New research presented at the annual meeting of the Orthopedic Research Society in San Francisco provides a more detailed elucidation of the molecular and cellular pathways by which cigarette smoke inhibits stem cell activity.

Bone healing is a two-step process which begins when stem cells differentiate into cartilage and ends when the cartilage has matured into bone. Cigarette smoking has been known to delay skeletal healing after fractures and other injuries by as much as 60%, thereby increasing the risk of further re-injury and chronic disability or pain. In 2005, researchers at the University of Rochester Medical Center in New York identified nicotine as a chemical which interferes with bone growth by altering gene expression. Now the same researchers have identified another chemical in cigarette smoke which also inhibits bone healing, namely, hydrocarbon benzo[a]pyrene (BaP). This polyaromatic hydrocarbon has now been found to prevent the stem cells from reaching the first step in the process, so that they are unable even to begin differentiating into cartilage. Through PCR – polymerase chain reaction, a technique which measures the level of gene expression – the scientists were able to use a mouse model in which they measured genetic changes that were caused by exposure to BaP. Gene expression is the process by which proteins are manufactured through the execution of instructions that are encoded within the genes. Many factors may influence gene expression, one of which is a transcription factor known as the "sex determining region Y-box 9" (SOX-9), which is required for the differentiation of cartilage from stem cells. In this study, the scientists were able to demonstrate by PCR that BaP interferes with SOX-9 expression in mesenchymal stem cells by blocking the conversion of the cells into cartilage and also by reducing levels of type II collagen gene expression. Previous studies have demonstrated that stem cells which are involved in cartilage formation contain proteins known as aryl hydrocarbon receptors (AhR) and that are known to react with BaP. When BaP binds with these receptors, SOX-9 activity is suppressed which results in a reduction both in the number of stem cells that differentiate into cartilage and in the amount of collagen that is produced. The receptors are believed to be involved in the cellular signaling pathways that are responsible for the metabolism of toxins. Staining experiments have also demonstrated that BaP inhibits the deposition of collagen nodules which are indicative of differentiated cartilage cells in vitro.

According to Michael Zuscik, Ph.D., associate professor in the Department of Orthopedics and Rehabilitation at the University of Rochester Medical Center, "Smoking reduces the rate at which the two sides of a fracture come together. We believe this new research will establish for the first time the mechanisms by which polyaromatic hydrocarbons interfere with the healing process."

Researchers at Baylor College of Medicine in Houston, Duke University Medical School, and the National Institute of Allergy and Infectious Disease have collaborated in a study which resulted in the discovery of a link between immunology and stem cells. That link is a gene which is known as the interferon-inducible GTPase Lrg-47 gene. The study was led by Dr. Margaret Goodell, Professor of Pediatrics and Director of the Stem Cells and Regeneration Center at Baylor.

When mice who lacked this gene were infected with a bacteria that resembled tuberculosis, the natural stem cells of the mice did not respond by making new blood cells as a defense against the infection, as they normally would in healthy mice who have the missing gene, which has been known for the role that it plays in stress response. These results therefore suggest that the gene might also be involved in the production of normal blood cells, which would be needed whenever infection and disease threaten an organism.

This is the first study to indicate a direct link between immunology and stem cell activation, as mediated by a common gene. The interferon gamma proteins are well known as regulators of the immune system, and the discovery that this gene is also regulated by interferon gamma was somewhat of a surprise to the researchers, who also found that mice who are bred to lack this gene have abnormally low blood counts and easily die of infection. Additionally, the mice also were found to have stem cells which did not function well, especially when subjected to chemical or pathological stress. Even bone marrow transplants that were performed on the mice were unsuccessful.

Additional research will investigate further exactly how this gene controls stem cell activity, as well as the precise role that interferons play in the cell. The genetic, molecular and cellular mechanisms at work in these pathways have many applications to human health and disease.

Scientists at the San Diego-based biotechnology company Novocell have reported that they were able to use human embryonic stem cells to produce insulin in diabetic mice, although the treatment also yielded some additional, undesirable consequences. In particular, as a result of receiving the embryonic stem cells, some of the mice developed a particular type of tumor known as a teratoma, which is the defining characteristic of embryonic stem cells. Unlike adult stem cells, embryonic stem cells are identified by their ability to form teratomas, and this remains the gold standard throughout the world by which embryonic stem cells are recognized. If a stem cell can form a teratoma, then it’s identified as an embryonic stem cell, and if it cannot form a teratoma then it is known to be something other than an embryonic stem cell. The study has therefore drawn criticism from experts who point out that embryonic stem cell “therapies” such as this are not applicable to humans, since adverse side effects such as the formation of tumors would also occur in human patients.

Embryonic stem cells have always been defined by their ability to form this particular type of tumor, a danger which does not exist with adult stem cells. But the formation of tumors is not the only risk inherent in embryonic stem cells, which have proven to be highly problematic in a multitude of ways. In 2006, scientists at Novocell reported that they had transformed human embryonic stem cells into insulin-producing beta cells in the laboratory, in vitro, although these cells were of no therapeutic value since they were unresponsive to glucose. In their most recent study, the Novocell scientists implanted “precursor” cells into the mice, where the cells were then allowed to mature in vivo into insulin-producing cells – a procedure which is unlikely to be approved by the FDA for use in humans, according to Dr. Mark Magnuson, Director of the Center for Stem Cell Biology at Vanderbilt University. Novocell representatives admit that any possible applicability of this potential therapy to humans still remains several years away.

Recent headlines have announced the news that ordinary adult non-stem cells were reprogrammed back into a more primitive state in which they resemble embryonic stem cells through a process known as de-differentiation (the reversal of a cell from a differentiated, specialized state to a non-differentiated, non-specialized state.) However, the specific molecular and cellular events that are involved in this cellular reprogramming have previously remained unknown. Now researchers at the Harvard Stem Cell Institute (HSCI) and the Massachusetts General Hospital (MGH) have been able to identify these events using cellular markers.

Previous work by other researchers involved adult mouse skin cells which were transformed into “induced pluripotent stem” (iPS) cells after being treated with 4 reprogramming factors, one of which was an oncogene (a gene that causes cancer) which was administered by retroviruses that act as “vectors” which deliver the gene into the target cells. Oncogenes and retroviruses are not applicable in human treatment, however, so the question remained as to whether or not the work could be translatable to human therapies.

Dr. Konrad Hochedlinger, one of the leaders of the current study, is assistant professor in Harvard’s new Department of Stem Cell and Regenerative Biology, and a recipient of the NIH Director’s New Innovator Award. He and his colleagues have now identified specific cell surface markers that are switched on and off at specific stages, thereby regulating the reprogramming of the cells. Dr. Doug Melton, who is Co-Director of the HSCI, described the discovery as, “an important first step in finding ways to create pluripotent stem cells from adult cells without the need for viruses or oncogenes.” In the future, it might be possible to use such cellular mechanisms to reprogram most types of adult non-stem cells, not only mature skin cells, into earlier cells which behave as pluripotent stem cells.

A recent paper by Bischoff et al. describes the effect of tissue acidification on mesenchymal stem cells. It is known that injured tissue is more acidic in comparison to healthy tissue, since injury is associated with reduced blood flow which results in hypoxic conditions and the accumulation of carbon dioxide, which lowers pH. In this study the investigators exposed mesenchymal stem cells that were derived from human bone marrow to various levels of acidity (7.4, 7.0, 6.7, and 6.4) from which they assessed the ability of the stem cells to generate interleukin-8 (IL-8), which is a pro-inflammatory chemotactic cytokine. The results correlated the lowest pH (6.4) with the maximum transcription of the IL-8 protein. Although the scientists were primarily interested in bone injury, their findings are applicable to a number of medical conditions including to the treatment of various types of tumors which are known to express a lower pH than non-malignant tissue. Further investigations will involve the administration of mesenchymal stem cells to patients with tumors, the lower acidity of which could possibly stimulate IL-8 which in turn could call in neutrophils that would eradicate the tumor via pro-inflammatory cytokines and other cellular mechanisms.