| The homeobox transcription factor Prox1 is known to be a master control gene for regulating lymphatic system formation during early embryonic development. This gene also plays an essential role in the development of other organs such as the liver, brain and eye. In addition to its physiological role, recent studies have revealed that Prox1 expression is lost in various tumor types due to epigenetic silencing or gene mutation. These findings have suggested a novel role for Prox1 as a candidate tumor suppressor gene; however, the underlying molecular mechanism remains to be described.;In the present work, we define the function of Prox1 as a tumor suppressor gene using papillary thyroid carcinoma (PTC) as a model system. We show that Prox1 is significantly downregulated in PTC at the level of gene expression, while the reduction in protein levels is relatively modest. We also demonstrate that the subcellular distribution of Prox1 is dramatically altered in PTC; while it is confined to the nucleus of normal thyroid epithelial cells, PTC cells display a diffuse, nucleocytoplasmic staining pattern for Prox1. Furthermore, we show that cytoplasmic mislocalization confers increased stability upon the Prox1 protein and explains why Prox1 protein levels exhibit only a slight reduction in PTC despite the notable decrease in mRNA levels.;We have investigated the underlying mechanism responsible for Prox1 protein mislocalization and show that it may be mediated via interactions with the thyroid hormone receptor. In addition, we have also determined that Prox1 gene repression may be due to upregulation of the Notch effector molecule Hey2 in PTC. We have defined the molecular basis of the tumor suppressive role of Prox1 by showing that it is capable of regulating cell proliferation and cell migration via modulation of its target genes p57kip2, ICAM-1 and uPAR.;In summary, we have characterized alterations in Prox1 expression and localization in a novel system, papillary thyroid carcinoma. We have identified pathways that may be responsible for gene repression and protein mislocalization, and have also demonstrated the functional consequences of protein mislocalization in terms of enhanced stability. We show that Prox1 is able to function as a tumor suppressor by its effects on specific cell proliferation and cell migration-associated genes. We also show that control of subcellular localization is a novel mechanism for inactivating Prox1 function in cancer. |
