Bilirubin interactions with phospholipid components of lung surfactant

Apoptosis in murine hepatoma hepa 1c1c7 wild-type, C12, and C4 cells mediated by bilirubin

Elevated serum and tissue bilirubin concentrations that occur in pathological conditions such as cholestasis, jaundice, and other liver diseases are known to stimulate cytotoxic responses. In preliminary studies, we noted that bilirubin seemed to cause apoptosis in murine hepatoma Hepa 1c1c7 wild-type (WT) cells. Consequently, we investigated apoptosis caused by bilirubin in WT, mutant C12 [aryl hydrocarbon receptor (AHR)-deficient], and C4 (AHR nuclear translocator-deficient) Hepa 1c1c7 cells. Three independent measures of apoptosis were used to quantify the effects of exogenous bilirubin (0, 1, 10, 25, 50, or 100 microM). Caspase-3 activity and cytochrome c release from mitochondria increased at 3 h post-treatment, before increased caspase-8 activity at 6 h, and nuclear condensation by 24 h after treatment with bilirubin. No differences in whole-cell lipid peroxidation were observed between the cell types; however, intracellular reactive oxygen species (ROS) production was greater in WT cells than C12 or C4 cells 3 h after bilirubin exposure. Pretreatment of cells for 1 h with 1 or 10 microM alpha-naphthoflavone, an AHR antagonist, before bilirubin exposure resulted in decreased caspase-3 activity at 6 h and nuclear condensation at 24 h in WT cells. These results indicate that bilirubin, a potential AHR ligand, causes apoptosis in murine Hepa 1c1c7 WT cells by a mechanism(s) partially involving the AHR, disruption of membrane integrity, and increased intracellular ROS production.

Immunohistochemistry of pulmonary surfactant-associated protein A in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a fatal complication in severe traumas and diseases. Although the contribution of pulmonary surfactant abnormality to the pathogenesis of ARDS has been clinically fairly well investigated, the histopathological evidence has not been established. The aim of this study was to clarify the immunohistochemical distribution of surfactant-associated protein A (SP-A) for early diagnosis of ARDS with special regard to hyaline membrane (HM) formation. Two-hundred-and-ten autopsy cases of prolonged death from various traumas and diseases were investigated. ARDS were observed in 23 cases, showing speckled SP-A immunostaining. During the early, exudative phase of ARDS, characteristic SP-A distribution showed intense staining in the intra-alveolar massive aggregates and thick 'peeling'-like substances accompanied with a lot of granular staining. During the proliferative phase, localized accumulation of granular SP-A and macrophages containing dense granular SP-A became predominant. During the final fibrotic phase, SP-A staining in HMs became weak, and disseminated granular staining was observed in the alveolar spaces. These findings provide morphological evidence of the increase of SP-A during the early phase of ARDS, including some molecular alteration and its decrease during the late phase. Characteristic SP-A distribution in the exudative phase appeared to be especially useful for early histopathological diagnosis of respiratory distress, even prior to the appearance of typical HMs.

Difference in susceptibilities of different cell lines to bilirubin damage

OBJECTIVE

To investigate if there are differences in susceptibilities to bilirubin toxicity of different cell lines.

METHODOLOGY

A modified 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) method was adopted to study the cytotoxic effect of bilirubin on several commercially available cell lines including human glioblastoma (ATCC CRL 1690, T98G), human neuroblastoma (ATCC HTB-10, SK-N-MC), human liver (ATCC CCL 13, Chang Liver, HeLa markers) and a mouse fibroblast (ATCC CCL-1, NCTC Colon 929).

RESULTS

Cytotoxicity was observed when certain bilirubin:albumin molar ratios were exceeded in the medium of a cell line in culture. Different cells exhibited different susceptibilities to the cytotoxic effects of bilirubin; neuroblastoma and glioblastoma were most susceptible, fibroblasts were the least vulnerable.

CONCLUSIONS

Our findings have confirmed the clinical impression that different cells sustain different degrees of cytotoxicities caused by bilirubin.

Influence of bilirubin on surface tension properties of lung surfactant

AIM

To investigate the influence of bilirubin on the surface tension activity of a porcine derived (Curosurf) and synthetic (Exosurf) surfactant.

METHODS

The captive bubble surfactometer at phospholipid doses of 0.5 mg/ml (low dose) and 1 mg/ml (high dose) in solutions of increasing bilirubin concentrations (0.25, 0.5, and 1.0 mg/ml) was used.

RESULTS

Curosurf (without bilirubin) showed a higher surface f1p4ion activity than Exosurf, as shown by area compression of 30 (SD 0.6)% compared with 76(1.4)% at low surfactant dose and 25 (0.9)% compared with 85 (0.5)% at high dose (P < 0.01). Bilirubin showed negligible surface activity at the concentrations studied. At low phospholipid dose (0.5 mg/ml Curosurf), bilirubin increased film area compression of lipid extract surfactant from 30 (0.6)% to 55 (1.6)%, 59 (0.1)%, and 68 (0.5)% at the three studied bilirubin concentrations, respectively (P < 0.01). At high phospholipid dose (1 mg/ml Curosurf), bilirubin had the same adverse, although less pronounced, effect on film area compression of porcine lipid extract surfactant (25 (0.9)% vs 26 (0.9)%, 39 (1.3)%, and 44 (1.1)%, respectively) (P < 0.01). Using synthetic surfactant (Exosurf), with a much lower original surface activity, bilirubin did not further inhibit its surface tension properties at any of the phospholipid doses studied.

CONCLUSION

These results indicate that in vitro bilirubin impairs the surface tension activity of porcine lipid extract surfactant, but does not affect synthetic surfactant activity.

Mechanisms of bilirubin toxicity

Possible mechanisms of bilirubin toxicity on the brain and lung were studied in animal experiments and in vitro. The uptake of tyrosine as precursor of dopamine in rat synaptosomes was evaluated to study the role of different bilirubin concentrations on synaptic neurotransmission. The results of this study show a statistically significant correlation between bilirubin levels and tyrosine uptake, supporting the hypothesis that the effect of bilirubin on neuronal excitability is dose-dependent. Concerning bilirubin toxicity on the lung, we studied the effect of different bilirubin concentrations on surface activity of modified natural surfactant (Curosurf) and synthetic surfactant (Exosurf), both in current clinical use for treatment of neonatal respiratory distress syndrome. Surface activity of Curosurf and Exosurf was investigated with the captive bubble surfactometer. The results of this study show that bilirubin impairs, in vitro, the surface tension activity of natural surfactant in a dose dependent manner and has no effect on surface tension activity of artificial surfactant. These data suggest that bilirubin interferes with surfactant proteins SP-B and/or SP-C, thus impairing surfactant activity at the air-liquid interface. We conclude that bilirubin shows its toxic effect reacting with different biological systems in a dose-dependent fashion.

Surfactant inhibition in experimental meconium aspiration

Human meconium components were separated into a water-methanol soluble phase containing mainly proteins and bilirubin and a chloroform soluble phase containing free fatty acids, triglycerides and cholesterol. Various amounts of original meconium, the water-methanol soluble fraction or the chloroform soluble fraction were added to standard suspensions of porcine surfactant (Curosurf). In a pulsating-bubble or Wilhelmy-balance system, meconium and its subfractions inhibited surfactant activity, but the chloroform soluble fraction had the highest specific inhibitory activity. Original meconium in saline (130 or 65 mg/ml) was instilled into the airways of ventilated near-term newborn rabbits (dose 6 ml/kg). Lung-thorax compliance was reduced by 38% in animals aspirating the higher dose and by 27% in the low-dose group. Histological sections showed intra-alveolar accumulation of fine meconium particles in experimental animals, but no plugging of larger airways. We conclude that respiratory failure from meconium aspiration may be mediated, in part, by inactivation of surfactant.