The PML gene is considered a growth regulator gene and plays a role in the maturation and activation of various cells. The product of this gene is a tumor suppressive protein, which is involved in the processes of cell differentiation and suppression of their proliferation, in a number of immunological processes associated, in particular, with the mechanisms of action of IFN-a. Thus, PML protein has been shown to stimulate the expression of class I antigens of the main histocompatibility complex and proteins involved in the movement of peptides to the cell surface in association with class I antigens. A number of studies have demonstrated that PML protein can induce an apoptosis process, both associated and not associated with a caspase mechanism. The PML protein is expressed mainly in differentiated cells in the postmitotic period.
The largest expression of this protein is found in endothelial cells, epithelial cells and macrophages. The PML protein is localized in the cell nucleus in the so-called nuclear bodies (nuclear bodies – NB) or PML-oncogenic domains (POD). These structures were described about 35 years ago, their presence is directly proportional to the level of protein synthesis and inversely proportional to the degree of cell differentiation. NBs are associated with the nuclear matrix, which plays a role in the movement of molecules and the organization of chromatin inside the nucleus. In acute promyelocytic leukemia with t (15; 17), PML protein moves from the NB and is visualized as fine material. After treatment with all-trans retinoic acid, PML is again localized in nuclear bodies. The researchers noted that an increase in the amount of PML protein in cells of the culture of acute promyelocytic leukemia (NB4-line) significantly suppresses its clonogenic activity and malignancy when conducting experiments on nude mice. This allowed the authors to suggest antagonism between the action of PML protein and the protein product of the chimeric PML-RARa gene.
The product of the chimeric PML-RARa gene is a pathological protein that retains the active functional domains of both the RARa protein and PML protein. In ALP, PML-RARa protein accumulates in the cytoplasm and nucleus of myeloid cells in a significantly larger number than normal RARa protein accumulates. The aberrant retinoid PML-RARa receptor with impaired DNA binding activity can attach to DNA in retinoic acid binding regions (RARE) as a homodimer, competing with normal RARa, which can bind to DNA, as indicated, only after heterodimerization with RXR.
It has also been proven that chimeric protein binds actively to RXR, displacing the normal RARa receptor. In the absence of retinoic acid, the chimeric receptor PML-RARa proves to be a stronger transcriptional repressor than the normal receptor.
This is explained by the fact that, forming a repressor protein complex, it is stronger than normal RARa and binds to the corepressor molecules N.COR and SMRT. These corepressor molecules, in turn, are associated with histoacetylases, which change the conformation of the DNA molecule and make it inaccessible for transcription factors. As a result, gene transcription is stopped. In order to cause dissociation of the RARa-corepressor-histone deacetylase complex, the ATRA concentration should be 10-6 mol / l. This significantly exceeds the physiological concentration of retinoic acid (10-9 mol / l), which is required for the dissociation of the complex, which includes the normal RARa receptor. Normally, after binding the ligand (ATRA) to the ligand-binding domain of RARa, corepressors detach (dissociation of the RARa-corepressor-histone deacetylase complex), the RARa receptor configuration changes, resulting in association domains with the TIF1 / TIF2 / CBP transcription coactivators. When APL under conditions of low physiological concentration of retinoic acid, the chimeric PML-RARa protein retains the corepressor deacetylase complex, which slows down the activation process of transcription and blocks the transcription of myeloid differentiation genes.
This block of differentiation can only be removed with a high concentration of retinoic acid, which is achieved during therapy with all-trans retinoic acid (ATRA). The effects of PML-RARa protein are associated not only with the differentiation unit, but also with the regulation of apoptosis and cell growth. Thus, in vitro, cells expressing this protein do not undergo apoptosis in situations where the factors necessary to maintain their viability are removed (serum or granulocyte-macrophage colony-stimulating factor — G-CSF), whereas in the control (if there is no expression of PML-RARa) cells die. From this it follows that this protein maintains the viability of tumor cells by blocking the mechanisms of apoptosis.