Crosstalk between Hippo and TGFß: Subcellular Localization of YAP/TAZ/Smad Complexes.

The Hippo pathway plays a crucial role in growth control, proliferation and tumor suppression. Activity of the signaling pathway is associated with cell density sensing and tissue organization. Furthermore, the Hippo pathway helps to coordinate cellular processes through crosstalk with growth-factor-mediated signaling pathways such as TGFß. Here we have examined the localization of interactions between proteins of the Hippo pathway (YAP/TAZ) and TGFß (Smad2/3) signaling pathway by using in situ proximity ligation assays. We investigated the formation of protein complexes between YAP/TAZ and Smad2/3 and examined how these interactions were affected by TGFß stimulation and cell density in HaCaT keratinocytes and in Smad4-deficient HT29 colon cancer cells. We demonstrate that TGFß induces formation of YAP/TAZ-Smad2/3 complexes in HaCaT cells. Under sparse cell conditions, the complexes were detected to a higher degree and were predominantly located in the nucleus, while under dense culture conditions, the complexes were fewer and mainly located in the cytoplasm. Surprisingly, we could not detect any YAP/TAZ-Smad2/3 complexes in HT29 cells. To examine if Smad4 deficiency was responsible for the absence of interactions, we treated HaCaT cells with siRNA targeting Smad4. However, we could still observe complex formation in the siRNA-treated cells, suggesting that Smad4 is not essential for the YAP-Smad2/3 interaction. In conclusion, this study shows localized, density-dependent formation of YAP/TAZ-Smad2/3 complexes in HaCaT cells and provides evidence supporting a crosstalk between the Hippo and the TGFß signaling pathways.

Detection, identification and in vivo treatment responsiveness of bone morphogenetic protein (BMP)-activated cell populations in the synovium of patients with rheumatoid arthritis.

OBJECTIVE: To characterise the bone morphogenetic protein (BMP) target cells positive for phosphorylated (P)-SMAD1/5, in rheumatoid arthritis (RA) synovium. METHODS: Synovial biopsies were obtained by needle arthroscopy. Anti-P-SMAD1/5 antibodies were used for Western blot (WB) on protein extracts from RA and normal synovium and for immunostaining of synovial biopsy sections. Positive cells were further identified by double staining for CD3, CD20, CD68, CD138, CD90, alpha smooth muscle actin (SMA), endoglin (CD105) and von Willebrand factor (VWF). In sections from early patients with RA taken before and under antirheumatic treatment, the degree of inflammation and activation of the BMP pathway were quantified. RESULTS: P-SMAD1/5 protein was detected by WB in RA and to a lesser extent in normal synovium. Different P-SMAD1/5 positive cell populations were identified in RA synovium, mainly in perivascular and sublining cells. P-SMAD1/5 positive perivascular cells were alphaSMA positive and located around VWF positive endothelial cells. Some CD90 positive synovial fibroblasts were P-SMAD1/5 positive, as was part of the CD68 positive synovial cells but other cells of the haematopoietic lineage showed no SMAD1/5 phosphorylation. Treatment resulted in an absolute but not relative decrease in BMP activation in the synovium. CONCLUSION: BMP-activated cells belong to distinct stromal compartments in RA synovium and some of them express markers associated with the mesenchymal progenitor cell lineage. Antirheumatic treatment effectively downregulates synovial inflammation, but BMP activation in the synovium does persist albeit reduced.

A sensitive detection of phospho-Smad1/5/8 and Smad2 in Western blot analyses.

The transforming growth factor-beta (TGF-beta) family is involved in a variety of physiological processes, and transmits signals through phosphorylation of Smad by the receptor complexes. In the present study, effects of blocking solution in Western blot analyses on detection of phosphorylated Smad1/5/8 and Smad2 were examined. When EzBlock was used as a blocking reagent, phosphorylated Smad1/8 and Smad2 were most efficiently detected. The anti-phospho-Smad2 antibody specifically recognized the phosphorylated form of Smad2, whereas the anti-phospho-Smad1/5/8 antibody also reacted to the unphosphorylated form. These antibodies did not react with the other Smads.

Reduced endothelial secretion and plasma levels of transforming growth factor-beta1 in patients with hereditary hemorrhagic telangiectasia type 1.

OBJECTIVE: To determine if patients with hereditary hemorrhagic telangiectasia (HHT) show alterations in transforming growth factor (TGF)-beta and its pathways. METHODS: Blood samples were obtained from HHT patients and controls, while endothelial cells were derived from umbilical veins of newborns (HUVEC) from HHT families. TGF-beta1 in plasma, or secreted by HUVEC, and plasma endoglin levels were measured by ELISA. Cellular levels of endoglin and receptor Smad proteins were tested by metabolic labeling and immunoprecipitation, mRNA levels for endoglin and TGF-beta1 by real-time PCR, and receptor Smad phosphorylation by Western blotting. RESULTS: TGF-beta1 and endoglin plasma levels analyzed in 197 individuals showed an inverse correlation with age. Circulating levels of TGF-beta1 were reduced in HHT1 patients (with Endoglin mutations) compared to control, but not in HHT2 patients (with ALK1 mutations). Endoglin levels were unchanged in plasma but decreased in activated monocytes and HUVEC with an HHT1 genotype. These HUVEC also expressed reduced levels of endoglin and TGF-beta1 mRNA, secreted less TGF-beta1, and showed normal receptor Smad expression and phosphorylation. CONCLUSIONS: Decreased plasma TGF-beta1 levels in HHT1 patients correlate with reduced production by endothelial cells. The lower endoglin expression in these cells may alter the regulation of TGF-beta1 via Smad-independent pathways.