C. trachomatis-infection accelerates metabolism of phosphatidylcholine derived from low density lipoprotein but does not affect phosphatidylcholine secretion from hepatocytes

Abstract

Background: Chlamydia trachomatis is a prevalent sexually transmitted disease and the leading cause of infectious blindness in developing nations. It was not known if C. trachomatis-infection influenced metabolism of lipoprotein-derived phospholipids. Nor was it known if C. trachomatis-infection altered phosphatidylcholine (PC) secretion from hepatocytes. In the current study, low density lipoprotein (LDL)-derived [methyl-3H]PC metabolism was examined in L929 cells infected with C. trachomatis to determine if PC derived from LDL could serve as a potential source of PC trafficked to C. trachomatis. In addition, release of endogenously synthesized [methyl-3H]PC into the medium was examined in rat liver hepatocytes infected with C. trachomatis to determine if infection altered PC secretion.

Results: L929 cells 20 h post infection exhibited a 39% (p < 0.05) reduction in radioactivity in PC but total radioactivity incorporation was unaltered compared to controls. Lysophosphatidyl [methyl-3H]choline (LPC) and aqueous [methyl-3H]choline metabolites were elevated 3.6-fold (p < 0.05) and 16.5-fold (p < 0.05), respectively, in C. trachomatis-infected cells and this was due to a 51% increase (p < 0.05) in calcium-dependent phospholipase A2 activity. Hepatocytes 22 h post infection then incubated for 16 h with [methyl-3H]choline showed elevated [methyl-3H]PC biosynthesis but [methyl-3H]PC secreted into the medium was unaltered compared to controls. In contrast, both cellular and medium lyso [methyl-3H]PC were elevated in C. trachomatis-infected cells.

Conclusion: This study is the first to show that metabolism of LDL-derived PC is accelerated in C. trachomatis infection and to support the notion that LDL-delivered PC may potentially serve as a source of PC trafficked to Chlamydia. In addition, C. trachomatis-infection does not inhibit PC secretion from hepatocytes indicating that the pool of newly synthesized PC destined for lipoprotein secretion may differ from the pool of PC used for C. trachomatis membrane biosynthesis.

Figure 1

Incorporation of [methyl-³H]choline labelled LDL into L929 cells, PC, LPC and aqueous fraction. L929 cells were infected with C. trachomatis at a multiplicity of infection of 3 to 5 CFU. At 22 h post infection (hatched bars) or post mock-infection (open bars), cells were incubated for 5 h with [³H]PC-LDL and radioactivity incorporated into A. L929 cells; B. PC; C. LPC; and D. aqueous fraction determined. Results represent the mean ± standard deviation of three dishes. *p < 0.05.

Figure 2

PLA₂ activity in C. trachomatis-infected cells. L929 cells were infected with C. trachomatis at a multiplicity of infection of 3 to 5 CFU. At 22 h post infection (hatched bars) or post mock-infection (open bars), cells were isolated and membrane fractions prepared and calcium-stimulated PLA₂ activity determined as described in Materials and Methods. Results represent the mean ± standard deviation of three dishes. *p < 0.05.

Figure 3

Cellular PC and LPC synthesis and their release into medium of hepatocytes infected with C. trachomatis. Isolated rat liver hepatocytes were infected with C. trachomatis at a multiplicity of infection of 5 to 20 CFU. At 20 h post infection or post mock-infection, cells were incubated for 16 h with [methyl-³H]choline and radioactivity incorporated into A. cellular PC, B. cellular LPC and C. medium PC (circles) and LPC (squares) determined. Results represent the mean of three dishes from two separate sets of hepatocytes.