XB130, a tissue-specific adaptor protein that couples the RET/PTC oncogenic kinase to PI 3-kinase pathway

Abstract

XB130 is a recently cloned 130 kDa-adaptor protein and Src kinase substrate, structurally similar to actin-filamentassociated protein. Here we show that XB130 is predominantly expressed in the thyroid. Given that XB130 is a thyroid-specific tyrosine kinase substrate, we asked whether it is targeted by RET/PTC, a genetically rearranged, constitutively active, thyroid-specific tyrosine kinase that plays a pathogenic role in papillary thyroid cancer. RET/PTC induced robust tyrosine phosphorylation of XB130, which promoted its subsequent association with the p85a subunit of phosphatidylinositol 3-kinase (PI 3-kinase). We identified tyrosine 54 of XB130 as the major target of RET/PTC-mediated phosphorylation and a critical binding site for the SH2 domains of p85a. Importantly, downregulation of XB130 in TPC1 papillary thyroid cancer cells, harboring the RET/PTC1 kinase, strongly reduced Akt activity without altering ERK1/2 phosphorylation, and concomitantly inhibited cell-cycle progression and survival in suspension. In conclusion, XB130 is a novel substrate of the RET/PTC kinase that links RET/PTC signaling to PI 3-kinase activation, and thereby plays an important role in sustaining proliferation and survival of thyroid tumor cells.

XB130, a Novel Adaptor Protein, Promotes Thyroid Tumor Growth

“…XB130 protein was identified to be expressed in the cytoplasm of thyroid follicular cells in normal thyroid tissue ( Figure 6A), as shown previously. 10 The respective staining also confirmed the presence of XB130 in papillary as well as insular forms of thyroid carcinoma ( Figure 6A). Semiquantitative analysis of cytoplasmic intensity scores demonstrated that the XB130 intensity in papillary thyroid carcinoma and anaplastic/insular carcinoma was significantly lower than that in normal and benign lesion (P Ͻ 0.001), and the intensity in the Hürthle cell carcinoma group was significantly lower than that in the normal group (P Ͻ 0.01) ( Figure 6B).…”

“…11,12 XB130 couples RET/PTC signaling to the phosphatidylinositol 3-kinase (PI3K)/Akt signaling through a specific binding site to the p85␣ subunit of PI3K, thereby regulating cell survival and proliferation. 10 We further found that XB130 has a strong affinity with the lamellipodial F-actin meshwork and is involved in the motility and invasiveness of tumor cells. 13 The objectives of the present study were to investigate whether XB130 is involved in tumor growth in vivo and to identify the pathogenetic molecular mechanisms.…”

“…10 Downregulation of XB130 expression in the TPC1 cell line derived from papillary thyroid carcinoma that carries the RET/PTC-1 rearrangement reduced proliferation and promoted anoikis. 10 The WRO cell line used in the present study also carries a RET/PTC-1 rearrangement (data not shown). Using the in vivo xenograft model we demonstrated that constitutive down-regulation of XB130 reduces growth of tumors derived from WRO cells.…”

“…9,10 Cells were transfected with 50 nmol/L pooled XB130 siRNAs using the oligofectamine reagent (Invitrogen, San Diego, CA), as described previously. 9,10,14 The medium containing siRNA was replaced with fresh medium after 24 hours. The siSTABLE V2 nontargeting siRNA#1 from Dharmacon (Lafayette, CO) was used as a negative control.…”

“…9 Our recent study revealed expression of XB130 in human thyroid tissue, and we found that XB130 is a downstream mediator of the signaling cascade propagated by RET/PTC, a genetically rearranged, constitutively active, thyroid-cancer-specific tyrosine kinase. 10 RET/PTC plays a pathogenic role and exhibits transforming ability by exerting its effects on differentiation and on mitogenic and metastatic potential in papillary thyroid cancer. 11,12 XB130 couples RET/PTC signaling to the phosphatidylinositol 3-kinase (PI3K)/Akt signaling through a specific binding site to the p85␣ subunit of PI3K, thereby regulating cell survival and proliferation.…”