Human Amniotic Fluid Stem Cells Can Act Like Heart Cells, Sort of

Human amniotic fluid-derived stem cells (AFSC) have a demonstrated ability to differentiate into several different adult cell types, and they also fail to form tumors in laboratory animals.

A previous study of AFSCs showed that if these stem cells were grown in culture with heart muscle cells from newly born rats, the AFSCs began to express heart-specific genes. While the AFSCs did not become full-fledged heart muscle cells, they began to differentiate in that direction.

Yang Gao and others in the laboratory of Jeffrey G. Jacot at Rice University tried this same experiment with human heart cells. They used a specific set of cell culture conditions that prevent the AFSCs from fusing with the heart cells, because the fusion of two cells can deceive researchers into thinking that the stem cells have actually become heart cells when in fact they have not.

Jacot and his coworkers discovered that when human AFSC made contact with human heart cells, they began to express proteins normally found in heart muscle that help them contract. One of these proteins, cardiac troponin T (cTnT), was definitely expressed in human AFSCs, even though this protein is rather specific to heart muscle cells. cTnT is also one of the proteins released into the bloodstream after a heart attack.  Further investigation uncovered absolutely no evidence of cell fusion. Thus when AFSCs touch human heart cells, they begin to make some heart-specific proteins.

Cardiac Troponin

Jacot and his group did an additional experiment. They tried culturing the human AFSCs on one side of the porous membrane and human heart cells on the other side. These conditions allow minimal contact between cells, but still exposes them the anything the cells might be secreting. Under these culture conditions, human AFSCs still showed a statistically significant increase in cTnT expression compared to culture conditions that without contact between the two cell types.  However, human AFSCs grown in the present of human heart cells still did not express the calcium modulating proteins that are so important for regulating heart muscle contraction. Additionally, the cells and did not have functional or morphological characteristics of mature heart muscle cells.

These data suggest that contact between heart cells and human AFSCs is a necessary but not sufficient condition to drive AFSCs to differentiate into heart cells. However, touching heart cells gets AFSCs part of the way. Maybe further research will provide other cues that will push these remarkable cells the rest of the way.

Primed Fat-Based Stem Cells Enhance Heart Muscle Proliferation

A Dutch group from the University of Groningen has shown that fat-based stem cells can enhance the proliferation of cultured heart muscle cells. The stem cells used in these experiments were preconditioned and this pretreatment greatly enhanced their ability to activate heart muscle cells.

This paper, by Ewa Przybyt, Guido Krenning, Marja Brinker, and Martin Harmsen was published in the Journal of Translational Medicine. To begin, Przybyt and others extracted human adipose derived stromal cells (ADSC) from fat tissue extracted from human liposuction surgeries. To do this, they digested the fat with enzymes, centrifuged and washed it, and then grew the remaining cells in culture.

Then they used rat neonatal heart muscle cells and infected them with viruses that causes them to glow when certain types of light was shined on them. Then Przybyt and others co-cultured these rat heart cells with human ADSCs.

In the first experiment, the ADSCs were treated with drugs to prevent them from dividing and then they were cultured with rat heart cells in a one-to-one ratio. The heart muscle cells grew faster with the ADSCs than they did without them. To determine if cell-cell contact was required for this stimulation, they used the culture medium from ADSCs and grew the heart cell on this culture medium. Once again, the heart cells grew faster with the ADSC culture medium than without it. These results suggest that the ADSCs stimulate heart cell proliferation by secreting factors that activate heart cell division.

Another experiment subjected the cultured heart cells to the types of conditions they might experience inside the heart after a heart attack. For example, heart cells were subjected to low oxygen tensions (2% oxygen), and inflammation – two conditions found within the heart after a heart attack. These treatments slowed heart cell growth, but this heart cell growth was restored by adding the growth medium of ADSCs. Even more remarkably, when ADSCs were grown in low-oxygen conditions or treated with inflammatory molecules (tumor necrosis factor-alpha or interleukin-1beta), the culture medium increased the fractions of cells that grew. Therefore, ADSCs secrete molecules that increase heart muscle cell proliferation, and increase proliferation even more after the ADSCs are preconditioned by either low oxygen tensions or inflammation.

In the next experiment, Przybyt and others examined the molecules secreted by ADSCs under normal or low-oxygen tensions to ascertain what secreted molecules stimulated heart cell growth. It was clear that the production of a small protein called interleukin-6 was greatly upregulated.

Could interleukin-6 account for the increased proliferation of heart cells? Another experiment showed that the answer was yes. Cultured heart cells treated with interleukin-6 showed increased proliferation, and when antibodies against interleukin-6 were used to prevent interleukin-6 from binding to the heart cells, these antibodies abrogated the effects of interleukin-6.

Przybyt and others then took these results one step further. Since the signaling pathways used by interleukin-6 are well-known, they examined these pathways. Now interleukin-6 signals through pathways, once of which enhances cell survival, and another pathway that stimulated cell proliferation. The cell proliferation pathway uses a protein called “STAT3” and the survival function uses a protein called “Akt.” Both pathways were activated by interleukin-6. Also, the culture medium of ADSCs that were treated with interleukin-6 induced the interleukin-6 receptor proteins (gp80 and gp130) in cultured heart muscle cells. This gives heart muscle cells a greater capacity to respond secreted interleukin-6.

This paper shows that stromal stem cells from fat has the capacity, in culture, to activate the growth of cultured heart muscle cells. Also, if these cells were preconditioned with low oxygen tensions or pro-inflammatory molecules, those fat-based stem cells secreted interleukin-6, which enhanced heart muscle cell survival, and proliferation, even if those heart muscle cells are exposed to low-oxygen tensions or inflammatory molecules.

This suggests that preconditioned stem cells from fat might be able to protect heart muscle cells and augment heart healing after a heart attack. Alternatively, cardiac administration of interleukin-6 after a heart attack might prove even more effective to protect heart muscle cells and stimulate heart muscle cell proliferation. Human trials anyone?