Untersuchungen zur Bedeutung der Peptidyl-Prolyl-Isomerase Pin1 bei der Pathogenese von Kopf-Hals-Tumoren

Squamous cell carcinoma of the head and neck (HNSCC) represent world-wide the eighth most common tumor entity. The 5-year-survival-rate for all tumor stages collectively is below 50%. Loco-regional control is the central aim for the treatment of this disease. Unfortunately most patients are diagnosed at advanced disease stages and have to be treated with chemo- and radiation therapy. In order to improve disease outcome of HNSCC patients it is essential to discover new target proteins embedded in oncogenesis that could possibly be treated by novel therapeutics. Pin1 is a peptidyl-prolyl isomerase (PPIase) belonging to the subfamily of the so-called parvulines. It is characterized by its unique substrate specifitiy, binding exclusively phosphorylated Ser/Thr-Pro residues. Because of its high substrate specifity Pin1 is linked to many cell cycle kinases that play a major role in oncogenesis. Pin1 is the only PPIase being essential for cell cycle function. It was found to be overexpressed in many human tumors, some of them expressing the unphosphorylated probably active form of the protein. Expression status of Pin1 at the protein and mRNA were assessed by western blot analyses, immunohistochemistry, immunocytochemistry and quantitative RT-PCR in 17 HNSCC cell lines and 41 HNSCC tissues-samples. The results were compared to normal mucosa of the oral cavity as a control. Isoelectric variants of Pin1 were observed after isoelectric focussing and 2D gel electrophoresis. Thus, the phosphorylation status of the PPIase could be defined in tumor tissues and compared to normal tissues. Furthermore, the effect of Pin1 inhibition with its naturally existing inhibitor Juglone as well as with Pin1-specific siRNA was examined by FACS analysis. In the analyzed HNSCC tumor tissues and cell lines Pin1 expression levels were found upregulated compared to normal control tissue samples. In addition, Pin1 was found hypophosphorylated and thus probably overactivated in tumor tissues in comparison to normal mucosa. Inhibition of Pin1 via juglone and via Pin1- specific siRNA showed increase in dead cells at cell cycle analysis. The results point to Pin1 as a key-player in carcinogenesis of head and neck squamous cell cancer. The PPIase was found overexpressed and is present typically in its active unphosphorylated form. However, no significant correlation between Pin1 expression levels and clinical data could be found. It still has to be investigated why Pin1 is frequently dephosphorylated in HNSCC tissues. No obvious increase of programmed cell death has been found in HNSCC cells following Pin1 inhibition with juglone and transfection with Pin1-specific siRNA but the fraction of cells in the sub-G1 phase of the cell cycle was increased. It will be a subject of further studies to discover what mechanisms lead to reduced Pin1 phosphorylation in tumor cells and which pathways could replace inhibited Pin1 function in a tumor cells in order to avoid apoptosis. Moreover, further exploration is necessary to define the role of Pin1 as a useful target for HNSCC therapy.