Protein Dephosphorylation and Protein Phosphorylation

FIG 21.5  Catalytic mechanism of tyrosine phosphatases (PTP1B). In the first step, there is transfer of an electron from C215 to the substrate phosphate. The catalytic cysteine has a low pKa (5.4) so that the sulfydryl group acts as a nucleophile (–S ).  This is coupled with protonation of the tyrosyl-leaving

group by the side chain of the conserved D181, leading to the formation of a cysteinyl-phosphate intermediate. In the second step involving Q262 and D181 there is hydrolysis of the catalytic intermediate with release of phosphate. The placement of the essential residues at the active site is depicted on the right. Note that the phosphotyrosine substrate is aligned and sandwiched between Y45 and F180. Y45 determines the depth of the catalytic cleft and is the major determinant of specificity for phosphotyrosine (1ptt10).

of counteracting the transforming effects of mutated, and therefore constitutively activated, PTKs and were therefore considered as possible tumour suppressors. Thus it came as quite a surprise when it was found that some PTPs, rather than opposing the actions of the kinases, actually cooperate with them to reinforce their signals. As an example, over-expression of PTP in rat embryo fibroblasts causes persistent activation of Src with concomitant cell transformation.13 In accordance with this, increased PTP mRNA levels have been detected in late-stage colorectal tumours and enhanced levels of the enzyme occur in one third of breast carcinomas. However, over-expression of PTP in breast carcinoma cells actually reduces tumour aggressiveness. The functional significance of PTP thus depends on the cellular context and the type of tumour. 19 PTP genes that map to chromosomal regions frequently deleted in human cancers have been identified, and also 4 PTP genes that map to regions frequently amplified in human cancers (see http://ptp.cshl.edu or http://science.novonordisk.com/ptp).

We now exemplify the different roles of tyrosine phosphatases.

PTP1B, diabetes, and obesity

Perhaps the most spectacular example of a link between a PTP and human disease is type 2 (mature onset, insulin-resistant) diabetes and obesity. Both

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Signal Transduction

FIG 21.6  PTPB1 down-regulates insulin signalling by dephosphorylation and inactivation of the receptor and its substrates (IRS). (a) PTP1B inhibits the action of insulin by dephosphorylating the cytosolic segment of the insulin receptor. Lack of PTP1B activity promotes glucose uptake due to elevated cell surface expression of its transporter, but (in rats at least) it also protects against the effects of insulin resistance and obesity induced by a high-fat diet. (b) PTP1B binds preferentially to the bisor tris-phosphorylated activation segment of the insulin receptor.19 pY-1162 is most readily recognized by the catalytic cleft of the phosphatase. Dephosphorylation renders the insulin receptor catalytically inactive (see page 554). Sequence comparison with other PTPs suggests that the features that confer the specificity of this reaction are unique to PTP1B and its close relative TCPTP (1ptt10 and 1irk20).

genetic and biochemical studies provide good evidence of a role of tyrosine phosphatase in the signalling events downstream of the insulin and leptin receptors.14

Dephosphorylation of the insulin receptor by PTPs is critical in the control of the cellular response to insulin. Numerous studies have demonstrated that in humans, and in animal models, the resistance to insulin in type 2 diabetes and obesity is accompanied by increases in PTP activity and increases in the level of expression of defined members of the PTP family (LAR and PTP1B).15 However, disruption of the LAR gene in mice yields a complex phenotype. There is a post-receptor defect in insulin signalling but, surprisingly, it is associated with impaired activation of downstream signals, such as PI 3- kinase, rather than the opposite. Matters are somewhat clearer for PTP1B

(Figure 21.6 and Table 21.1). Here, for Xenopus oocytes injected with the purified enzyme, insulin-induced activation of S6 kinase and transition through the G2/M checkpoint of the cell cycle is inhibited.16,17 When over-expressed

in Rat1 fibroblasts, PTP1B reduces insulin-induced phosphorylation of its own receptor and the downstream phosphorylation of components of the signalling cascade. It also reduces the translocation of GLUT-4 (glucose

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Protein Dephosphorylation and Protein Phosphorylation

Table 21.1  Consequences of the absence of PTP1B

transporter) to the membrane and glycogen synthesis.18 Conversely, loading of hepatoma cells with neutralizing antibodies to PTP1B enhances insulininduced phosphorylation of its receptor and of the receptor substrate IRS-1.

An unequivocal link between insulin signalling and the tyrosine phosphatase was established through the use of PTP1B knockout mice; these animals remained healthy and displayed an enhanced sensitivity to insulin.21 Moreover, when fed a high-fat diet, they failed to become obese and retained their normal sensitivity to insulin. By contrast, their wild-type (PTP1B / ) litter mates suffered rapid weight gain and onset of insulin resistance,

which coincided with a reduced level of tyrosine phosphorylation of the insulin receptor.22 All this suggests that the insulin receptor -subunit acts as a substrate of PTP1B. Surprisingly, the knockout mice did not show any predisposition to cancer despite the potential of PTP1B to regulate growth factor receptor tyrosine kinase signalling and to counteract the transforming

effects of the kinases Src and Neu.23,24 The activity of ERK present in fibroblasts obtained from these mice was only slightly enhanced and there was no effect on the activation level of PKB. A possible explanation comes from the finding that, due to a lack of PTP1B, expression of RasGAP and phosphorylation of p62Dok are elevated and that this leads to attenuation of the Ras–MAPK

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