YB-1 is a eukaryotic proteins with numerous intra- and extracellular functions based on its ability to interact with RNA DNA and many proteins. from inhibition of the mTOR signaling pathway with inhibitor PP242 but not rapamycin. Experiments on reporter constructs showed that dependence of mRNA translation on activity of the mTOR signaling pathway was dictated by 5′ untranslated regions of this mRNA irrelatively of the TOP-like sequences at the beginning of 5′ UTR. Introduction YB-1 belongs to proteins with a cold shock domain and performs many functions in the cell (reviewed in [1]). It interacts with many other RNU2AF1 proteins and with DNA and RNA. It acts as a chaperon of nucleic acids [2] thereby restoring their conformational mobility lost upon a temperature decrease. By binding to nucleic acids YB-1 regulates virtually all DNA- and mRNA-dependent events in eukaryotic cells including replication and reparation of DNA [3]-[5] as well as transcription [1] splicing of mRNA [6] [7] and mRNA translation [8]-[11]. In other words it performs both overall and specific regulation of gene expression at differential levels. The amount of YB-1 is especially high in cancer cells rendering it a pronounced marker of tumors [12] [13]. YB-1 translocation through the cytoplasm towards the nucleus stimulates transcription of several genes encoding protecting protein including those in charge of multiple drug level of resistance [14]. When involved with DNA reparation in the nucleus YB-1 plays a part in cell level of resistance against ionizing xenobiotics and rays [15]. Which means nuclear localization of YB-1 is known as to be an early on marker of multiple medication resistance of tumor cells [12] [16]. Its raised focus HOE-S 785026 in the cytoplasm may prevent oncogenic change from the cell due to triggered PI3K/Akt kinase signaling pathway [17]. Besides YB-1 can promote changeover of differentiated epithelial cells into mesenchymal stem types with an increased migration ability permitting their dispersion somewhere else [18]. As demonstrated by independent research an elevated manifestation of YB-1 in tumors correlates with improved tumor dissemination making YB-1 an early marker of metastasis [19]-[21]. Also YB-1 was shown to be able to secret HOE-S 785026 from cells and to bind to Notch receptors on the cell surface thereby stimulating cell proliferation similar to the epidermal growth factor [22] [23]. In spite of considerable achievements in studying YB-1 functions regulation of YB-1 synthesis in the cell remains poorly understood. The reported studies are mostly devoted to regulation of expression at the level of transcription [24]-[28]. There are only a few papers on expression regulation at the post-transcriptional level [29]-[32] although it is quite possible. Here using Western and Northern blotting we detected amounts of YB-1 and mRNA in rabbit organs and in some cultivated cell lines. As found these two values frequently show no correlation with each other in rabbit organs. In cultivated cells they are slightly variable with the bulk of mRNA usually observed in free mRNPs i.e. poorly translated. Using [35S]-Met pulse labeling of cell proteins followed by immunoprecipitation of YB-1 we determined the rate of YB-1 synthesis in the cell. It was shown to depend on conditions affecting the rate of cell division (cell culture density serum starvation). Finally we have shown that inhibition of the mTOR kinase results in specific negative regulation of both endogenous YB-1 synthesis and translation of reporter constructs containing mRNA 5′ UTR. Results Analysis of YB-1 and mRNA Amounts in Cultivated Cells and Rabbit Organs Prior to determining the level of mRNA translation we estimated the amounts of YB-1 and mRNA in several eukaryotic HOE-S 785026 cell cultures and various rabbit organs. The amount of YB-1 was determined using Western blot with antibodies against its C-terminal peptide while Northern blot was used to detect mRNA. As seen from Fig. 1A in total lysates of eukaryotic cell lines YB-1 varies only slightly and amounts to about 20-30 ng per 15 μg of the total protein. The amount of mRNA shows no considerable variability either ranging from 0.4 to 0.6 ng per 10 μg of the total RNA (Fig. 1C). The only exception is amounts of mRNA and YB-1 detected in 3T3 cells where mRNA surpasses the level regular of various other cell lines as the quantity of YB-1 will not approach it. Body 1 Evaluation of YB-1 and mRNA quantities in HOE-S 785026 the cell. Concerning YB-1 and mRNA quantities in rabbit body organ lysates they change from organ to body organ (Fig. 1B). Rabbit.