This thesis describes a complex signaling system that is based on reversible phosphorylation at tyrosine residues mediated by protein tyrosine kinases (PTK) and subsequent dephosphorylation by protein tyrosine phosphatases (PTP). Both enzymes have important regulatory control over cell growth, proliferation and differentiation and for that reason, imbalanced PTK and/or PTP activity has been linked to various disease states. Multiple studies have assessed the involvement of PTPs in disease and here we describe four PTPs that have been found to colocalize with genetic susceptibility loci connected to type II diabetes, one PTP that has been implicated in autoimmunity, 6 PTPs that have been identified in a mutational analysis and that seem to be involved in colorectal cancer and a total of 8 PTPs that have been specifically implicated in breast cancer. Specifically the role of PTP1B is described in breast tumorigenesis, where this protein has been implicated in both tumorsuppressor activities as well as in oncogenic processes. Research on PTP1B function either in PTP1B deficient mice generated through genetic manipulations or through pharmacological inhibition of the protein, showed that PTP1B deficiency resulted in a delay of ErbB2-induced mammary tumorigenesis and protected from lung metastasis creating strong evidence that PTP1B functions as an oncogene in mammary breast tumorigenesis. To date, it often remains unclear what role PTPs play in tumorigenesis: oncogene or tumorsuppressor. In order to discriminate between the two, additional research on the precise function of these proteins is needed
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