Cell surface expression of receptor protein tyrosine phosphatase RPTP mu is regulated by cell-cell contact.

SHP-2 is an intracellular SH2 domain-containing protein-tyrosine phosphatase with an essential role in cell signaling. Here we demonstrate that localization of SHP-2 is regulated by cell density in a cell adhesion-dependent manner. When cells were plated at low densities, SHP-2 was distributed in Triton X-100-insoluble fractions, whereas it was totally soluble when cells were plated at high densities or when low density cells approached confluency. In all cases, the total protein level of SHP-2 was not changed. Fluorescent cell staining revealed that SHP-2 was co-localized with actin stress fibers to the cell peripheral at low cell densities but was diffused in the entire cytoplasm at high cell densities. Transient transfection of cells with truncated forms of SHP-2 demonstrated that the catalytic domain of the enzyme was responsible for the density-regulated distribution of SHP-2, but the catalytic activity was not required. An in vitro co-sedimentation study demonstrated direct binding of full-length and SH2 domaintruncated forms of SHP-2 to F-actin. The data indicate that SHP-2 is regulated by cell density and that it may have a role in assembling and disassembling of the actin network.

Section: Discussionmentioning

confidence: 99%

Association of Tyrosine Phosphatase SHP-2 with F-actin at Low Cell Densities

Zhao

Peng

et al. 2001

“…1A). Computer-based scanning of full-length pVHL30 indicated multiple potential phosphorylation sites over the entire open reading frame and three predicted CK2 phosphorylation sites in the acidic domain at Ser 33 , Ser…”

Section: Resultsmentioning

confidence: 99%

Tumor Suppression by the von Hippel-Lindau Protein Requires Phosphorylation of the Acidic Domain

Lolkema

Gervais

Snijckers

et al. 2005

The tumor suppressor function of the von HippelLindau protein (pVHL) has previously been linked to its role in regulating hypoxia-inducible factor levels. However, VHL gene mutations suggest a hypoxia-inducible factor-independent function for the N-terminal acidic domain in tumor suppression. Here, we report that phosphorylation of the N-terminal acidic domain of pVHL by casein kinase-2 is essential for its tumor suppressor function. This post-translational modification did not affect the levels of hypoxia-inducible factor; however, it did change the binding of pVHL to another known binding partner, fibronectin. Cells expressing phospho-defective mutants caused improper fibronectin matrix deposition and demonstrated retarded tumor formation in mice. We propose that phosphorylation of the acidic domain plays a role in the regulation of proper fibronectin matrix deposition and that this may be relevant for the development of VHL-associated malignancies.

“…Earlier studies have demonstrated increased total PTP activity in lysates or membrane fractions derived from high density cultures, as well as increased levels of different transmembrane PTPs (3)(4)(5)(6)(7)(8). However, no density-dependent differences in the net levels of tyrosine phosphorylation in vivo of key signaling molecules have been demonstrated.…”

Section: Ptp-mediated Differences In Pdgf-induced Tyrosine Phosphorylmentioning

confidence: 99%

“…Cell density-dependent up-regulation of the receptor-like PTPs density-enhanced phosphatase-1 and PTP-at the protein level has been demonstrated (6,7). Comparison of mRNA levels of PTPs in growing and contact-inhibited cells also revealed higher mRNA levels for PTP-in dense cells (8).…”

mentioning

confidence: 92%

Protein-tyrosine Phosphatase-mediated Decrease of Epidermal Growth Factor and Platelet-derived Growth Factor Receptor Tyrosine Phosphorylation in High Cell Density Cultures

Sörby

Östman

1996

Contact-induced growth inhibition is a characteristic feature of normal cells grown in monolayer. The importance of reversible tyrosine phosphorylation in mitogenic signaling, together with earlier reports of increased levels of protein-tyrosine phosphatases (PTPs) in densely cultured cells, has led to the proposal that PTPs may be involved in mediating contact inhibition of cell growth. We have compared net levels of ligand-induced tyrosine phosphorylation of the epidermal growth factor (EGF) receptor in mink lung epithelial cells cultured under sparse or dense conditions. The levels of net tyrosine phosphorylation of the stimulated EGF receptor was found to be more than 4-fold higher in sparse cultures. This difference was greatly reduced when cells were pretreated with the PTP inhibitor phenyl arsine oxide. Monitoring of dephosphorylation rates in vivo of the stimulated EGF receptors revealed increased EGF receptor-directed PTP activity in dense cultures. The platelet-derived growth factor ␤-receptor, expressed in stably transfected porcine aortic endothelial cells, also displayed lower levels of ligand induced net tyrosine phosphorylation in cells from dense cultures. This density-dependent difference in tyrosine phosphorylation was reduced by pretreatment of cultures with the PTP inhibitor orthovanadate. A PTP-mediated decrease of the in vivo net levels of ligand induced tyrosine phosphorylation of EGF and plateletderived growth factor receptors in cells at high density have thus been demonstrated. Loss of this previously unnoticed regulatory pathway may be involved in cellular transformation.Contact-induced growth inhibition is a characteristic feature of normal cells grown in monolayer. This negative growth regulatory mechanism is lost in many tumor cells. The molecular events underlying contact inhibition remain largely unknown. The importance of reversible tyrosine phosphorylation in growth factor-induced mitogenic signaling, together with the potential of protein-tyrosine phosphatases (PTPs) 1 to antagonize tyrosine kinase signaling, has led to the proposal that PTPs may be involved in contact inhibition (for a recent review see Ref. 1). The finding that treatment of normal rat kidney cells with the PTP inhibitor orthovanadate relieved cells from contact inhibition of cell growth provided early experimental support for this hypothesis (2). During recent years additional data have been obtained that support this idea.Increased PTP activity in membrane fractions or cell lysates from cells harvested at high cell densities has been reported (3-5). Cell density-dependent up-regulation of the receptor-like PTPs density-enhanced phosphatase-1 and PTP-at the protein level has been demonstrated (6, 7). Comparison of mRNA levels of PTPs in growing and contact-inhibited cells also revealed higher mRNA levels for PTP-in dense cells (8). The demonstration of specific homophilic interactions of the extracellular domains of the transmembrane PTP-and -also points toward a role in cell contact-induced signaling (9 -11)...