RV, & March KL (2004). Secretion of angiogenic and antiapoptotic factors by human adipose stromal cells. Circulation, 109 (10), 1292-8 PMID: 14993122

Recent Study Raises Questions About Using Adult Stem Cells for Chronic Heart Disease

A recent clinical study (POSEIDON Randomized Trial) investigated the effects of transplanting bone marrow derived adult stem cells into patients with known heart disease. The results were presented at the 2012 American Heart Association (AHA) meeting in Los Angeles and also published in the article “Comparison of Allogeneic vs Autologous Bone Marrow–Derived Mesenchymal Stem Cells Delivered by Transendocardial Injection in Patients With Ischemic Cardiomyopathy: The POSEIDON Randomized Trial“. The article by Dr. Joshua Hare and colleagues appeared in the online edition of the Journal of the American Medical Association on November 6, 2012.

The primary goal of the study was to compare whether adult stem cells from other donors (allogeneic cells) are just as safe as the stem cells derived from the patients’ own bone marrow (autologous cells). Thirty patients with a prior heart attack and reduced cardiac function received either allogeneic or autologous cells. The injected cells were mesenchymal stem cells (MSCs), an adult stem cell type that resides within the bone marrow and primarily gives rise to bone, fat or cartilage tissue. MSCs are quite distinct from hematopoietic stem cells (HSCs) which are also present in the bone marrow but give rise to blood cells. In the POSEIDON study, patients underwent a cardiac catheterization and the MSCs were directly injected into the heart muscle. Various measurements of safety and cardiac function were performed before and up to one year after the cell injection.

The good news is that in terms of safety, there was no significant difference when either autologous or allogeneic MSCs were used. Within the first month after the cell injection, only one patient in each group was hospitalized for what may have been a major treatment related side effect. In the long-run, the number of adverse events was very similar in both groups. The implication of this finding is potentially significant. It suggests that one can use off-the-shelf adult stem cells from a healthy donor to treat a patient with heart disease. This is much more practical than having to isolate the bone marrow from a patient and wait for 4-8 weeks to expand his or her own bone marrow stem cells.

The disappointing news from this study is that one year following the stem cell injection, there was minimal improvement in the cardiac function of the patients. The ejection fraction of the heart is an indicator of how well the heart contracts and the normal range for healthy patients is roughly 55-60%. In the current study, patients who received allogeneic cells started out with an average ejection fraction of 27.9% and the value increased to 29.5% one year after the cell injection. The patients who received autologous cells had a mean ejection fraction of 26.2% prior to the cell transplantation and a mean ejection fraction of 28.5% one year after the stem cell therapy. In both groups, the improvement was minimal and not statistically significant. A different measure of the functional capacity of the heart is the assessment of the peak oxygen consumption. This measurement correlates well with the survival of a patient and is also used to help decide if a patient needs a heart transplant. There was no significant change in the peak oxygen consumption in either of the two groups of patients, one year after the treatment. Some other measures did indicate a minor improvement, such as the reduction of the heart attack scar size in both patient groups but this was apparently not enough to improve the ejection fraction or oxygen consumption.

One of the key issues in interpreting the results is the fact that there was no placebo control group. The enrollment in a research study and the cell injection procedure itself could have contributed to minor non-specific or placebo benefits that were unrelated to the stem cell treatments. One odd finding was that the patient sub-group which showed a statistically significant improvement in ejection fraction was the group which received the least stem cells. If the observed minor benefits were indeed the result of the injected cells turning into cardiac cells, one would expect that more cells would lead to greater functional improvement. The efficacy of the lowest number of cells points to non-specific effects from the cell injection or to an unknown mechanism by which the injected cells activate cardiac repair without necessarily becoming cardiac cells themselves.

The results of this study highlight some key problems with current attempts to use adult stem cells in cardiovascular patients. Many studies have shown that adult stem cells have a very limited differentiation potential and that they do not really turn into beating, functional heart cells. Especially in patients with established, long-standing heart disease, the utility of adult stem cells may be very limited. The damage that the heart of these patients has suffered is probably so severe that they need stem cells which can truly regenerate the heart. Examples of such regenerative stem cells are embryonic stem cells or induced pluripotent stem cells which have a very broad differentiation potential. Cardiac stem cells, which exist in very low numbers within the heart itself, are also able to become functional heart cells. Each of these three cell types is challenging to use in patients, which is why many current studies have resorted to using the more convenient adult bone marrow stem cells.

Human embryonic stem cells can develop into functional heart cells, but there have been numerous ethical and regulatory concerns about using them. Induced pluripotent stem cells (iPSCs) appear to have the capacity to become functional heart cells, similar to what has been observed for human embryonic stem cells. However, iPSCs were only discovered six years ago and we still have a lot to learn more about how they work. Lastly, cardiac stem cells are very promising but isolating them from the heart requires an additional biopsy procedure which can also carry some risks for the patients. Hopefully, the fact that adult bone marrow stem cells showed only minimal benefits in the POSEIDON study will encourage researchers to use these alternate stem cells (even if they are challenging to use) instead of adult bone marrow stem cells for future studies in patients with chronic heart disease.

One factor that makes it difficult to interpret the POSEIDON trial is the lack of a placebo control group. This is a major problem for many stem cell studies, because it is not easy to ethically justify a placebo group for invasive procedures such as a stem cell implantation. The placebo patients would also have to receive a cardiac catheterization and injections into the heart tissue, but instead of stem cells, the injections would just contain a cell-free liquid solution. Scientifically, such a placebo control group is necessary to determine whether the stem cells are effective, but this scientific need has to be weighed against the ethics of a “placebo” heart catheterization. Even if one were to ethically justify a “placebo” heart catheterization, it may not be easy to recruit volunteer patients for the study if they knew that they had a significant chance of receiving “empty” injections into their heart muscle.

There is one ongoing study which is very similar in design to the POSEIDON trial and it does contain a placebo group: The TAC-HFT trial. The results of this trial are not yet available, but they may have a major impact on whether or not bone marrow stem cells have a clinical future. If the TAC-HFT trial shows that the bone marrow stem cell treatment for patients with chronic heart disease has no benefits or only minor benefits when compared to the placebo group, it will become increasingly difficult to justify the use of these cells in heart patients.

In summary, the POSEIDON trial has shown that treating chronic heart disease patients with bone marrow derived stem cells is not yet ready for prime time. Bone marrow cells from strangers may be just as safe as one’s own cells, but if bone marrow stem cells are not very effective for treating chronic heart disease, than it may just be a moot point.

 

Image credit: Wikipedia

The Importance of Being Embryonic

Human ESC colony – Wikimedia

There are three broad categories of human stem cells: 1) adult stem cells, 2) embryonic stem cells (ESCs) and 3) induced pluripotent stem cells (iPSCs). Adult stem cells can be found in selected adult tissues, such as the hematopoietic stem cells in the bone marrow which give rise to a variety of blood cells on a daily basis in an adult. Such adult stem cells are quite rare and, when compared to ESCs, somewhat limited in the type of cells they can generate. Hematopoietic stem cells, for example, routinely produce leukocytes (white blood cells) and erythrocytes (red blood cells), but most researchers agree that they cannot give rise heart muscle cells. On the other hand, human ESCs are pluripotent, which refers to the fact that they can differentiate into nearly all cell types, from neurons to insulin-producing pancreatic cells or even heart muscle cells.

Human ESCs are usually derived from human eggs that were created in an in vitro fertilization clinic but never implanted in a woman. Such clinics often generate far more fertilized human eggs than they actually implant, because it is difficult to predict how many implantation attempts are necessary before a successful pregnancy can be achieved. The “back-up” eggs remain in a freezer at the in vitro fertilization clinic and the donors can then decide whether they want these eggs to be used for the generation of human ESCs, which can be used for either research or ESC-based therapies. The informed consent of the donors is critical and needs to be documented before the ethics committees at the research institutions permit their usage. In spite of these regulations, some religious groups in the US have voiced concerns about using the ESCs, because they feel that even though the donated fertilized egg was never implanted in a woman, it could have been implanted and that its fertilized state already indicates a degree of personhood that requires protection. When the fertilized egg is cultured in a lab and ESCs are derived from it, the fertilized egg is invariably destroyed and from a certain religious perspective, this constitutes a destruction of a human life. Due to concerns about the ethics of using human ESCs, multiple US-based Christian groups have championed the use of adult stem cells to help repair injured tissues and organs. However, since adult stem cells are very rare and limited in their differentiation potential, most stem cell biologists do not see adult stem cells as a suitable alternative to ESCs.

A landmark paper published by Shinya Yamanaka’s group in 2007 provided a new perspective in the gridlock between demands of Christian groups to ban human ESC research and the desire of stem cell biologists to use human ESCs for regenerative medicine.  Yamanaka and his colleagues were able to show that human adult skin fibroblasts could be converted into embryonic-like stem cells (induced pluripotent stem cells or iPSCs). The iPSCs were not adult stem cells with, but actually exhibited the broad differentiation capacity that was previously only seen in human ESCs. From an ethical perspective, iPSCs seemed like a perfect solution since they could be generated without the destruction of any fertilized eggs. Shinya Yamanaka and John Gurdon, whose earlier work had set the stage for Yamanaka’s discovery, received the 2012 Nobel Prize for these exciting findings. Yamanaka’s work was not only lauded by fellow scientists, but also by religious groups, who felt that his work abolished the need for human ESCs. What these religious organizations did not understand was that human ESC research provided the foundation for Yamanaka’s research. All the factors used to reprogram adult skin cells into iPSCs were derived from a careful analysis of ESCs and the culture of human iPSCs was only made possible after the culture of human ESCs had been established in the late 1990s. To this day, the comparison of human ESCs and iPSCs is a topic of active investigation. In many ways, iPSC research is still – pardon the pun – in its embryonic stage. We are still in the process of understanding how an adult cell can be reprogrammed into an iPSC and whether the reprogramming process leaves any kind of marks or blemishes that would affect the generated iPSC.

To understand the biology and nature of iPSCs, researchers routinely use them side-by-side with human ESCs, which still serve as the “gold-standard” for a pluripotent stem cell. At a symposium of the International Society of Stem Cell Research (ISSCR) in San Francisco on August 24, 2012, Yamanaka showed the results of a new study in which he compared the gene expression profiles of 49 different human iPSC lines and 10 different human ESC lines. The comparison revealed that the majority of iPSC lines are indistinguishable from human ESCs, but that there is a minority of iPSC cell lines that behave very differently from human ESCs. Other stem cell researchers have also shown both similarities and differences between ESCs and iPSCs, and definitive conclusions about whether human ESCs and iPSCs are equally suitable for regenerating human tissues and organs cannot yet be drawn.

These new studies remind us that human ESC research is still a very active area of investigation and that in the years to come, research on both ESCs and iPSCs is needed. This was also emphasized in a recent statement by the ISSCR:

 

Yamanaka’s recent and exciting advances demonstrate that it is possible to reprogram cells in adult human tissues into cells that very closely resemble, but may not be identical to, ES cells. Along with recent progress on redirecting cell fate to enhance tissue repair, these experiments have captured the imagination of the scientific community worldwide. While many scientists are very optimistic about the future of this new research, some people in political circles have incorrectly interpreted this enthusiasm as a verdict that research on human ES cells is no longer necessary. This conclusion is not yet scientifically justified.

At present, and in the foreseeable future, there is a strong scientific and medical consensus that continued research on all types of stem cells is critical to developing research strategies that will ultimately provide new therapies. Supporting all forms of stem cell research is in the best long-term interests of a broad spectrum of patients with debilitating diseases and injuries. In fact, predictions about what might or might not be possible cannot substitute for careful and rigorous research to discover what strategy will provide the most successful therapeutic intervention for a given disease or condition. The basic tools for these discoveries include human ES cells, which remain the benchmark for assessing pluripotency and the ability of cells to develop into all the different cell types of the body.

In the wake of the announcement of the Nobel Prize, the ISSCR (whose current president is Shinya Yamanaka) wanted to pre-empt any attempts to dismiss the importance of human ESC research, which remains a cornerstone of stem cell biology and regenerative medicine. I applaud the ISSCR for this pro-active approach. Taking ethical concerns into account is important, but one also needs to make sure that scientific discoveries are not misused to put forward political or religious agendas. In the next years or decades, we may indeed discover that iPSCs can completely replace human ESCs. On the other hand, we may discover that iPSCs and ESCs will play distinct and complementary roles in the future of regenerative medicine. We will not know the answer to the question until we conduct the research and keep an open mind when we assess the results. The nascent biology of iPSCs and ESCs is a journey into the unknown and this is what makes it such an exciting area of research.