PTPRT/PTPρ is the most highly mutated tyrosine phosphatase in human colon carcinomas. We and others found that PTPRT/ PTPρ is also mutated in lung, gastric and skin (melanoma) cancers (Forbes et al. 2008
; Wang et al. 2004
). The spectrum of mutations, which included nonsense mutations and frameshifts, suggested that these mutations were inactivating (Wang et al. 2004
). Biochemical analyses demonstrated that missense mutations in the catalytic domains of PTPRT diminished its phosphatase activity and overexpression of PTPRT inhibited colon cancer cell growth. Moreover, we showed previously that the tumor-derived mutations in the MAM and Ig domains of PTPρ are defective in cell-cell adhesion (Yu et al. 2008
). All these data suggest that PTPRT/PTPρ normally functions as a tumor suppressor gene. The experiments presented in this manuscript evaluate the contribution of the individual extracellular domains of PTPρ to PTPρ-mediated adhesion. We demonstrate that in addition to the MAM and Ig domains, the FNIII repeats are required for PTPρ-mediated adhesion. Importantly, we also show that PTPρ-mediated adhesion is impaired by human tumor-derived mutations in those FNIII repeats. These studies indicate that all the tumor-derived mutations located in the extracellular domain of PTPRT are loss-of-function mutations, thereby providing further evidence that PTPRT is a tumor suppressor. This notion is further supported by our recent study showing that PTPRT knockout mice are highly susceptible to colon specific carcinogen azoxymethane (AOM)-induced colon cancer (Zhao et al.).
To gain insight into how the point mutations may alter the adhesive function of PTPρ, we modeled the human tumor derived PTPρ mutations onto the crystal structure of PTPμ (). The crystal structure of PTPμ includes only the first three FNIII repeats (Aricescu et al. 2007
). Four of these mutations fall within the second FNIII (FNIII-2) repeat (I395V, Y412F, R453C, Q479E). It is important to note that FNIII-2 is required for efficient cell-cell adhesion (Aricescu et al. 2006
). Two of the mutations fall in the third FNIII (FNIII-3) repeat (S492F, N510K). The mutation within the linker region between FNIII-1 and FNII-2 (Y412F) could alter the flexibility or positioning between the two domains. The other mutations (I395V R453C, Q479E, S492F, N510K) appear to be clustered in surface exposed regions and/or near the linker regions between FNIII-2 and FNIII-3 that could alter flexibility, position of individual FNIII repeats and/or protein-protein interactions. While these mutations do not correspond to the adhesive interface hypothesized for PTPμ based upon a single static crystal structure (Aricescu et al. 2007
), some of the mutations fall within the FNIII-2 repeat which is required for adhesion, and all of these surface modifications would likely alter either the three-dimensional topology and/or cis/trans interactions of PTPρ.
Figure 5 Structural modeling of the point mutations of PTPρ. The point mutations (indicated in red) evaluated in this manuscript were modeled onto the equivalent sites in the crystal structure of PTPμ. The PTPρ mutations likely alter cell-cell (more ...)
Other type IIB RPTPs also function as tumor suppressor genes, by regulating adhesion and/or tyrosine phosphorylation of effector proteins. Expression of PTPμ is reduced in human glioblastomas (GBM) and cell lines (Burgoyne et al. 2009a
). Reduction in full-length PTPμ expression with a concomitant increase in a proteolytically processed cytoplasmic fragment of PTPμ is observed in GBM cells and both are correlated with decreased adhesion and increased migration observed in the disease (Burgoyne et al. 2009a
; Burgoyne et al. 2009b
). Over-expression of the full-length form of PTPμ suppressed migration of GBM cells and reduces their survival (Burgoyne et al. 2009b
). Of note, at the same time that stable cell adhesion is reduced due to the loss of full-length PTPμ expression in GBM cells, a catalytically active cytoplasmic fragment of PTPμ is released and translocates to the nucleus (Burgoyne et al. 2009b
). Catalytic activity of this fragment is also necessary for cell migration and viability of GBM cells (Burgoyne et al. 2009b
). Therefore, both the tyrosine phosphatase activity of the cytoplasmic fragment of PTPμ and the loss of PTPμ-mediated adhesion may contribute to the invasiveness of GBM cells.
PTPκ is also implicated in tumor progression. PTPκ expression is reduced in melanoma cell lines and human tissue biopsies (McArdle et al. 2001
), and in Hodgkin’s lymphoma cells (Flavell et al. 2008
). Over-expression of PTPκ in Hodgkin lymphoma cells reduces cellular proliferation and survival (Flavell et al. 2008
). Like PTPμ, PTPκ is proteolytically processed (Anders et al. 2006
), potentially as a result of aberrant glycosylation (Kim et al. 2006
). It is not clear whether adhesion or catalytic activity of PTPκ is required for its tumor suppressor activity (Flavell et al. 2008
; Kim et al. 2006
). Of note, the cleaved cytoplasmic domain fragment of PTPκ is catalytically active, translocates to the nucleus and promotes β-catenin-mediated transcription (Anders et al. 2006
). Signaling via the PTPκ cytoplasmic fragment might augment the loss of stable cell-cell adhesion and may promote tumor progression.
Although speculative, it is interesting to postulate that tyrosine phosphatase activity and or cleavage of PTPρ will also be an important element of its tumor suppressor activity, as has been demonstrated for PTPμ and PTPκ. Interestingly, we identified and validated STAT3 as a direct substrate of PTPRT (Zhang et al. 2007
). STAT3 is latent transcription factor that translocates from the cytoplasm to the nucleus after being phosphorylated at the Y705 residue. It is possible that the cleaved intracellular fragment of PTPρ, which contains the phosphatase domains, translocates to nucleus and dephosphorylates STAT3. Besco and colleagues found that PTPRT associates with the adhesion molecule E-cadherin and its binding partner p120 catenin, which are PTPRT substrates (Besco et al. 2006
). p120 also translocates to the nucleus and associates with the Kaiso transcription factor (Daniel and Reynolds 1999
). Given the frequency and distribution of mutations in PTPRT/PTPρ in human colon and other cancers, a more thorough understanding of its biological function is warranted.