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The mechanisms that enable cells to regulate their gene expression and thus their metabolism, proliferation or cellular behaviour are not only important to understand the basic biology of a living cell, but are also of crucial interest in cancerogenesis. Highly interwoven and tightly regulated pathways are the basis of a robust but also flexible regulatory network. Interference with these pathways can be either causative for tumorigenesis or can modify its outcome. The receptor tyrosine kinase (RTK) and RAS dependent pathways leading to AKT or ERK1/2 activation are of particular interest in melanoma. These signaling modules are commonly activated by different mutations that can be found in various pathway components like NRAS, BRAF or PTEN. The first part of this work deals with the diverse and versatile functions of the ERK1/2 pathway feedbackregulator MKP2 in different cellular, melanoma relevant settings. In addition, a functional role of the AP1-complex member FOSL1, an ERK1/2 transcriptional target being implicated in the regulation of proliferation, is demonstrated. Secondly, aspects of direct pharmacological inhibition of the ERK1/2 pathway with regard to the induction of apoptosis have been analysed. Due to the high frequency of melanoma related mutations occurring in the RAS/RAF/MEK/ERK pathway (e.g. NRASQ61K, BRAFV600E), inhibition of this signaling cascade is deemed to be a promising therapeutic strategy for the treatment of malignant melanoma. However, although in clinical trials mono-therapeutic treatment with MEK- or RAF inhibitors was successful in the short run, it failed to show satisfactory long-lasting effects. Hence, combination therapies using a MAPK pathway inhibitor and an additional therapy are currently under investigation. I was able to demonstrate that inhibition of MEK using the highly specific inhibitor PD184352 can have a protective effect on melanoma cells with regard to their susceptibility towards the apoptosis inducing agent cisplatin. Single application of cisplatin led to strong DNA damage and the induction of caspase-dependent apoptosis. Additional administration of the MEK inhibitor, however, strongly reduced the apoptosis inducing effect of cisplatin in several melanoma cell lines, These cells displayed an increased activation of the serine/threonine kinase AKT after MEK inhibition. This AKT activation concomitantly led to the phosphorylation of FOXO transcription factors, attenuating the cisplatin induced expression of the BH3-only protein PUMA. PUMA in turn was important to mediate the apoptosis machinery after cisplatin treatment. My results also indicate a participation of RTKs, in particular EGFR, in mediating MEK inhibitor induced activation of AKT. These results demonstrate that inhibition of the RAS/RAF/MEK/ERK signaling pathway in melanoma cell lines does not necessilary have favourable effects in a cytotoxic co-treatment situation. Instead, it can even enhance melanoma survival under pro-apoptotic conditions.
Mathematical optimization framework allows the identification of certain nodes within a signaling network. In this work, we analyzed the complex extracellular-signal-regulated kinase 1 and 2 (ERK1/2) cascade in cardiomyocytes using the framework to find efficient adjustment screws for this cascade that is important for cardiomyocyte survival and maladaptive heart muscle growth. We modeled optimal pharmacological intervention points that are beneficial for the heart, but avoid the occurrence of a maladaptive ERK1/2 modification, the autophosphorylation of ERK at threonine 188 (ERK\(^{Thr188}\) phosphorylation), which causes cardiac hypertrophy. For this purpose, a network of a cardiomyocyte that was fitted to experimental data was equipped with external stimuli that model the pharmacological intervention points. Specifically, two situations were considered. In the first one, the cardiomyocyte was driven to a desired expression level with different treatment strategies. These strategies were quantified with respect to beneficial effects and maleficent side effects and then which one is the best treatment strategy was evaluated. In the second situation, it was shown how to model constitutively activated pathways and how to identify drug targets to obtain a desired activity level that is associated with a healthy state and in contrast to the maleficent expression pattern caused by the constitutively activated pathway. An implementation of the algorithms used for the calculations is also presented in this paper, which simplifies the application of the presented framework for drug targeting, optimal drug combinations and the systematic and automatic search for pharmacological intervention points. The codes were designed such that they can be combined with any mathematical model given by ordinary differential equations.