CD74-dependent Deregulation of the Tumor Suppressor Scribble in Human Epithelial and Breast Cancer Cells

Abstract

The γ subunit of the major histocompatibility complex (MHC) class II complex, CD74, is overexpressed in a significant proportion of metastatic breast tumors, but the mechanistic foundation and biologic significance of this phenomenon are not fully understood. Here, we show that when CD74 is overexpressed in human cancer and noncancerous epithelial cells, it interacts and interferes with the function of Scribble, a product of a well-known tumor suppressor gene. Furthermore, using epithelial cell lines expressing CD74 under the control of tetracycline-inducible promoter and quantitative high-resolution mass spectrometry, we demonstrate that, as a result of CD74 overexpression, the phosphorylation pattern of the C-terminal part of Scribble undergoes specific changes. This is accompanied with a translocation of the protein from the sites of cell-to-cell contacts at the plasma membrane to the cytoplasm, which is likely to effectively enhance the motility and invasiveness of the cancer cells.

The Scribble–Dlg–Lgl Module in Cell Polarity Regulation

“…Indeed, phosphorylation of Scribble following CD74 amplification in epithelial cancer leads to its phosphorylation and mislocalization to the cytosol (Metodieva et al 2013), Dlg phosphorylation by JNK can regulate its localization in mammalian cells , and Lgl is hyperphosphorylated and mislocalized in glioblastoma in a PTEN-dependent manner (Gont et al 2013). The regulation of Lgl function by phosphorylation is well documented, whereby phosphorylation by aPKC causes conformational changes in Lgl that leads to its dissociation from the cell cortex (Grifoni et al 2013).…”

Studying Interrelation of PDLIM4 with the Model of CD74-Mediated Development of Breast Cancer

“…Mechanisms of CD74-mediated BC formation have been poorly studied; the only existing model today is the concept of Scribble-dependent formation of tumors. According to this model, an increased amount of intracellular CD74 influences functional Scribble activity, the product of the well-known tumor suppressor Scrib, which is crucial for maintaining polarity of epithelial cells (Qin et al, 2005;Metodieva et al, 2013). In humans, Scribble is required for maintaining Ecadherin-dependent intercellular interactions: suppression of Scribble expression, or loss of its activity results in decreased expression of E-cadherin, violation of adhesive contacts and acquisition of the mesenchymal phenotype by the cells, related to high migratory activity and invasiveness (Qin et al, 2005;Su et al, 2012).…”

“…According to a number of researches, an increased CD74 level in cells does not result in considerable decrease of Scribble expression; however, it causes inactivation of this protein (Metodieva et al, 2013). During Scribble interaction with CD74, the status of phosphorylation of the specific sites of the C-terminal site of Scribble changes, which leads to decreased functional activity.…”

“…The active Scribble form is normally located on a basallateral membrane that is required for its ability to inhibit G1/S transition. However, in course of CD74-dependent Scribble inactivation, dephosphorylated protein moves from sections of intercellular contacts to the cytoplasm, which further leads to acquisition of mesenchymal phenotype by the cells (Metodieva et al, 2013). Thus, for analyzing the interrelation of the level of PDLIM4 expression with Scribble activity, it was possible to trace the intracellular localization of Scribble in PDLIM4 (+) and PDLIM4 (-) cells.…”

“…This nature of intracellular localization allows to make a conclusion about the functional activity of the protein: The close-to-membrane localization is characteristic of the phosphorylated form of Scribble and is required for its normal functioning (Metodieva et al, 2013). At the same time, in the cells characterized by high level of PDLIM4 expression, one can see delocalization of Scribble from the locations of intercellular contacts, which indicates the loss of functional activity of the protein.…”