The intracellular localization and the physiological properties of type 2 phosphatidic acid phosphatase.

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Lipid phosphate phosphatases(LPPs) are integral membrane proteins with six transmembrane domains that act as ectoenzymes dephosphorylating a variety of extracellular lipid phosphates. Using polarized MDCK cells stably expressig human LPP1 and LPP3, we found that IPP1 was located exclusively at the apical surface whereas LPP3 was distributed mostly in the basolateral subdomain. We identified a novel apical sorting signal at the N-terminus of LPP1 composed of F(2) DKTRL(7). In the case of LPP3, a dityrosine motif present in the second cytoplasmic portion was identified as basolateral targeting signal. Our work shows that LPP1 and LPP3 are equipped with distinct sorting signals that cause them to differentially localize to the apical vs. the basolateral subdomain, respectively. The different targeting of LPP1 from LPP3 may be due to their different raft associations. LPP1 not but LPP3 was solubilized with Triton X-100, while CHAPS never solubilized both LPP isozymes. Sucrose density gradient centrifugation assay revealed that LPP1 and LPP3 were associated with lipid rafts in different manners. To understand the physiological roles of LPP1 and LPP3, we investigated the effects of LPP expression in the human ovarian cancer cell lines. In the cell lines, extracellular lysophosphatidic acid(LPA) constitutively produced induce heparin-binding EGF-like growth factor(HB-EGF) ectodomain shedding, EGFR transactivation, and the cancer cell proliferation. Both LPP1 and LPP3 decreased the constitu1tive shedding of HB-EGF, suggesting LPPs kept the extracellular LPA level low. Whether the distinct intracellular localization between LPP1 and LPP3 affects their physiological roles is an important issue to understand in the future.