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

磷脂磷酸酶(LPP)是一类具有六个跨膜结构域的完整膜蛋白，它们作为胞外酶，对多种胞外脂磷酸盐进行脱磷。利用稳定表达人LPP1和LPP3的极化MDCK细胞，我们发现IPP1仅位于根尖表面，而LPP3主要分布在基底外侧亚区。我们在LPP1的N端发现了一个由F(2)DKTRL(7)组成的新的顶端分选信号。在LPP3的情况下，存在于第二细胞质部分的二酪氨酸基序被鉴定为基侧靶向信号。我们的工作表明，LPP1和LPP3都配备了不同的排序信号，使它们分别差异地定位于顶端亚域和基侧亚域。LPP1和LPP3靶向的不同可能是由于它们的RAFT结合不同所致。LPP1不能，但LPP3能被Triton X-100增溶，而CHAPS不能增溶这两种LPP同工酶。蔗糖密度梯度离心法显示LPP1和LPP3以不同的方式与脂筏结合。为了了解LPP1和LPP3的生理作用，我们研究了LPP在人卵巢癌细胞系中表达的影响。在细胞系中，细胞外合成的溶血磷脂酸(LPA)可诱导肝素结合的EGF样生长因子(HB-EGF)的胞外区脱落、EGFR反式激活和癌细胞的增殖。LPP1和LPP3均能减少HB-EGF的结构性脱落，提示LPPs使细胞外LPA水平保持在较低水平。LPP1和LPP3之间不同的细胞内定位是否会影响它们的生理作用是未来需要了解的重要问题。

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.