Melanoma originates in the epidermis and becomes metastatic after invasion into the dermis. Prior interactions between melanoma cells and dermis are poorly studied. Here, we show that melanoma cells directly affect the formation of the dermal tumour niche by microRNA trafficking before invasion. Melanocytes, cells of melanoma origin, are specialized in releasing pigment vesicles, termed melanosomes. In melanoma in situ, we found melanosome markers in distal fibroblasts before melanoma invasion. The melanosomes carry microRNAs into primary fibroblasts triggering changes, including increased proliferation, migration and pro-inflammatory gene expression, all known features of cancer-associated fibroblasts (CAFs). Specifically, melanosomal microRNA-211 directly targets IGF2R and leads to MAPK signalling activation, which reciprocally encourages melanoma growth. Melanosome release inhibitor prevented CAF formation. Since the first interaction of melanoma cells with blood vessels occurs in the dermis, our data suggest an opportunity to block melanoma invasion by preventing the formation of the dermal tumour niche.

The effector protein of the canonical Wnt pathway is β-catenin, which is regulated by the ubiquitin system. This study shows that the E3 ubiquitin ligase EDD ubiquitinates β-catenin, leading to up-regulation of β-catenin's expression levels and activity. Thus the results demonstrate a role for the ubiquitin system in up-regulation of the Wnt pathway.

Adenomatous polyposis coli (APC) is a multifunctional tumour suppressor protein that negatively regulates the Wnt signalling pathway. The APC gene is ubiquitously expressed in tissues and organs, including the large intestine and central nervous system. The majority of patients with sporadic and hereditary colorectal cancer have mutations in the gene encoding APC. Approximately 30% of these mutations are single nucleotide changes that result in premature stop codons (nonsense mutations). A potential therapeutic approach for treatment of this subset of patients is the use of aminoglycosides and macrolides that induce nonsense mutation read-through and restore levels of full-length protein. We have used reporter plasmids and colorectal cancer cell lines to demonstrate that several aminoglycosides and tylosin, a member of the macrolide family, induced read-through of nonsense mutations in the APC gene. In xenograft experiments and in the Apc(Min/+) mouse model, these compounds ameliorated the tumorigenic clinical symptoms caused by nonsense mutations in the APC gene.

Members of the low density lipoprotein receptor family (LDLR), LRP5/6, were shown to interact with the Frizzled (Fz) receptors and to function as Wnt coreceptors. Here we show that mLRP4T100, a minireceptor of LRP1, another member of the LDLR family, interacts with the human Fz-1 (HFz1), previously shown to serve as a receptor transmitting the canonical Wnt-3a-induced signaling cascade. However, in contrast to LRP5/6, mLRP4T100, as well as the full-length LRP1, did not cooperate with HFz1 in transmitting the Wnt-3a signaling but rather repressed it. mLRP4T100 inhibitory effect was displayed also by endocytosis-defective mLRP4T100 mutants, suggesting that LRP1 repressive effect is not attributable to LRP1-mediated enhanced HFz1 internalization and subsequent degradation. Enforced expression of mLRP4T100 decreased the capacity of HFz1 cysteine-rich domain (CRD) to interact with LRP6, in contrast to HFz1-CRD/Wnt-3a interaction that was not disrupted by overexpressing mLRP4T100. These data suggest that LRP1, by sequestering HFz1, disrupts the receptor/coreceptor complex formation, leading to the repression of the canonical Wnt signaling. Thus, this study implies that the ability to interact with Fz receptors is shared by several members of the LDLR family. However, whereas some members of the LDLR family, such as LRP5/6, interact with Fz and serve as Wnt coreceptors, others negatively regulate Wnt signaling, presumably by sequestering Fz. The low density lipoprotein receptor (LDLR)1 related protein-1 (LRP1), a member of the LDLR endocytic receptor family, is a large multifunctional cysteine-rich receptor that binds and endocytoses a variety of structurally and functionally distinct ligands (1, 2). The LDLR family comprises two major subfamilies: one comprised of "small" receptor members of ϳ120 kDa (the LDL receptor, apoE receptor-2, and the very low density lipoprotein receptor) and a second one encompassing "large" receptor members of ϳ600 kDa (LRP1, LRP1B, and LRP2 (Megalin)). In addition, two less related members, LRP5 and LRP6, were recently identified as Wnt coreceptors (3-6).LRP1 is synthesized as a single chain precursor that undergoes post-translational proteolytic processing within the transGolgi compartment by a furin-like protease (7). This results in the formation of mature LRP1 as a noncovalently associated heterodimer, consisting of an extracellular 515-kDa subunit and a transmembrane 85-kDa subunit with a transmembrane domain and a cytoplasmic tail.

Adenomatous polyposis coli (APC) is a multifunctional tumor suppressor protein that negatively regulates the Wnt signaling pathway. The APC gene is ubiquitously expressed in various tissues, especially throughout the large intestine and central nervous system. Mutations in the gene encoding APC have been found in most colorectal cancers and in other types of cancer. The APC gene product is a large multidomain protein that interacts with a variety of proteins, many of which bind to the well conserved armadillo repeat domain of APC. Through its binding partners, APC affects a large number of important cellular processes, including cell-cell adhesion, cell migration, organization of the actin and microtubule cytoskeletons, spindle formation and chromosome segregation. The molecular mechanisms that control these diverse APC functions are only partly understood. Here we describe the identification of an additional APC armadillo repeat binding partner - the Striatin protein. The Striatin family members are multidomain molecules that are mainly neuronal and are thought to function as scaffolds. We have found that Striatin is expressed in epithelial cells and co-localizes with APC in the epithelial tight junction compartment and in neurite tips of PC12 cells. The junctional localization of APC and Striatin is actin-dependent. Depletion of APC or Striatin affected the localization of the tight junction protein ZO-1 and altered the organization of F-actin. These results raise the possibility that the contribution of APC to cell-cell adhesion may be through interaction with Striatin in the tight junction compartment of epithelial cells.