1. ABSTRACT Myosins are proteins that convert chemical energy stored in ATP into mechanical force that is applied on an actin filament. Nuclear myosin 1 (NM1) was the first myosin detected in the cell nucleus. Together with nuclear actin they were shown to play important roles in DNA transcription and chromatin remodeling. However, the molecular details of the NM1 functions are largely unknown. To expand our knowledge about this molecular motor we studied tissue expression, mechanism of nuclear localization and molecular interactions of this myosin motor. In the first part we examined the expression pattern of NM1 in various mouse tissues. We demonstrated that NM1 is present in cell nuclei of all mouse tissues examined except for cells in terminal stages of spermatogenesis. Quantitative PCR and western blots demonstrated that the expression of NM1 in tissues varies, with the highest levels in the lungs. NM1 is a nuclear isoform of earlier identified myosin 1c (Myo1c), which was described initially as a cytosolic, and plasma membrane associated protein. The only known difference between these two proteins was the presence of additional 16 amino acids at the N-terminus of NM1. Next we focused on the influence of NM1 domains, including the N-terminus, on the subcellular localization of this protein. We found...

Tumor suppressor genes play a role as a growth regulator and a gatekeeper of a cell. Their inactivation is often detected in malignant tumors. Identification of novel tumor suppressor gene candidates may help to further understand tumorigenesis and aid in the discovery of a new treatment leading toward cure of cancer. This PhD research project aimed to understand functional significance of a novel tumor suppressor gene candidate, myosin IC (MYO1C) and to identify potential interaction(s) of the MYO1C protein with key components of the signaling pathways involving in cancer development. In an experimental rat model for endometrial carcinoma (EC), detailed molecular genetic analysis of a candidate tumor suppressor region located distal to the tumor protein 53 (Tp53) suggested the myosin IC gene (Myo1c) as the best potential target for deletion of the genetic material. The question arising was whether and how MYO1C could function as a tumor suppressor gene. By using qPCR, Western blot or immunohistochemistry analyses, we examined MYO1C protein level in panels of well-stratified human colorectal cancer (CRC) and EC respectively. We found that MYO1C was significantly down-regulated in these cancer materials and that for the EC panel, the observed down-regulation of MYO1C correlated with tumor stage, where tumors at more advanced stages had less expression of MYO1C. In cell transfection experiments, we found that over-expression of MYO1C significantly decreased cell proliferation, and silencing MYO1C with siRNA increased cell viability. Additionally, knockdown of MYO1C impaired the ability of cells to migrate, spread and adhere to the surface. Recent published studies suggested a potential interplay between MYO1C and the phosphoinositide 3-kinase (PI3K)/AKT pathway. To examine this hypothesis, we analyzed the expression and/or activation of components of the PI3K/AKT and RAS/ERK signaling pathways in vivo in CRC samples, and in vitro in cells transfected with the MYO1C gene expression construct or MYO1C-targeted siRNA. To identify other potential pathways/ mechanisms through which MYO1C may exert its tumor suppressor activity, we additionally performed new sets of MYO1C-siRNA knockdown experiments. At different time points post transfection, we performed microarray global gene expression experiments followed by bioinformatics analysis of the data. Altogether, the results suggested an early PI3K/AKT response to altered MYO1C expression. We additionally identified several cancer-related genes/pathways with late response to MYO1C knockdown. All things considered, the identification of MYO1C-expression impact on cell proliferation, migration, and adhesion in combination with its interplay between several cancer-related genes and signaling pathways provide further evidence for the initial hypothesis of a tumor suppressor activity of MYO1C. / Cellular, Molecular and Functional Characterization of the Tumor Suppressor Candidate MYO1C