Role of the PI3-kinase signaling pathway in trafficking of the surfactant protein A receptor P63 (CKAP4) on type II pneumocytes

Abstract

factant protein A (SP-A) plays an important role in the maintenance of lung lipid homeostasis. Previously, an SP-A receptor, P63 (CKAP4), on type II pneumocyte plasma membranes (PM) was identified by chemical cross-linking techniques. An antibody to P63 blocked the specific binding of SP-A to pneumocytes and the ability of SP-A to regulate surfactant secretion. The current report shows that another biological activity of SP-A, the stimulation of surfactant uptake by pneumocytes, is inhibited by P63 antibody. cAMP exposure resulted in enrichment of P63 on the cell surface as shown by stimulation of SP-A binding, enhanced association of labeled P63 antibody with type II cells, and promotion of SP-A-mediated liposome uptake, all of which were inhibited by competing P63 antibody. Incubation of A549 and type II cells with SP-A also increased P63 localization on the PM. The phosphatidylinositol 3-kinase (PI3-kinase) signaling pathway was explored as a mechanism for the transport of this endoplasmic reticulum (ER)-resident protein to the PM. Treatment with LY-294002, an inhibitor of the PI3-kinase pathway, prevented the SP-A-induced PM enrichment of P63. Exposure of pneumocytes to SP-A or cAMP activated Akt (PKB). Blocking either PI3-kinase or Akt altered SP-A-mediated lipid turnover. The data demonstrate an important role for the PI3-kinase-Akt pathway in intracellular transport of P63. The results add to the growing body of evidence that P63 is critical for SP-A receptor-mediated interactions with type II pneumocytes and the resultant regulation of surfactant turnover. lung; phospholipid; plasma membrane; lipid turnover ALVEOLAR TYPE II cells produce and secrete lung surfactant, the complex mixture of phospholipids and proteins that functions to reduce surface tension and prevents collapse of the alveoli during expiration. These specialized cells store surfactant components in a membrane-bound lamellar body. Surfactant protein A (SP-A), the most abundant surfactant protein present in the lung surfactant, belongs to the mammalian C-type lectin family of proteins (28,35). In monomeric form, it is a 28-to 32-kDa hydrophilic protein that organizes into trimers and can further oligomerize into higher order forms (35). The primary structure of the protein contains a collagen-like region connected by a neck region to a globular carbohydrate-recognition domain. SP-A has a calcium-dependent activity that can: bind to dipalmitoylphosphatidylcholine (DPPC), the major phospholipid present in the lung; increase the aggregation and molecular order of phospholipids; bind to peroxiredoxin 6 and regulate its phospholipase A 2 activity (59); and, in conjunction with SP-B, promote tubular myelin formation in lung surfactant (30, 37). As a member of the collectin family of host defense proteins, SP-A also contributes to innate pulmonary immunity (56). In fact, SP-A may have multiple roles in respiratory physiology, as related protein forms are present in all vertebrate classes, including most primitive amphibious fish (48).Either a defi...

Cytoskeleton-Associated Protein 4: Functions Beyond the Endoplasmic Reticulum in Physiology and Disease

“…Further evidence supporting the interaction between CKAP4 and SP-A, and consistent with a physiological function of CKAP4 in surfactant turnover, comes from studies performed by Kazi et al [18]. They hypothesized that the phosphatidylinositol-3-kinase (PI3-kinase) signaling pathway was a possible mechanism for the transport of CKAP4 from the ER to the plasma membrane.…”

“…(c) The binding of SP-A to CKAP4 inhibits the secretion of surfactant [12]. (d) An interaction between cAMP and Epac causes a signal cascade stimulating PI3-Kinase which in turn activates Akt and increases the transport of CKAP4 from the ER to the plasma membrane [18]. (e) The SP-A/CKAP4 interaction is recycled via a clathrin pathway and is stimulated by the cAMP signaling cascade [18].…”

“…(d) An interaction between cAMP and Epac causes a signal cascade stimulating PI3-Kinase which in turn activates Akt and increases the transport of CKAP4 from the ER to the plasma membrane [18]. (e) The SP-A/CKAP4 interaction is recycled via a clathrin pathway and is stimulated by the cAMP signaling cascade [18]. (f) CKAP4 binds to the CCN2 promoter via its C-terminal domain (residues 126-501 which includes the bZIP-like DNA-binding domain) in an APF-dependent fashion [13].…”

“…For example, CKAP4 has been shown to be required for APF-mediated signaling [11,[13][14][15][16], for gentamycin-induced cytotoxicity [17], for regulation of tPA function [10], and for SP-A-induced surfactant uptake by pneumocytes [18]. This evidence for CKAP4 mediating a wide array of cellular responses important in physiological and pathological processes, such as interstitial cystitis, druginduced cytotoxicity, pericullar proteolytic activity, and lung lipid homeostasis, provides new insights into this classically ER-resident protein.…”

CKAP4 is a Dickkopf1 receptor and is involved in tumor progression

“…Although we are not sure how CKAP4 interacts with p85α in a DKK1-dependent manner, it is intriguing to speculate that in the absence of DKK1, the proline-rich region of CKAP4 is masked and the closed region is opened by from the ER to cell surface membrane through AKT activation (45). tPA binds to CKAP4 on rat vascular smooth muscle cells, and anti-CKAP4 antibody inhibits tPA binding and decreased tPA activity (27).…”