Is the epithelial-to-mesenchymal transition clinically relevant for the cancer patient?

Epithelial-to-mesenchymal transition (EMT) is a transdifferentiation process by which a fully differentiated epithelial cell acquires mesenchymal traits, and therefore, mesenchymal abilities such as motility and invasiveness. It is a pivotal physiological process involved in embryogenesis (Type 1 EMT) and in wound healing and tissue remodeling (Type 2 EMT), which, some authors claim, but there are still some controversies, has also been co-opted by tumor cells to increase their malignant potential (Type 3 EMT). Many biomarkers of Type 3 EMT have been characterized and classified into functional categories (i.e., extracellular proteins, cell surface molecules, cytoskeletal markers, transcriptional factors, and, recently, micro RNAs). The extra and intracellular signals that lead to EMT are only starting to be understood, but there is a consensus that Ras and TGF-beta signaling must converge with NF-κB in order to achieve a full EMT. The most classical experimental model is the induction of EMT by TGF-beta in cultures of epithelial cells. Other pathways involving GSK3b, and Wnt/beta-catenin, are also implicated. Ultimately, every EMT-inducing pathway will activate any of the E-cadherin transcriptional repressors (ZEB1, ZEB2, Twist, Snail or Slug). Although in the pre-clinical setting, EMT has also been related to an accelerated tumor progression and to an increased resistance to conventional chemotherapy. In this sense, several groups are beginning to use EMT as a predictive marker of response to treatment. Finally, two chemicals targeting TGF-beta are in clinical trials and many laboratories have initiated studies to use other EMT-related molecules as a therapeutic target for the cancer patient with some modest, but encouraging results.

Protein kinase C delta inhibits the production of proteolytic enzymes in murine mammary cells.

In previous studies we have determined that protein kinase C (PKC) delta, a widely expressed member of the novel PKC serine-threonine kinases, induces in vitro changes associated with the acquisition of a malignant phenotype in NMuMG murine mammary cells. In this study we show that PKCdelta overexpression significantly decreases urokinase-type plasminogen activator (uPA) and matrix metalloproteinase-9 (MMP-9) production, two proteases associated with migratory and invasive capacities. This effect is markedly enhanced by treatment with phorbol 12-myristate 13-acetate (PMA). On the other hand, depletion of PKCdelta using RNAi led to a marked increase in both uPA and MMP-9 secretion, suggesting a physiological role for PKCdelta in controlling protease secretion. The MEK-1 inhibitor PD98059 reverted the characteristic pattern of proteases secretion and phospho-ERK1/2 up-regulation observed in PKCdelta overexpressors, suggesting that the PKCdelta effect is mediated by the MEK/ERK pathway. Our results suggest a dual role for PKCdelta in murine mammary cell cancer progression. While this kinase clearly promotes mitogenesis and favors malignant transformation, it also down-modulates the secretion of proteases probably limiting metastatic dissemination.