Pleiotrophin regulates serine phosphorylation and the cellular distribution of β-adducin through activation of protein kinase C

Abstract

Pleiotrophin (PTN) was found to regulate tyrosine phosphorylation of ␤-adducin through the PTN͞receptor protein tyrosine phosphatase (RPTP)␤͞ signaling pathway. We now demonstrate that PTN stimulates the phosphorylation of serines 713 and 726 in the myristoylated alanine-rich protein kinase (PK) C substrate domain of ␤-adducin through activation of either PKC ␣ or ␤. We also demonstrate that PTN stimulates translocation of phosphoserine 713 and 726 ␤-adducin either to nuclei, where it associates with nuclear chromatin and with centrioles of dividing cells, or to a membrane-associated site, depending on the phase of cell growth. Furthermore, we demonstrate that PTN stimulates the degradation of ␤-adducin in PTN-stimulated cells. Phosphorylation of serines 713 and 726 in ␤-adducin is known to markedly reduce the affinity of ␤-adducin for spectrin and actin and to uncouple actin͞spectrin͞ ␤-adducin multimeric complexes needed for cytoskeletal stability. The Ptn gene also is a proto-oncogene (12, 13). Its high-level importance in human malignancies is suggested because expression of the endogenous Ptn gene is seen frequently in many different highly malignant human neoplasms (14-22), and introduction of a dominant negative Ptn gene or targeted ribozymes into cells from malignancies with constitutive expression of the Ptn gene reverses the malignant phenotype to the phenotype of the premalignant cells both in vivo and in vitro, indicating that PTN signaling is responsible for the ''switch'' of the premalignant cell to the highly malignant phenotype (11,12,21,23,24).Recent studies have uncovered clues to the mechanisms by which PTN initiates this striking diversity of phenotypes in PTN-stimulated cells. Pleiotrophin signals through two functionally and structurally independent signaling domains (refs. 5, 11, and 12 and N. Zhang, R. Zhong, Z. Y. Wang, and T.F.D., unpublished data), each of which binds heparin (25), and each of which signals very different phenotypes. The N-terminal domain, when expressed with the endogenous signal peptide, transforms murine fibroblasts, whereas the C-terminal domain does not; however, the C-terminal domain of PTN induces rapid growth and a striking angiogenic phenotype when it is expressed with the endogenous signal peptide in the premalignant cell. Thus, the separate domains of PTN function independently of each other, signal different phenotypes, and, a priori, need to signal through two separate receptor-like proteins (5,11,12).One receptor that initiates PTN signaling is the transmembrane receptor protein tyrosine phosphatase (RPTP) ␤͞. The interaction of PTN with RPTP␤͞ induces receptor dimerization and inactivates the endogenous tyrosine phosphatase activity of RPTP␤͞, thereby disrupting the balanced activity of RPTP␤͞ and an unknown but constitutively active tyrosine kinase(s) on the mutual substrates of RPTP␤͞ and the tyrosine kinase and thus initiating a sharp and rapid increase in the steady state levels of tyrosine phosphorylation (26). The first substrate of RPTP␤͞ to be id...

Pleiotrophin disrupts calcium-dependent homophilic cell–cell adhesion and initiates an epithelial–mesenchymal transition

“…␤-adducin also was identified as a target of the PTN͞RPTP␤͞ -signaling pathway (3,4), and PTN disrupts ␤-adducin-actin-spectrin complexes supporting cystoskeletal stability.…”

“…In these cases, because the malignant cells constitutively express Ptn, the malignant cells have a ''stable EMT'' or perhaps more appropriately, an ''arrested EMT.'' Pleiotrophin signals through inactivation of the endogenous protein tyrosine phosphatase activity of RPTP␤͞ ; it increases tyrosine phosphorylation of the different substrates of RPTP␤͞ through the persistent activity of tyrosine kinases acting at the same sites as RPTP␤͞ (2)(3)(4)(5). Pleiotrophin was the first natural ligand to be discovered for this class of receptor-type transmembrane tyrosine phosphatases (2) and the PTN͞RPTP␤͞ signaling pathway triggered by PTN is unique.…”

“…2, PTN induced a rapid increase in tyrosine phosphorylation of ␤-catenin when U373 cells were stimulated with PTN at concentrations from 0 to 10 ng͞ml and slightly higher levels of tyrosine phosphorylation were seen as the concentration of PTN was increased to 25 ng͞ml. In cells stimulated with 50 and 100 ng͞ml PTN, concentrations of PTN previously found to be in excess of saturating concentrations (2,3), the levels of tyrosine phosphorylation of ␤-catenin fell somewhat (Fig. 1).…”

“…The diverse substrates of the receptor protein tyrosine phosphatase (RPTP)␤͞ (2) include ␤-catenin (2), ␤-adducin (3,4), Fyn (5), GIT1͞Cat-1 (6), and P190RhoGAP (7), indicating that RPTP␤͞ is promiscuous in substrate specificity, but through its activity, is critically positioned to coordinately regulate the steady-state levels of tyrosine phosphorylation of proteins in different signaling networks and cellular systems. Pleiotrophin (PTN the protein and Ptn the gene) is a secreted, highly conserved cytokine (8,9) that signals through inactivation of RPTP␤͞ ; as a consequence, PTN coordinately increases tyrosine phosphorylation of the many substrates of RPTP␤͞ through persistent activity of the tyrosine kinases that phosphorylate the same sites that are dephosphorylated by RPTP␤͞ in cells not stimulated by PTN.…”

Anaplastic lymphoma kinase is expressed in different subtypes of human breast cancer

“…However, these data were not reconciled with earlier studies that demonstrated that the Receptor Protein Tyrosine Phosphatase (RPTP) β/ζ is the functional receptor of PTN [15]; in those earlier studies, PTN was shown to inactivate RPTPβ/ζ and to increase tyrosine phosphorylation of the substrates of RPTPβ/ζ, which results from phosphorylation of these substrates by kinases that phosphorylate the same sites dephosphorylated by RPTPβ/ζ when cells are not stimulated by PTN. The levels of tyrosine phosphorylation of β-catenin [15] and of other substrates of RPTPβ/ζ, including β-adducin [16,17], Fyn [18], GIT1/Cat-1 [19], and P190RhoGAP [20], all are increased in PTNstimulated cells.…”