Integrated lipidomics in the secreted phospholipase A(2) biology

Abstract

Mammalian genomes encode genes for more than 30 phospholipase A(2)s (PLA(2)s) or related enzymes, which are subdivided into several subgroups based on their structures, catalytic mechanisms, localizations and evolutionary relationships. More than one third of the PLA(2) enzymes belong to the secreted PLA(2) (sPLA(2)) family, which consists of low-molecular-weight, Ca(2+)-requiring extracellular enzymes, with a His-Asp catalytic dyad. Individual sPLA(2) isoforms exhibit unique tissue and cellular localizations and enzymatic properties, suggesting their distinct pathophysiological roles. Recent studies using transgenic and knockout mice for several sPLA(2) isoforms, in combination with lipidomics approaches, have revealed their distinct contributions to various biological events. Herein, we will describe several examples of sPLA(2)-mediated phospholipid metabolism in vivo, as revealed by integrated analysis of sPLA(2) transgenic/knockout mice and lipid mass spectrometry. Knowledge obtained from this approach greatly contributes to expanding our understanding of the sPLA(2) biology and pathophysiology.

Keywords: knockout mouse; lipidomics; phospholipase A2; phospholipid; transgenic mouse.

Figure 1.

Pla2g5-Tg mice display fatal respiratory failure due to lung surfactant destruction. (a and b) ESI-MS of surfactant PC (a) and PG (b) from Pla2g5-Tg mice (right) and WT mice (left); Molecular peaks shown in red, such as PC32:0 (16:0–16:0) and PC32:1 (16:0–16:1) (a) as well as PG32:0 (16:0–16:0), PG34:1 (16:0–18:1), PG36:4 (16:0–20:4) and PG38:6 (16:0–22:6); (b) were dramatically reduced in Pla2g5-Tg mice compared with WT mice. IS, internal standard; (c) RT-PCR of sPLA₂-V mRNA in cultured human bronchial epithelial cells with or without stimulation for 12 h with pro-inflammatory cytokines. sPLA₂-V was induced by IL-1β or TNFα but not by IFNγ; (d) A schematic model of the role of sPLA₂-V in lung surfactant hydrolysis. sPLA₂-V is secreted from bronchial epithelial cells and alveolar macrophages stimulated with pro-inflammatory cytokines, and aberrant hydrolysis of surfactant PC and PG by sPLA₂-V leads to airway injury. For details, see [8].

Figure 2.

Pla2g10-Tg mice display alopecia during the postnatal hair cycle. (a) Pla2g10-Tg mice displayed temporary alopecia during 2–3 wk after birth, a period of the postnatal hair cycle; (b) Histology of 3-wk-old Pla2g10-Tg mice, in comparison with that of WT mice, revealed hair follicle distortion as well as epidermal hyperplasia, sebaceous gland enlargement, and cyst formation; (c) ESI-MS of skin phospholipids indicated that PE molecular species with PUFA (arachidonic acid (C20:4) and docosahexaenoic acid (C22:6)), but not PC, was markedly decreased in Pla2g10-Tg mice compared with WT mice; (d) A schematic model of the sPLA₂-X action in the skin. sPLA₂-X is converted by certain skin proteases to its active form, which then hydrolyzes PE in skin membranes to liberate PUFA that is further metabolized to skin-acting eicosanoids; (e) Endogenous sPLA₂-X is localized in the ORS of hair follicles (shown in red). ORS, outer root sheath; IRS, inner root sheath. For details, see [38].

Figure 3.

Pla2g3-Tg mice display systemic inflammatory and atherosclerotic phenotypes. (a) ESI-MS of PC and PS in splenocytes from Pla2g3-Tg (III-Tg) and wild-type (WT) mice. Major peaks are indicated by arrows. Peaks altered in III-Tg mice relative to WT mice are shown in red. SM, sphingomyelin; (b) HPLC profile of plasma lipoproteins in III-Tg and WT mice; (c) Increased atherosclerosis in III-Tg mice on the ApoE^−/− background (male, 24-wk-old). Atheroslcerotic lesions were visualized by oil red O staining. Areas positive for the staining were quantified. For details, see [51,55].

Figure 4.

Pla2g3^−/− mice have multiple defects in epididymal sperm maturation. (a) Altered eicosanoid levels in Pla2g3^−/− mice. The levels of individual eicosanoids in the epididymis were determined by LC-ESI-MS/MS. sPLA₂-III is selectively coupled with 12/15-LOX and CYP450 pathways; (b) Unusual accumulation of PC molecular species in the epididymal fluids from Pla2g3^−/− mice relative to Pla2g3^+/+ mice, as assessed by ESI-MS; (c) The roles of sPLA₂-III in epididymal sperm maturation are summarized. For details, see [57].