Regulation of Telomerase Activity by Rab5 Guanine Nucleotide Exchange Factors

Many cancerous cells display abnormalities in the signal transduction pathways responsible for responding to extracellular growth factors. Growth factors, such as insulin-like growth factor I, represent a major class of mitogenic ligands that can initiate the mitogen activated protein kinase (MAPK) pathway. The role of the MAPK pathway in transducing growth signals to the interior of the cell and subsequently stimulating cell growth and proliferation is highlighted by the fact that roughly one quarter of all human tumors contain mutant forms of Ras proteins. Ras interference 1 (Rin1) is involved in key steps of receptor mediated endocytosis and can potentially moderate signaling through the MAPK pathway. The possible connection between Rin1, an effector of the active form of Ras, and subsequent telomerase gene expression and activity is of particular interest as telomerase is active in roughly 85% of all cancers. In this study, we determine the effect of the expression of Rin1 on cellular proliferation as well as on telomerase gene expression and activity in several different human cancer cells. MDA-MB 231, MCF7, and MCF-12A breast cell lines expressing Rin1 exhibit reduced levels of proliferation of up to 31% following exposure to the IGF-1 growth factor. Telomerase activity and gene expression were also reduced by factors of up to 1.5 and 2.55 fold in the three breast cell lines studied when compared to control cells. Similarly, overexpression of Rin1 in human Yusik melanoma cells leads to reduced telomerase activity. Telomerase activity was determined by the telomeric repeat amplification protocol (TRAP) assay while gene expression was measured by RT-qPCR. Furthermore, our observations suggest that overexpression of the Rin1 Y561F mutant and Rin1 delta splice variant in MDA-MB 231 cells results in increased proliferation and telomerase activity in these cancer cells when compared to control cells. MDA-MB 231 cells expressing wild type Rin1 displayed lowered levels of phosphorylation for the p-44/42 (ERK), STAT3, and Ets2 transcription factors. Additionally, overexpression of the C-terminus region of Rin1 in these cells greatly reduced ERK phosphorylation. In summary, Rin1 may play a novel tumor suppressor role in modulating signaling through the Ras/MAPK pathway upon IGF-1 stimulation

A hypothesis for a novel role of RIN1-the modulation of telomerase function by the MAPK signaling pathway

Cancerous cells display abnormalities in the signal transduction pathways responsible for responding to extracellular growth factors, or mitogens. Mutations that alter proteins involved in these types of pathways can lead to inappropriate or unregulated cell growth, and therefore predispose the cell to become malignant. The critical role of the Ras/mitogen-activated protein kinase (MAPK) pathway in transducing growth signals to the interior of the cell and subsequently stimulating cell growth and proliferation is underscored by the fact that roughly one quarter of all human tumors contain mutant forms of Ras proteins. A particular focus on the signaling and membrane trafficking adaptor protein known as Ras interference 1 (RIN1) will reveal how this protein can potentially play a significant role in the development of the cancerous phenotype in certain cell types. Of equal interest is the possible connection between the Ras/MAPK pathway, and subsequent expression and enzymatic activity of telomerase-a key enzyme known to be overexpressed in roughly 85% of all cancers. RIN1 is a 783 amino acid (84 kDa) cytosolic protein that is involved in key steps of growth factor receptor-mediated endocytosis and can potentially moderate signaling through the MAPK pathways. RIN1, with its unique ability to compete directly with Raf for activation by Ras, could potentially influence signaling through more than one of the MAPK pathways. If so, RIN1 may then be able to exert a precise and selective effect on the downstream signal(s) of a MAPK target such as telomerase