The reversible phosphorylation of tyrosine residues in proteins, which is governed by the balanced action of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs), is a key element of the signaling pathways that are involved in the control of cell proliferation. Deregulation of either of these key regulators leads to abnormal cell signaling, which is largely associated with human pathologies including cancer. This review focuses on recent studies on the role of the protein tyrosine phosphatase SHP-1 on cell-cycle regulation and its possible roles in tumour onset and progression. SHP-1 is a PTP with two SH2 domains that is expressed in haematopoietic cells and, moderately, in many other cell types, especially malignant epithelial cells. SHP-1 regulates cell proliferation, whether it is by controlling mitogenic pathways activated by receptors with tyrosine kinase activity, or by regulating components of the cell-cycle machinery such as CDK2, p27 and cyclin D1. Since several inhibitors targeting SHP-1 have demonstrated their value in cancer treatment, this phosphatase has been proposed as a therapeutic target for this pathology.

The primary focus of this investigation was to study the relationship between neuroendocrine (NE) differentiation and epidermal growth factor (EGF) because both have been implicated in the progression of prostate cancer. For this purpose, we used gefitinib and trastuzumab, which are inhibitors of EGF receptor (EGFR) and ErbB2, respectively. EGF prevents NE differentiation induced by androgen depletion. This effect is prevented by gefitinib, which blocks the activation of EGFR and ErbB2, stimulation of mitogen-activated protein kinase (MAPK), and cell proliferation induced by EGF. Conversely, trastuzumab does not inhibit the effect of EGF on EGFR phosphorylation, MAPK activity, cell proliferation, and NE differentiation, although it reduces ErbB2 levels specifically, suggesting that ErbB2 is not necessary to inhibit NE differentiation. Prevention of NE differentiation by EGF is mediated by a MAPK-dependent mechanism and requires constitutive Akt activation. The abrogation of the PI3K/Akt pathway changes the role of EGF from inhibitor to inductor of NE differentiation. We show that EGFR tyrosine kinase, MAPK, and PI3K inhibitors inhibit the cell proliferation stimulated by EGF but induce the acquisition of NE phenotype. Altogether, the present data should be borne in mind when designing new clinical schedules for the treatment of prostate cancer, including the use of ErbB receptors and associated signaling pathway inhibitors.

SHP-1, a haematopoietic cell-specific tyrosine phosphatase, is also expressed in human prostate. In this study, we report that SHP-1 depletion in PC-3 cells induced by small interfering RNAs causes G1 phase cell-cycle arrest accompanied by changes in some components of the cellcycle machinery. SHP-1 knockdown increases p27 Kip1 (p27) protein stability, its nuclear localization and p27 gene transcription. These effects could be mediated by PI3K-AKT pathway as SHP-1 interacts with PI3K regulating its activity and p110 catalytic subunit phosphorylation. The increase in p27 protein stability could also because of reduced cyclin-dependent kinase (CDK2) activity. SHP-1 knockdown decreases the CDK6 levels, inducing retinoblastoma protein hypophosphorylation, downregulation of cyclin E and thereby a decrease in the CDK2 activity. However, the codepletion of SHP-1 and p27 does not produce re-entry into the cycle, implying that p27 is not required to maintain cell-cycle arrest induced by SHP-1 depletion. The maintenance of the PC-3 cell antiproliferative response after p27 loss could be because of mislocalization of CDK2 induced by SHP-1 knockdown. This study shows that SHP-1 depletion promotes cellcycle arrest by modulating the activity of cell-cycle regulators and suggests that SHP-1 may be required for the proper functioning of events governing cell-cycle progression.

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A phosphoryl protein tyrosine phosphatase (PTPase) activity has been characterized in rat pancreatic acinar membranes using 32P-labeled poly(Glu,Tyr) as substrate. Acinar membranes exhibited a high affinity for the substrate, with an apparent K , of 0.46 pM and an apparent V,,, of 0.9 nmol . mg protein-. min-Acinar membrane PTPase activity displayed specific characteristics of other PTPases; it was inhibited by the inhibitors Zn2+, orthovanadate and by the divalent cations Mn2+ and Mg2+, and was stimulated by the reducing-agent dithiothreitol. It was also inhibited by soybean trypsin inhibitor and stimulated by trypsin. Gel permeation of pancreatic acinar membranes gave a single peak of enzyme activity with an apparent molecular mass of 70000 Da. Further purification by HPLC on DEAE revealed two peaks of PTPase activity at 120 mM and 180 mM NaCl. These two peaks reacted in a Western-blot procedure with anti-(peptide) serum directed towards conserved domain of PTPase as a common 67-kDa form associated with lower-molecular-mass proteolytic fragments (31 -56 kDa). Incubation of pancreatic acini with somatostatin analogues, SMS 201-995 or BIM 23014, resulted in a stimulation of membrane PTPase activity. The stimulation was rapid and transient, with a maximal level reached within 15 min of addition. The two analogs stimulated PTPase activity in a dose-dependent manner with half-maximal activation occurring at 7 pM and 37 pM and maximal activation at 0.1 nM and 0.1 -1 nM for SMS 201-995 and BIM 23014, respectively. The stimulated-membrane PTPase activity also eluted at an apparent molecular mass of 70 kDa in gel-permeation chromatography. The two analogs inhibited the binding of [1251-Tyr3]SMS 201-995 to pancreatic acinar membranes with similar relative potencies to that observed on stimulation of PTPase activity. We conclude that pancreatic acinar membranes possess a low-molecular-mass PTPase which is stimulated by somatostatin analogs at concentrations involving activation of membrane somatostatin receptors.Protein tyrosine phosphorylation plays a crucial role in cellular regulation of various functions such as proliferation, differentiation and transformation and is controlled by two sets of enzymes; protein tyrosine kinases (PTK) and protein tyrosine phosphatase (PTPase). Whereas the field of PTK has grown explosively over the last 10 years, less is known about PTPase.Recently, a cytosolic PTPase from human placenta (PTPIB) was purified as a 37-kDa soluble protein [l, 21 and its amino acid sequence showed an unexpected similarity to cytoplasmic domains I and I1 of human leukocyte commun antigen CD45 [3], and leukocyte-commun-antigen-related molecule (LAR) [4]. CD45 and LAR have been subsequently shown to have intrinsic PTPase activity in their cytoplasmic domain [5, 61. The two proteins resemble receptor molecules since they have a transmembrane domain, a cytoplasmic domain composed of two repeated PTPase domains and contain different extracellular domains. More recently, a novel recep-