Phosphatase Subfamily PTPRK

Phosphatase Classification: Fold CC1:Superfamily CC1: Family PTP: Subfamily PTPRK

PTPRK is a chordate-specific subfamily that regulates cell-cell adhesion. It is involved in human cancer and in the nervous system.

Evolution

PTPRK is found in chordates (see technical notes). There are four copies in human, which have similar genomic organization, despite great disparities in gene size due to variations in intron length [1]. PTPRK may be orthologous to some invertebrate phosphatases, including the Ptp36E subfamily

Domain Structure

PTPRK have dual intracellular catalytic domains and an extracellular region consisting of a MAM domain, 1 Ig domain and 4 FN3 domains [2, 3, 4]. The MAM domain is essential for homophilic cell-cell interaction and helps determine the specificity of these interactions. Truncated PTPRM is properly expressed at the cell surface but fails to promote cell-cell adhesion. Homophilic cell adhesion is fully restored in a chimeric PTPRM molecule containing the MAM domain of PTPRK. However, this chimeric protein does not interact with either PTPRK or PTPRM [2]. The Ig domain of PTPRM contains the homophilic binding site and regulates subcellular localization [5].

Functions

PTPRK members mediate homophilic cell-cell interaction [2, 6, 7, 8, 9]. They are putative tumor suppressors in various types of cancer. They also function in the nervous system.

Human PTPRKs

PTPRK (R-PTP-kappa)

PTPRK is a putative tumor suppressor [10] in various types of cancer, such as breast cancer [11], prostate cancer [12], lymphoma [13] and glioma [14]. It plays its function as tumor suppressor through different mechanisms. PTPRK influences transactivating activity of beta-catenin in non-tumoral and neoplastic cells by regulating the balance between signaling and adhesive beta-catenin which is a molecule endowed with a dual function being involved both in cell adhesion and in Wnt signaling pathway [15]. PTPRK is a key factor in coordinating apoptosis via the regulation of MAPK pathways, in particular the JNK pathway in prostate cancer cells [12]. PTPRK dephosphorylates Epidermal growth factor receptor (EGFR) and thereby regulates EGFR tyrosine phosphorylation and subsequent promotes human keratinocyte survival and proliferation [16]. It is worthy pointing out that PTPRK is the target of transforming growth factor {beta} (TGF-{beta})-Smad, which inhibits proliferation and promotes cell migration [13, 17, 18]. PTPRK also dephosphorylates Src [18].

PTPRK regulates CD4+ T cell development through ERK1/2-mediated signaling [19].

PTPRM (PTP mu)

PTPRM mediates homophilic cell-cell adhesion [2, 8, 20, 21]. It associates with and dephosphorylates cadherin and catenin [22, 23, 24]. It also binds and recruits the scaffolding protein RACK1, a scaffolding protein, to cell-cell contacts [25, 26].

PTPRM functions in developing nervous system. In particular, by interacting with E-cadherin, it regulates retinal ganglion cells neurite outgrowth and the lamination of the retina by Brady-Kalnay's group [27, 28, 29]. In addition, two other proteins have been reported to interact with PTPRM by Brady-Kalnay's group: i) IQGAP1, a known regulator of the Rho GTPases, Cdc42 and Rac1 [30], and ii) BRCA2 and CDKN1A interacting protein (BCCIP), a gene involved in cell cycle arrest and DNA repair [31].

PTPRM is involved in tumorigenesis and metastasis. It is proteolytically cleaved in glioblastoma multiforme (GBM), which yields PTPRM fragments that promote dispersal of GBM cells [32]; In breast cancer, it was observed that decreased expression of PTPRM was correlated with poor prognosis and inversely correlated with disease free survival, and knockdown of PTPRM increased proliferation, adhesion, invasion and migration of breast cancer cells [33].

PTPRM is also expressed in specific vascular endothelial bed [34, 35].

PTPRT (RPTPrho)

PTPRT is restrictively expressed in the central nervous system and functions in regulating cadherin-mediated cell adhesion cellular adhesion in the central nervous system [9, 36, 37]. In brain, PTPRT regulates synapse formation through interaction with cell adhesion molecules, and this function and the phosphatase activity are attenuated through tyrosine phosphorylation by the synaptic tyrosine kinase Fyn [38].

PTPRT is a putative tumor suppressor in colon cancer. PTPRT specifically dephosphorylated signal transducer and activator of transcription 3 (STAT3) at Y705 is essential for the function of STAT3 (Note: PTPRD dephosphorylate STAT3 Y705, as well). Overexpression of normal PTPRT in colorectal cancer cells reduced the expression of STAT3 target genes [39]. PTPRT knockout mice exhibit increased levels of colonic paxillin phosphorylation at residue Y88, which is found as a common feature of human colon cancers [40].

PTPRT is the most frequently mutated PTP in different types of human cancers and is believed to be a tumor suppressor in colon cancer [41] and head and neck squamous cell carcinoma [42], but it does not play a critical role in the development of common human cancers [43]. It is worthy pointing out that more than half of the identified tumor-derived mutations are located in the extracellular part, particularly MAM domain and Ig domain, which mediates the homophilic interaction of cell-cell adhesion [44].

PTPRU (PTP-RO/hPTP-J/PTP pi/PTP lambda)

PTPRU is predominantly expressed in adult brain, lung, and kidney [45, 46]. It localizes to the adherens junctions with cell adhesion molecules like beta-catenin and E-cadherin [7]. PTPRU directly binds and dephosphorylates beta-catenin, which is a key molecule involved in both cell adhesion and Wnt signaling pathway [47, 48]. PTPRU-deficient mice exhibited hypertension and low glomerular filtration rate [49].

Weak PTPRU expression was detected in peripheral blood lymphocytes, thymus, and spleen even though gene expression was relatively high in the Jurkat T lymphoma cell line. Moreover, PTPRU gene expression was down-regulated after Jurkat cells (an immortalized line of human T lymphocyte cells) were stimulated by either Phorbol myristate acetage (PMA) or calcium ionophore [50, 51].

Both catalytic domains of PTPRU have substitutions that predict inactivity - the D1 domain has a D->E change in the WPD motif and a Y->E in the KNRY motif, while the D2 domain has E->A and F->S at these motifs. However, both domains have been reported to have in vitro catalytic activity [3], and mutation of the catalytic cysteines in both domains blocks PTPRU ability to block beta catenin activation [48].

External links

OrthoDB links:

References

All Medline abstracts: PubMed | HubMed

Phosphatase Subfamily PTPRK

Evolution

Domain Structure

Functions

Human PTPRKs

PTPRK (R-PTP-kappa)

PTPRM (PTP mu)

PTPRT (RPTPrho)

PTPRU (PTP-RO/hPTP-J/PTP pi/PTP lambda)

External links

References

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