Peptidases in the asthmatic airways

Reverse transcription polymerase chain reaction (RT-PCR) analysis of proteolytic enzymes in cultures of human respiratory epithelial cells

BACKGROUND

Pancreatic proteolytic digestive enzymes are a major extracellular barrier to the sucessful systemic delivery of biopharmaceuticals via the oral route, whereas in health in the lungs these powerful proteases are virtually absent from the extracellular fluids. Despite this, the absorption of some (but not all) natural peptides and proteins from the lungs may be poor, and one has to acknowledge that information on the activity and spatial distribution of proteolytic enzymes in the human lung is scarce. Here, we investigated expression patterns of a series of proteolytic enzymes in several human respiratory cell types on mRNA level in an attempt to better understand the fate of inhaled biopharmaceuticals.

METHODS

The mRNA expression of proteolytic enzymes (i.e., carboxypeptidases: CPA1, CPA2, CPB, CPM; gamma-glutamyltransferases: GGT1, GGT2; angiotensin-converting enzymes: ACE, ACE2; aminopeptidases: APA, APB, APN, APP1, APP2, APP3; endopeptidases: 24.11 (neprilysin), 24.15 (thimet oligopeptidase), 24.18 (meprin A); enteropeptidase; trypsin 1, trypsin 2; neutrophilic elastase; dipeptidyl peptidase 4; gamma-glutamylhydrolase) was investigated by semiquantitative RT-PCR in human bronchial (hBEpC, Calu-3, 16HBE14o-) and alveolar (A549) epithelial cells, respectively. Gastrointestinal Caco-2 cells were used as comparison.

RESULTS

Obvious differences were observed in proteinases' expression pattern between the investigated cell types. Although considered to be of bronchial epithelial phenotype, neither Calu-3 nor 16HBE14o- cells matched the mRNA expression pattern of hBEpC in primary culture. Of all investigated cell lines, Caco-2 expresses the highest number of proteases and peptidases.

CONCLUSIONS

Although mRNA expression does not necessarily signify enzyme functionality, our results provide the first comprehensive analysis of peptidase and protease expression and distribution in human lung epithelial cells and are the basis for further investigations.

Regulation of expression and function of dipeptidyl peptidase 4 (DP4), DP8/9, and DP10 in allergic responses of the lung in rats

The expression of dipeptidyl peptidase 4 (DP4, CD26) affects T-cell recruitment to lungs in an experimental rat asthma model. Furthermore, the gene of the structural homologous DP10 represents a susceptibility locus for asthma in humans, and the functional homologous DP8/9 are expressed in human leukocytes. Thus, although several mechanisms may account for a role of DP4-like peptidases in asthma, detailed information on their anatomical sites of expression and function in lungs is lacking. Therefore, bronchi and lung parenchyma were evaluated using immunohistochemistry and histochemical/enzymatic activity assays, as well as quantitative real-time PCR for this family of peptidases in naïve and asthmatic rat lungs derived from wild-type F344 and DP4-deficient F344 rat strains. Surprisingly, results show not only that the induction of experimental asthma increases DP4 enzymatic activity in the bronchoalveolar lavage fluid and parenchyma, but also that DP8/9 enzymatic activity is regulated and, as well as the expression of DP10, primarily found in the bronchial epithelium of the airways. This is the first report showing a differential and site-specific DP4-like expression and function in the lungs, suggesting a pathophysiologically significant role in asthma.

Insulin disposition in the lung following oral inhalation in humans : a meta-analysis of its pharmacokinetics

BACKGROUND

Oral inhalation of insulin potentially offers non-invasive treatment and better glycaemic control in diabetes by virtue of its apparently faster absorption into the systemic circulation compared with subcutaneous injection. Nevertheless, the lung kinetics of inhaled insulin in humans have yet to be fully clarified because of the complexity of insulin-glucose (patho)physiology and the difficulty in approximating the inhaled dose. As a result, there remains considerable debate on the mechanisms of absorption and metabolism of insulin in the lung.

OBJECTIVES

To develop and apply a physiologically realistic insulin-glucose kinetic model to a meta-analysis of insulin-glucose profiles from well-controlled clinical studies of inhaled insulin published in the literature, and thereby, to derive the kinetic descriptors of insulin in the lung following inhalation through curve fitting.

MODEL DEVELOPMENT

The model assumed first-order absorption (k(a,L)) and parallel non-absorptive loss (k(mm,L)), the latter primarily occurring via metabolism and mucociliary clearance in the lung, alongside two systemic compartments. Where necessary, glucose-dependent endogenous pancreatic insulin secretion was also taken into account by using blood glucose data as the second independent variable.

RESULTS

Despite the model's simplicity and the use of mean data, 16 insulin-glucose profiles from ten clinical studies were successfully fitted to the model, yielding values for the rate constants k(a,L) and k(mm,L). Whole serum insulin profiles were rate-determined by k(a,L) and k(mm,L) combined, representing 'flip-flop' pharmacokinetics. The best estimate for k(a,L) was found to be 0.020-0.032 h(-1), effectively unchanged across doses (0.3-1.8 IU/kg), formulations (powder vs liquid) and subjects (healthy vs diabetic), suggesting passive diffusive absorption of insulin from the lung. In contrast, the values for k(mm,L) were much larger (0.5-1.6 h(-1)) and decreased with increasing inhaled dose. Therefore, it is likely that dose-dependent saturable lung metabolism controls the value of k(mm,L), alongside mucociliary clearance. As a result, the absolute bioavailability ranged from 1.5% to 4.8%. The modelling analysis also enabled derivation of increased values for both k(a,L) and k(mm,L) as a possible cause of faster absorption for deep inspiratory manoeuvres and increased absorption in smokers, and faster and increased absorption for insulin lispro.

CONCLUSIONS

Although some of these results need to be substantiated experimentally, it appears that this modelling analysis has enabled unification of the literature information associated with the kinetics and mechanisms of insulin disposition in the lung following inhalation in humans.