Role of the metastasis suppressor tetraspanin CD82/KAI 1 in regulation of signalling in breast cancer cells

Four transmembrane domain proteins of the tetraspanin superfamily are the organisers of specific microdomains at the membrane (tetraspanin-enriched microdomains (TERM)) that incorporate various transmembrane receptors and modulate their activities. Tetraspanin CD82 is frequently downregulated or absent in the metastatic cancers. In human prostatic cancer, downregulation of CD82 has been correlated with tumour progression, providing evidence for its role as a metastasis suppressor. We have shown recently that the overexpression of metastasis suppressor tetraspanin CD82/KAI1 led to the attenuation of epidermal growth factor receptor (EGFR) signalling, to an increased internalisation rate of the receptor and to the redistribution of EGFR at the plasma membrane [1]. Moreover, our latest data suggested that the effect of CD82 on the EGFR signalling is mediated by gangliosides [2]. Gangliosides (a subclass of glycosphingolipids) are essential structural components of distinct microdomains at the membrane. In addition, these glycosphingolipids are also involved in the regulation of signalling and tumour progression.

We currently demonstrate that inhibition of the glycosphingolipid biosynthetic pathway with specific inhibitors of glucosylceramide synthase (NB-DGJ and PPMP) resulted in specific weakening of the interactions involving tetraspanin CD82, including CD82–EGFR association. Furthermore, ectopic expression of the plasma membrane-bound sialidase Neu3 in mammary epithelial cells destabilised CD82-containing complexes. The destabilisation of these complexes correlated with the redistribution of the proteins within the plasma membrane. Importantly, depletion of gangliosides affected EGF-induced signalling only in the presence of CD82. Taken together our results provide strong evidence that gangliosides play an important role in supporting the integrity of CD82-enriched microdomains [3]. Furthermore, these data demonstrate that the association between different proteins in TERM in mammary epithelial cells is controlled by distinct mechanisms.

In further experiments we are going to investigate the role of TERM, and specifically CD82-enriched microdomains, in the signalling through the ErbB3 receptor. The ErbB3 receptor is considered a major partner for the ErbB2 receptor and is involved in the progression of breast cancer.