The sequences of the human genome that do not carry instructions for making proteins, which a decade ago were called junk DNA, have increasingly been shown to be important in understanding the cellular machinery involved in biological processes, including in certain diseases. A gene located on human chromosome 12 and known by the name HOTAIR generates a very long RNA, comprising 2,200 nucleotides, but does not give rise to any protein. However, recent studies indicate that this portion of the genetic sequence appears to play an important role in regulating metastasis, the cellular mechanism that allows cancer to spread from one organ to another, thus leading to tumors in other parts of the body. A new study by researchers at the Center for Cell-Based Therapy (CTC) at the Ribeirão Preto School of Medicine of the University of São Paulo (FMRP-USP) shows in detail the important role of RNA HOTAIR in this process.
The study indicates that this RNA is responsible for activating the so-called epithelial-mesenchymal transition (EMT) in tumors, a process that alters the morphology and functionality of a portion of the cancer cells. “Thus, the tumor’s epithelial cells are transformed into mesenchymal cells and start to behave as if they were cancer stem cells,” says Wilson Araújo da Silva Junior, a CTC geneticist and author of an article on the subject to be published in the September 2013 issue of the journal Stem Cells. “Cancer cells gain the ability to detach from the original tumor, migrate through the bloodstream and adhere to other organs and generate new cancers.” In addition to promoting the spread of the disease throughout the body via metastasis, the EMT also helps to perpetuate the cells of the original tumor.
The epithelial-mesenchymal transition is a transformation that typically occurs in the early stages of embryonic development and is involved in generating various types of body tissue. It is also associated with healing processes that include fibrosis formation and wound regeneration. In these situations, EMT is beneficial to the preservation of life. The problem is that in the case of tumors this transformation also appears to be useful to the development of tumors. Epithelial cells form the body’s external covering (skin) and internal lining (mucosa) of its cavities. They are unable to break away from other cells, spread throughout the body and become other types of cells. Their appearance and functions differ from mesenchymal cells, which are able to spread throughout the body and transform into other types of cells. In other words, according to this line of reasoning, if there were no EMT, it would be harder for a tumor to spread throughout the body.
Sometimes chemotherapy and radiation kill most cancer cells, but not those that are part of the epithelial-mesenchymal transition, such as cancer stem cells. It is through them that the original tumor returns or appears elsewhere. “Tumor cells are heterogeneous,” says Marco Antonio Zago, another author of the article and coordinator of the CTC, one of the Research, Innovation and Dissemination Centers (RIDCs) maintained by FAPESP. “In the experiment, when we suppressed the HOTAIR, we saw that EMT did not occur.” It is still too early to say for sure, but blocking the HOTAIR action may be a way to combat metastasis.
The USP researchers worked with human tumor cells of the breast and colon. “These forms of cancer are widely used models in this type of study,” says Cleidson Pádua Alves, a biologist who did his postdoctoral studies at the USP center and is the first author of the article. With this work, scientists discovered that by administering TGF -ß1 (a transformation factor) in cancer cells grown in vitro, the RNA HOTAIR was activated; there were changes in how a number of genes functioned and the epithelial-mesenchymal transition occurred. The more the HOTAIR was activated, the more intense this process became. However, if the gene that produces the RNA HOTAIR was neutralized, EMT simply did not happen. “This RNA is part of the cellular programming required for metastasis to occur,” says Silva. Prior to their study, there was evidence that both HOTAIR, which belongs to a new class of RNAs (the lincRNAs or long intergenic non-coding RNAs), and the EMT mechanism were related to cancer progression. What was not known was that HOTAIR was essential to activating the epithelial-mesenchymal transition.
Center for Cell-Based Therapy (CTC) (No. 2013/08135-2); Grant Mechanism Research, Innovation and Dissemination Centers (RIDC); Coordinator Marco Antonio Zago / FMRP-USP; Investment R$4.5 million per year for the entire RIDC (FAPESP).
ALVES, C.P. et al. The lincRNA HOTAIR is required for the epithelial-to-mesenchymal transition and stemness maintenance of cancer cell lines. Stem Cells. In press.