Regulation and Function of the Mitogen-activated Protein Kinase Signaling Module MEK1/2 - ERK1/2 in MDCK Cell Transdifferentiation

As differentiated cells usually maintain their specialised character, gene expression in differentiated cells was long thought to be irreversibly fixed and stable propagated during replication. However, even highly specialised cells can alter their properties due to certain stimuli or environmental influences leading to a deranged control of cell differentiation and/or cell proliferation. In addition, in some conditions such as reparative regeneration, tumongenesis and carcinogenesis, conversion of cellular specificities can frequently be observed. Renal tubulointerstitial fibrosis, for example, is known to be an important contributor to the progression of renal disease and in the kidney several cell types have been implicated in fibrogenesis including tubulointerstitial fibroblasts, vascular adventitial cells, and tubular epithelial cells. Although several factors have been implicated in induction of renal epithelial cell transdifferentiation, little information is available to date about the intracellular signalling molecules involved in this event. Different members of the mitogen-activated protein kinase (MAPK) family play a central role in cellular processes such as growth control, cell differentiation, and response to external stress. Expression of a constitutively active mutant of the MAPK activator MEKI leads to transformation of fibroblasts but neuronal differentiation of PC 12 cells. During the last years a cellular model that would allow direct studies on the importance of single MAPK isoforms for epithelial cell differentiation and/or growth was not available. However, based on our recent studies we were able to establish MDCK-C7 cells as an excellent cellular model for studying intracellular signals involved in the regulation of renal epithelial cell differentiation and proliferation. Moreover, utilising constitutively active MEK1 mutants and a stable transfection approach, we recently demonstrated an association of increased basal and serum-stimulated activity of the MEK1-ERK2 signalling module with epithelial transdifferentiation and growth inhibition in renal MDCK-C7 cells. Thus, the primary objective of the present grant proposal is to continue our in vitro studies of the intracellular signalling events responsible for the regulation of renal epithelial differentiation and proliferation, i.e. the function of distinct members of the MAPK family of protein kinases in MDCK-C7 cell differentiation as well as their effects on the expression of different cytoskeletal proteins, which might be associated with an epithelial to mesenchymal transdifferentiation of these cells. These studies could ultimately lead to the understanding of the pathophysiological mechanisms underlying those renal diseases, which are associated with alterations of the differentiation and/or proliferation of renal epithelial cells.

Although differentiated cells will usually maintain their specialized character, conversion of cellular specificities can be observed during adaptation or reparative regeneration. In pathological conditions, such as inflammation and carcinogenesis, even highly specialized cells can alter their properties leading to a deranged control of cell differentiation and/or proliferation. MAP kinases are central regulators of these processes. As differentiated cells usually maintain their specialized character, gene expression in differentiated cells was long thought to be irreversibly fixed and stably propagated during replication. However, even highly specialized cells can alter their properties due to certain stimuli or environmental influences leading to a deranged control of cell differentiation and/or cell proliferation. In addition, in some conditions such as reparative regeneration, tumorigenesis and carcinogenesis, conversion of cellular specificities can frequently be observed. Renal tubulointerstitial fibrosis, for example, is known to be an important contributor to the progression of renal disease and in the kidney several cell types have been implicated in fibrogenesis. Although several factors have been implicated in induction of renal epithelial cell transdifferentiation, until recently little information was available about the intracellular signaling molecules involved in this event. In FWF Project No. P13295-MED we utilized Madin-Darby canine kidney (MDCK) cell lines to study the role of signaling molecules of the MAP Kinase family in the regulation of renal epithelial cell differentiation, proliferation, and invasion. Evidence from several independent experimental approaches in these cells now suggests that sustained activation of the MEK1-ERK2 signaling module leads to epithelial dedifferentiation, failure of morphogenesis and expression of a highly invasive cell phenotype. By switching the phenotype of once differentiated epithelial cells to dedifferentiated myofibroblasts, tubular epithelial cells are likely to play a pathophysiological role in those renal diseases, which are associated with alterations in epithelial cell differentiation and/or proliferation such as renal fibrogenesis or carcinogenesis.