Regulation of Receptor Tyrosine Kinase Signaling by Protein Tyrosine Phosphatase-1B

Abstract

Receptor tyrosine kinases (RTKs) are key regulators of cellular homeostasis. Based on in vitro and ex vivo studies, protein tyrosine phosphatase-1B (PTP1B) was implicated in the regulation of several RTKs, yet mice lacking PTP1B show defects mainly in insulin and leptin receptor signaling. To address this apparent paradox, we studied RTK signaling in primary and immortalized fibroblasts from PTP1B ؊/؊ mice. After growth factor treatment, cells lacking PTP1B exhibit increased and sustained phosphorylation of the epidermal growth factor receptor (EGFR) and the platelet-derived growth factor receptor (PDGFR). However, Erk activation is enhanced only slightly, and there is no increase in Akt activation in PTP1B-deficient cells. Our results show that PTP1B does play a role in regulating EGFR and PDGFR phosphorylation but that other signaling mechanisms can largely compensate for PTP1B deficiency. In-gel phosphatase experiments suggest that other PTPs may help to regulate the EGFR and PDGFR in PTP1Bfibroblasts. This and other compensatory mechanisms prevent widespread, uncontrolled activation of RTKs in the absence of PTP1B and probably explain the relatively mild effects of PTP1B deletion in mice.Regulation of cellular proliferation, adhesion, and migration is pivotal for maintaining homeostasis. Multiple peptide growth factors direct these processes to differing extents in primary fibroblasts and fibroblast cell lines. These growth factors signal via receptors with intrinsic protein-tyrosine kinase activity, termed receptor tyrosine kinases (RTKs).1 Ligand binding activates the intrinsic kinase activity of these receptors, resulting in the phosphorylation of multiple receptor tyrosyl residues. These serve as docking sites to recruit signal relay molecules containing src homology-2 and/or phosphotyrosine binding domains, most of which also are RTK substrates. Based largely on experiments in which wild-type PTPs or their catalytically impaired (dominant-negative) mutants were overexpressed, multiple PTPs, including LAR (7), DEP-1 (8), TC-PTP (9, 10), Shp-1 (11, 12), Shp-2 (13), and PTP1B (14 -17) have been implicated in the dephosphorylation of various RTKs. Which, if any, of these PTPs regulate RTK signaling under physiologically relevant conditions of expression has remained largely unclear. Also unknown is the extent to which RTK dephosphorylation, as opposed to other down-regulatory mechanisms such as RTK degradation and/or inhibitory seryl phosphorylation, plays the (a) key role in receptor inactivation.PTP1B is a widely expressed non-receptor PTP that is localized on intracellular membranes via a hydrophobic C-terminal targeting sequence (18,19). A role for PTP1B in the regulation of many cellular functions has been suggested, including integrin (20 -23), cadherin (24, 25), and cytokine receptor signaling (26 -28), cell cycle regulation (29 -31), and the response to cellular stress (32). Multiple studies indicated that PTP1B dephosphorylates the EGFR (16, 17) and the insulin receptor (IR) (14,(33)(34)(35). Analy...

Protein Tyrosine Phosphatase 1B (PTP1B) Deficiency Accelerates Hepatic Regeneration in Mice

“…30 In this step, PTP1B deficiency increased tyrosine autophosphorylation of EGFR and HGFR in hepatocytes and regenerating liver. This result agrees with the increased phosphorylation of EGFR reported in PTP1B Ϫ/Ϫ fibroblasts, 3 whereas sustained HGFR tyrosine phosphorylation in response to an apoptotic trigger has been described in the livers of PTP1B Ϫ/Ϫ mice. 42 Moreover, our results revealed that PTP1B modulates the early signaling pathways activated downstream of receptors of the tyrosine kinase superfamily in hepatocytes.…”

“…Both HGFR and EGFR are members of the PTP1B substrate family. 3 Thus, we hypothesized that tyrosine phosphorylation of these receptors would be increased immediately after PH in the livers of PTP1B Ϫ/Ϫ mice compared with controls. To test this hypothesis, we performed Western blot analysis with anti-phospho-specific antibodies that recognize the activated forms of EGFR or HGFR.…”

“…1 Protein tyrosine phosphatase 1B is a key regulator of metabolism and cell growth by its ability to dephosphorylate receptors of the tyrosine kinase superfamily and modulate the duration and intensity of the signals emerging from these activated receptors. As demonstrated by genetic and biochemical approaches, PTP1B dephosphorylates and inactivates the epidermal growth factor receptor (EGFR), 2 the platelet-derived growth factor receptor, 3 the hepatocyte growth factor receptor (HGFR), 4 the insulin receptor (IR), 5 and the insulin-like growth factor-1 receptor. 6 All of them are implicated in the control of proliferation, survival, and/or metabolic functions of hepatocytes 7,8 and many other cellular models.…”

The epidermal growth factor receptor family: Biology driving targeted therapeutics

“…Tyrosine dephosphorylation of active ErbB receptors can occur through phosphatases, including density-enhanced phosphatase-1 (DEP1) [80] and protein tyrosine phosphatase PTP1B [81]. Dephosphorylation has the capacity to down-regulate activated receptors through abolishment of binding sites for signaling intermediates and adaptor proteins.…”

Glucose lowers the threshold for human aortic vascular smooth muscle cell migration: inhibition by protein phosphatase-2A

“…Since HG increases polyol pathway flux and activates PKCβ II [8,28], the influence of this pathway and PKCβ II on receptor level were investigated. A major finding was that at the higher PDGF-BB concentration (1,800 pmol/l) in HG, PKC inhibitors (calphostin C: general PKC inhibitor; or LY379196: PKCβ II -specific inhibitor at 200 nmol/l [8]) or AR inhibitors (alrestatin and sorbinil) increased receptor levels, eliminating the difference between NG and HG.…”