Fat instead of a tumor: details

The plasticity of cancer cells was used against them, turning them into fat cells

Vyacheslav Kalinin, "Elements"

Cancer cells have plasticity – the ability to respond to signals from their environment and transform into a non-specialized state in which they spread throughout the body, giving metastases. Scientists from the University of Basel have shown that one of the main mechanisms that ensure plasticity can be used to fight a tumor, causing the differentiation of cancer cells into ordinary adipocyte fat cells. This suppresses the growth of the tumor into the surrounding tissues and the formation of metastases. Research is still far from clinical application, but so far the new method demonstrates great potential for the treatment of cancer patients.

Fig. 1. The general scheme of differentiation of cancer cells into adipocytes, as a result of which cancer tumors lose their ability to invade and metastasize. EMT – epithelial-mesenchymal transition (EMF), MET – mesenchymal-epithelial transition (MEP, this is the reverse process of EMF). Epithelial cells that have turned into malignant (cancer cells) can undergo EMF and go into an undifferentiated state in which they can spread throughout the body, settling in different organs and forming metastases. In the article under discussion, it is shown that with the help of pharmacological means, further artificial differentiation can be carried out, turning such undifferentiated cancer cells into adipocytes (post-mitotic cancer-adipocyte) – adipose tissue cells that do not have the ability to further transform (no plasticity) and are not capable of reverse dedifferentiation into cancer cells (no reversal). As a result, primary cancer (primary tumor) loses the ability to metastasis (no metastasis). A drawing from the discussed article in Cancer Cell.

The plasticity of cancer cells – their ability to respond to the state of their microenvironment and biochemical signals and change in response to these signals – allows them to acquire resistance to therapeutic agents and elude the body's immune system, which causes the progression of the tumor and its ability to form metastases. One of the main mechanisms that ensure the plasticity of cancer cells (including in carcinomas that form in epithelial tissues) is the so–called epithelial-mesenchymal transition (EMF), in which epithelial cells, usually located in one or more layers, turn into a volumetric structure of pluripotent mesenchymal cells (similar in properties to stem cells cells), capable of further differentiating into cells of various types. Normally, this mechanism is active during the development of the embryo, in the adult body it participates in wound healing. But if failures occur in the cellular programs that control EMF (and this is a fairly common situation in cancer, since cellular self-control systems are generally very "loose" in cancer cells), then this allows the tumor to grow faster and be more resistant to specialized therapy: it is known that in cases when a cancer tumor could "to subdue EMF, metastasis is more active, and patient survival is lower (M. A. Nieto et al., 2016. EMT: 2016).

Since EMF is associated with the development of cancer, various scientific groups have attempted to suppress tumors using the reverse process – mesenchymal-epithelial transition (MEP). It is quite difficult, and gradually accumulates evidence that metastasis may increase as a result. An alternative approach – therapy by stimulating further differentiation of cells that have committed EMF – has been successful in at least one form of leukemia (H. de Thé, 2018. Differentiation therapy revisited).

Scientists from the group of Gerhard Cristofori (Gerhard M. Christofori) from the University of Basel (Switzerland) published an article describing another successful case of this approach. They suggested that the spread of cancer cells of a solid tumor, which requires plasticity, can be suppressed if their further differentiation into adipocytes (fat cells) is stimulated.

The first series of experiments was carried out in vitro on various cultured cell lines derived from mouse breast tumors, which are used to simulate cancer and metastases. Adipogenesis was induced by combined exposure to rosiglitazone (rosiglitazone, known primarily as a remedy for diabetes, but its ability to inhibit receptors activated by peroxisomal proliferators that are involved in the regulation of cellular differentiation) and the protein BMP2 (which belongs to the family of transforming growth factor-beta, TGF-β) is important here. In a few days, the cells were almost completely differentiated into adipocytes (Fig. 2). Moreover, all the tested molecular and biochemical markers showed that the adipocytes that arose from tumor cells fully corresponded to true adipocytes.

Fig. 2. Above is the protocol of therapy with rosiglitazone and BMP2. From below – micrographs of the state of cultured in vitro breast cancer cells of mice on the corresponding days. It can be seen that in just a few days almost all cancer cells turn into adipocytes: the light spots on the two right photos are droplets of fat accumulated in adipocytes. The length of the scale segment is 100 microns. A picture from the discussed article in Cancer Cell.

Next, the scientists found out which transcription factors are activated or suppressed in EMF and how they can be influenced in terms of regulation of adipogenesis. At this stage of the study, they worked with a culture of mouse cells that had already undergone EMF.

It is known that proteins from the TGF-β family play an important role in the regulation of EMF (as well as MEP) in the body. They work as the strongest inducers of EMF, and TGF-β signaling pathways are activated in metastatic cancer cells. But the proteins of this family affect adipogenesis in different ways: the already mentioned BMP2 protein (and other bone morphogenetic proteins) activate this process, and TGF-β itself (which gave the name to the family), on the contrary, suppresses it. To deal with this suppressive effect of TGF-β, scientists began to look at which signaling pathways associated with TGF-β remain active in the presence of adipogenesis inducers. A detailed analysis showed that the MEK/ERK signaling pathway (aka MAPK/ERK, one of the key and most well-studied ways of transmitting signals from the cell surface to the nucleus) is most "to blame": it inhibits the differentiation of mesenchymal cancer cells into adipocytes. In the presence of the pharmacological substance PD98059 inhibiting this signaling pathway, differentiation occurred even in the presence of active TGF-β.

To test to what extent the effects observed in cell cultures are reproduced on living organisms, experiments were conducted on model mice. They were injected with breast cancer cells. The introduction of a combination of rosiglitazone and PD98059 in different doses to animals led to the fact that some cancer cells had signs characteristic of adipocytes: fat bubbles formed inside them, and characteristic markers (FABP4 and adiponectin) were synthesized. Such cells were localized mainly along the periphery of the tumor – this is the part of it where invasion zones (tumor germination into surrounding tissues) are formed.

Then the researchers suggested that adipogenic differentiation could suppress tumor invasion. Combination therapy with rosiglitazone and trametinib was used for testing (trametinib, this substance also inhibits the MEK/ERK signaling pathway and, unlike PD98059, it has passed all clinical trials and is approved for use). The assumption was brilliantly confirmed: this combination of drugs was not toxic to mice, suppressed the development of tumor invasion zones and stimulated the formation of adipocytes. Further studies have shown that such therapy suppresses tumor metastasis as well.

Finally, scientists conducted preclinical trials that necessarily precede the use of new drugs and methods of treatment to people: xenotransplantation (transplantation of body tissue of another type) was carried out standard for such cases. Fragments of the tumor of a breast cancer patient were transplanted to several mice. 4 weeks after transplantation of tumor fragments to mice, they were divided into 4 experimental groups: mice from the first group were not injected with drugs, the second group was administered only rosiglitazone, the third – trametinib, the fourth – both drugs. Over the next 8 weeks, scientists monitored the growth of tumors, then the primary tumors were extracted and their mass was determined.

Immunohistological studies have revealed the formation of human adipocytes in tumors, especially pronounced in the case of the combination "trametinib + rosiglitazone". The results obtained corresponded to what was obtained during transplantation of cultured cells. After removal of tumors, therapy was continued, and 4 months after transplantation, mice were euthanized and their lungs were examined for the presence and size of metastases. And the number of metastases, and especially their sizes, were smaller with simultaneous therapy with both drugs (Fig. 3).

Fig. 3. Histological sections of mouse lungs 4 months after xenotransplantation of human breast tumor fragments. One slice from different experimental groups is shown: control; mice that were given only rosiglitazone; mice that were given only trametinib, and mice that were given both drugs. Metastases when stained with hematoxylin and eosin look like darker spots. It is clearly noticeable that therapy with both drugs suppresses the development of metastases. A picture from the discussed article in Cancer Cell.

The results of the work under discussion show that the combination therapy with trametinib and rosiglitazone has great potential for the treatment of breast cancer and, possibly, other oncological diseases (this still needs to be checked). In preclinical trials, drugs that are already being used to treat humans have been used, which may facilitate the further implementation of this approach. Of course, the presented approach must still pass comprehensive tests: it is necessary to determine its effectiveness and the presence of possible side effects. It may be possible to find more active inducers of differentiation of cancer cells into adipose tissue.

Source: Dana Ishay-Ronen et al., Gain Fat-Lose Metastasis: Converting Invasive Breast Cancer Cells into Adipocytes Inhibits Cancer Metastasis // Cancer Cell. 2019.

Portal "Eternal youth" http://vechnayamolodost.ru