Changes in lipid structure produced by surfactant proteins SP-A, SP-B, and SP-C

Biosynthetic routing of pulmonary surfactant proteins in alveolar type II cells

Surfactant proteins A, B, and C (SP-A, SP-B, and SP-C) are synthesized in alveolar type II cells. SP-B and SP-C are both synthesized as large precursor molecules that are proteolytically processed to their mature sizes. In a previous immunoelectron microscopic study, we showed that precursor SP-B is processed to its mature size in multivesicular bodies. In the present study, using a specific antibody against precursor SP-C, we demonstrate that precursor SP-C is present in the same intracellular compartments of the biosynthetic pathway, i.e., endoplasmic reticulum, Golgi complex, and multivesicular bodies, as precursor SP-B. Since mature SP-C is known to be present in multilamellar bodies, this suggests a biosynthetic routing and site of processing of this protein similar to those of SP-B. Double-labeling experiments using antibodies against SP-A, precursor SP-B, precursor SP-C, and an antibody against HA I, an adaptor protein involved in the budding of transport vesicles from the Golgi complex, showed that the different surfactant proteins traverse and exit the Golgi complex via the same route. The surfactant proteins do not exit the Golgi complex via HA I-positive coated buds or vesicles. These data are in accordance with the concept that SP-A, SP-B, and SP-C are transported together through the same biosynthetic pathway via multivesicular bodies to multilamellar bodies.

Changes in exogenous surfactant in ventilated preterm lamb lungs

Preterm lambs were treated with either a surfactant from bovine lung (Survanta) or three synthetic surfactants (Exosurf), a 69:22:9 mixture of dipalmitoylphosphatidylcholine, phosphatidylglycerol, and palmitic acid prepared by heat annealing (Lipid Mixture 1) or with glass beads (Lipid Mixture 2). After 5 h of ventilation, large and small aggregate surfactant fractions were isolated from alveolar washes by centrifugation. SP-A was used as an indicator for the association of endogenous surfactant components with the treatment surfactants. The large aggregate fraction from Survanta-treated lambs contained more SP-A than did the fractions from the lambs treated with the other surfactants (p < 0.05). The surfactants used to treat the sheep and the large aggregate surfactants from alveolar washes increased compliances when tested in surfactant-deficient, immature rabbits, relative to that in control animals. The large aggregate fractions in alveolar washes from lambs treated with Survanta, Lipid Mixture 1, and Lipid Mixture 2 improved compliances in the preterm rabbits to a greater extent than did the surfactants used to treat the lambs. The small aggregate fractions were inactive as surfactants. The function of exogenous surfactant can be improved after exposure to the preterm lung. The improvement may result from the association of exogenous surfactant with components of endogenous surfactant.

Exposure of rabbit fetal lung to glucocorticoids in vitro does not enhance transcription of the gene encoding pulmonary surfactant-associated protein-B (SP-B)

We have investigated the ontogeny and hormonal regulation of both synthesis rates and cellular accumulation of the mRNA for surfactant-associated protein B (SP-B) in rabbit fetal lung. The developmental pattern for SP-B mRNA synthesis increased as a function of gestational age and paralleled that for SP-B mRNA levels except on days 22-26 of gestation where relatively higher levels of gene transcription were observed. Time-course studies with explants from 26- and 30-day fetal lung maintained in culture revealed a gradual increase in mRNA levels and a much smaller increase in gene transcription relative to adult values. Within 48 h of exposure of 26-day explants to dexamethasone at 10(-8) M there was a rapid increase in SP-B mRNA levels to 7-fold adult levels. A similar overall although somewhat slower and attenuated pattern was observed with 30-day explants. Dexamethasone at 10(-8) M had no effect on SP-B gene transcription with explants of either gestational age. We conclude that the major effect of dexamethasone treatment in vitro on SP-B mRNA levels appears to be post-transcriptional and there are small but distinct differences in the effects of glucocorticoids on SP-B mRNA levels with explant cultures from early and late stages of fetal lung maturation.

Adsorption, compression and stability of surface films from natural, lipid extract and reconstituted pulmonary surfactants

A pulsating bubble surfactometer was used to study the surface activities and surface film stabilities of bovine pulmonary surfactants (10 mg/ml) and a reconstituted surfactant (10 mg/ml). Pulmonary surfactants were natural surfactant (NS), lipid extract surfactant [LES(chol)] and lipid extract surfactant without neutral lipids (LES). NS is composed of phospholipids, neutral lipids and surfactant-associated proteins (SP-A, SP-B and SP-C). Both LES(chol) and LES are organic solvent extracts of NS. LES(chol) retains all the components of NS except SP-A. Reconstituted surfactant was dipalmitoylphosphatidylcholine (DPPC): 1-palmitoyl-2-oleoyl-phosphatidylglycerol (POPG): SP-B/7:3:1%. All three pulmonary surfactants attained the equilibrium surface tension almost instantaneously at 37 degrees C. The adsorption rates of NS and LES(chol) at 24 degrees C were similar to those at 37 degrees C, while LES exhibited a lower adsorption rate at 24 degrees C. Reconstituted surfactant adsorbed slower than any of the pulmonary surfactants. Film stability was studied by recording the spontaneous increase in the pressure gradient of a static bubble at the minimum size (Rmin) once near zero surface tension was attained. The order of surface film stabilities were: reconstituted surfactant > > NS > LES > LES(chol). Surface films of NS and LES could be stabilized by prolonged pulsation, while film stability of LES(chol) was only moderately affected by pulsation. These results indicate that SP-A in NS promotes formation of some unique structure, possibly tubular myelin, which induces selective adsorption of lipids into the surface.

Treatment responses to surfactants containing natural surfactant proteins in preterm rabbits

The in vivo function of surfactants reconstituted using natural surfactant lipid and protein constituents was evaluated in 27-day-gestation preterm rabbits. The animals were treated with protein-free surfactant lipids (LH-20), LH-20 + 5% SP-A, LH-20 + 1% SP-B, LH-20 + 1% SP-C, LH-20 + 5% SP-A + 1% SP-B + 1% SP-C (SP-ABC), natural sheep surfactant, or 4 ml/kg 0.45% NaCl (control) and then ventilated with tidal volumes of 8 ml/kg and 3 cm H2O positive end-expiratory pressure (PEEP). Ventilatory pressures (peak pressures minus PEEP) and dynamic compliances of the LH-20 + SP-C rabbits were greater (p < 0.01) than those of control, LH-20, and LH-20 + SP-A groups but lower (p < 0.05) than in the LH-20 + SP-B, LH-20 + SP-ABC, and sheep surfactant groups. Recoveries of intravascular labeled albumin in the lungs were comparable in the LH-20 + SP-B, LH-20 + SP-C, LH-20 + SP-ABC, and sheep surfactant groups and less (p < 0.01) than in LH-20 + SP-A rabbits, which had lower (p < 0.05) recoveries than did the control and LH-20 groups. The postventilation pressure-volume curves for LH-20 + SP-B and LH-20 + SP-ABC rabbits had significantly lower opening pressures, larger maximal lung volumes, and larger retained volumes on deflation relative to the LH-20 + SP-C, LH-20, and control groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of phospholipids with proteins and peptides. New advances III

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.

Lung surfactant proteins, SP-B and SP-C, alter the thermodynamic properties of phospholipid membranes: a differential calorimetry study

The ability of the low molecular weight lung surfactant-associated proteins, SP-B and SP-C, to alter the thermotropic properties of synthetic multilamellar vesicles was tested using differential scanning calorimetry (DSC). The presence of either SP-B or SP-C in dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphosphatidylglycerol (DPPG) multilamellar vesicles broadened the DSC thermogram and reduced the enthalpy of transition in a concentration-dependent manner. With both proteins, the temperature at which the peak of the phase transition (Tm) was detected was shifted to a higher value. The increase in Tm caused by both proteins was greater with DPPG than DPPC. We have interpreted these results as implying the presence of a protein-perturbed domain of lipid. Both SP-B and SP-C were found to influence the surface activity of the phospholipids in a concentration-dependent fashion. We speculate that instability of lipid packing predicted to occur at protein-created lipid domain boundaries may be important for the expression of surface activity in pulmonary surfactant.

Alveolar surfactant and adult respiratory distress syndrome. Pathogenetic role and therapeutic prospects

The adult respiratory distress syndrome (ARDS) is characterized by extended inflammatory processes in the lung microvascular, interstitial, and alveolar compartments, resulting in vasomotor disturbances, plasma leakage, cell injury, and complex gas exchange disturbances. Abnormalities in the alveolar surfactant system have long been implicated in the pathogenetic sequelae of this life-threatening syndrome. This hypothesis is supported by similarities in pulmonary failure between patients with ARDS and preterm babies with infant respiratory distress syndrome, known to be triggered primarily by lack of surfactant material. Mechanisms of surfactant alterations in ARDS include: (a) lack of surface-active compounds (phospholipids, apoproteins) due to reduced generation/release by diseased pneumocytes or to increased loss of material (this feature includes changes in the relative composition of the surfactant phospholipid and/or apoprotein profiles); (b) inhibition of surfactant function by plasma protein leakage (inhibitory potencies of different plasma proteins have been defined); (c) "incorporation" of surfactant phospholipids and apoproteins into polymerizing fibrin upon hyaline membrane formation; and (d) damage/inhibition of surfactant compounds by inflammatory mediators (proteases, oxidants, nonsurfactant lipids). Alterations in alveolar surfactant function may well contribute to a variety of pathophysiological key events encountered in ARDS. These include decrease in compliance, ventilation-perfusion mismatch including shunt flow due to altered gas flow distribution (atelectasis, partial alveolar collapse, small airway collapse), and lung edema formation. Moreover, more speculative at the present time, surfactant abnormalities may add to a reduction in alveolar host defense competence and an upregulation of inflammatory events under conditions of ARDS. Persistent atelectasis of surfactant-deficient and in particular fibrin-loaded alveoli may represent a key event to trigger fibroblast proliferation and fibrosis in late ARDS ("collapse induration"). Overall, the presently available data on surfactant abnormalities in ARDS lend credit to therapeutic trials with transbronchial surfactant administration. In addition to the classical goals of replacement therapy defined for preterm infants (rapid improvement in lung compliance and gas exchange), this approach will have to consider its impact on host defense competence and inflammatory and proliferative processes when applied in adults with respiratory failure.

Surfactant protein B deficiency: radiographic manifestations

Surfactant is a complex structure primarily composed of phospholipids, but containing essential proteins as well. Congenital deficiency of Surfactant Protein-B (SPB) has recently been documented for the first time in two siblings. The pathologic findings in these infants was that of congenital pulmonary alveolar proteinosis and the radiographic manifestations were strikingly similar to hyaline membrane disease.

The surfactant system of the adult lung: physiology and clinical perspectives

Pulmonary surfactant is synthesized and secreted by alveolar type II cells and constitutes an important component of the alveolar lining fluid. It comprises a unique mixture of phospholipids and surfactant-specific proteins. More than 30 years after its first biochemical characterization, knowledge of the composition and functions of the surfactant complex has grown considerably. Its classically known role is to decrease surface tension in alveolar air spaces to a degree that facilitates adequate ventilation of the peripheral lung. More recently, other important surfactant functions have come into view. Probably most notable among these, surfactant has been demonstrated to enhance local pulmonary defense mechanisms and to modulate immune responses in the alveolar milieu. These findings have prompted interest in the role and the possible alterations of the surfactant system in a variety of lung diseases and in environmental impacts on the lung. However, only a limited number of studies investigating surfactant changes in human lung disease have hitherto been published. Preliminary results suggest that surfactant analyses, e.g., from bronchoalveolar lavage fluids, may reveal quantitative and qualitative abnormalities of the surfactant system in human lung disorders. It is hypothesized that in the future, surfactant studies may become one of our clinical tools to evaluate the activity and severity of peripheral lung diseases. In certain disorders they may also gain diagnostic significance. Further clinical studies will be necessary to investigate the potential therapeutic benefits of surfactant substitution and the usefulness of pharmacologic manipulation of the secretory activity of alveolar type II cells in pulmonary medicine.