Phosphatase Subfamily PTPRK

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

PTPRG is a receptor PTP family involved in metazoan neural development and maybe cancer.

Evolution

PTPRK subfamily is vertebrate specific. There are four copies in human, which have similar genomic organization, despite great disparities in gene size due to variations in intron length [1].

Domain Structure

PTPRK subfamily has dual intracellular catalytic domains. All human members of PTPRK subfamily has the same extracellular domain combination: a MAM domain, 1 Ig domain and 4 FN3 domain [2, 3, 4].

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 RPTP mu does not interact with either PTPRK or PTPRM [2].

Functions

PTPRK subfamily mediates homophilic cell-cell interaction [2, 5, 6, 7, 8]. They functions in nervous system. They are putative tumor suppressors in various types of cancer.

PTPRK (LAR)

PTPRK is a putative tumor suppressor [9] in various types of cancer, such as breast cancer [10], prostate cancer [11], lymphoma [12] and glioma [13]. 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 [14]. PTPRK is a key factor in coordinating apoptosis via the regulation of MAPK pathways, in particular the JNK pathway in prostate cancer cells [11]. PTPRK dephosphorylates Epidermal growth factor receptor (EGFR) and thereby regulates EGFR tyrosine phosphorylation and subsequent promotes human keratinocyte survival and proliferation [15]. 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 [12, 16, 17]. PTPRK also dephosphorylates Src [17].

PTPRK regulates CD4+ T cell development through ERK1/2-mediated signaling [18].

PTPRM

PTPRM mediates homophilic cell-cell adhesion [7].

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 [8, 19, 20]. 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 [21].

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 [22]. PTPRT knockout mice exhibit increased levels of colonic paxillin phosphorylation at residue Y88, which is found as a common feature of human colon cancers [23].

PTPRT is the most frequently mutated PTP in different types of human cancers and is believed to be a tumor suppressor in colon cancer [24] and head and neck squamous cell carcinoma [25], but it does not play a critical role in the development of common human cancers [26]. 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 [27].

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

PTPRU is predominately found in adult brain, lung, and kidney [28, 29]. PTPRU is glycosylated and expressed at the cell surface. PTPRU localizes to the adherens junctions with cell adhesion molecules like catenin and E-cadherin [6]. PTPRU directly binds and dephosphorylates beta-catenin, which is a key molecule involved in both cell adhesion and Wnt signaling pathway [30, 31]. PTPRU-deficient mice associated with hypertension and low glomerular filtration rate [32].

PTPRU was weakly detected in the 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 [33, 34].

References

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