PERSPECTIVE
Gene Therapy for Pulmonary Edema

Jahar Bhattacharya

Pulmonary edema, abnormal liquid accumulation in the lung, continues to be a clinically difficult condition for which the mortality rate stubbornly refuses to dip below the 30% mark despite recent therapeutic advances such as the use of mechanical ventilation at low tidal volumes (11). The malignant form of the edema, commonly referred to as the "permeability" type, results from the hyperfiltration and alveolar flooding that follow breakdown of barrier properties of the microvascular and alveolar membranes. However, current therapy is restricted to general protocols for cardiovascular and respiratory support and significantly lacks specific approaches for barrier repair.

The report by Kaner and colleagues in this issue (1) provides compelling evidence that gene therapy approaches require consideration in the management of permeability-type pulmonary edemas. Using adenoviral gene therapy protocol for delivery of the human vascular endothelial growth factor (VEGF) transgene, these authors overexpressed VEGF messenger (m)RNA in mouse lung and successfully induced prolonged pulmonary edema, as indicated by lung water increases sustained for several days. Microvascular barrier deterioration was also evident in increased parenchymal accumulation of plasma albumin, a pathognomonic feature of pulmonary edemas of the permeability type.

An extensive literature attests to the hyperpermeability effect of VEGF in microvessels and cultured endothelial cells (2). Previously, adenovirus-mediated transfer of the VEGF gene in the hindlimb was shown to induce localized edema formation (3). However, cellular mechanisms underlying the hyperpermeability remain presently unclear, although there are indications that molecular profiles of the endothelial tight junction could be altered. Of the several protein types present at the endothelial tight junction, one, namely occludin, requires attention. Since occludin-depleted tight junctions undergo loss of barrier properties (4), VEGF's hyperpermeability effect may be attributed to its ability to leach occludin from endothelial junctions (5). However, other endothelial tight junctional proteins such as the newly recognized claudins (6) may also play a role.

Significantly, all of the lung injury could be abrogated (also by gene therapy) by inducing lung expression of a truncated form of flt-1, the gene for the human VEGF receptor prior to induction of the VEGF gene. The protein expressed by the truncated gene is soluble, yet it binds VEGF and competitively inhibits VEGF binding to cell-surface receptors. Hence, protection from pulmonary edema resulted because of soluble phase ligation of VEGF by the induced flt-1 receptor. The important conclusion to be drawn from these findings is that receptor-mediated edemagenic processes may be inhibited by lung overexpression of soluble receptor. Such "receptor" therapy may provide a means for reducing blood levels of unbound and thus potentially barrier-injurious ligands released under edema-predisposing conditions.

From the experimental standpoint, Kaner and coworkers demonstrate the feasibility of establishing gene therapy models of pulmonary edema. The important advance here is the generation of a model in which pulmonary edema is sustained for almost a week. Clinically, pulmonary edema develops either acutely within hours or subacutely within days. Although experimental models of pulmonary edema commonly replicate the acute time course, few address the subacute's, probably because soluble edemagenic factors used frequently for generating the model are difficult to deliver in a consistent and regulated manner over prolonged periods. Lung overexpression of targeted genes delivered by gene therapy may provide a way out of this constraint.

The lung microvascular membrane consists of a single layer of endothelial cells of the so-called continuous type. Apical tight junctions of these cells are critically responsible for microvascular barrier function. A large number of studies associated with, for example, immune, thrombocoagulative, or stress-induced processes have been to shown to cause lung microvascular barrier deterioration and pulmonary edema. These studies have identified potential edemagenic factors such as reactive oxygen species, cytokines, and bacterial products, as well as cellular mechanisms, which may protect against or promote the edemagenic process. Kaner and associates bring into focus a new player, namely VEGF, which now must be included in the clutch of substances known to be potentially edemagenic in the lung.

Given these considerations, the question of native expression of lung VEGF becomes important particularly under edema-inducing conditions. The literature is both conflicting and inadequate. Thrombin, which induces lung hyperpermeability and pulmonary edema, sensitizes human umbilical endothelial cells to VEGF by upregulating VEGF receptors (7). To the extent that these findings apply to the lung, increased VEGF ligation on lung microvascular endothelial cells could amplify thrombin's edemagenic effects. However, other edemagenic factors, such as bacterial endotoxin (8) and hyperoxia (9), have been shown to decrease lung expression of VEGF mRNA, thus detracting from the VEGF role in these etiologies of pulmonary edema. Also of note is a clinical report that in burn or traumatwo well recognized causes of pulmonary edemaincreases of plasma VEGF levels in fact promoted recovery (10), whereas absence of such increases elevated the incidence of sepsis and respiratory distress. Counterintuitively, these clinical observations appear to argue in favor of a VEGF effect that is protective for microvessels rather than deleterious. Because VEGF is expressed in alveolar cells in proximity to lung microvascular endothelial cells (9), clearly much more needs to be learned regarding its role in lung microvascular regulation, as well as that in the natural history of pulmonary edema.

	Footnotes

Address correspondence to: Jahar Bhattacharya, M.D., Ph.D., Lung Biology Laboratory, St. Luke's Roosevelt Hospital Center, 1000 Tenth Avenue, New York, NY 10019. E-mail: jb39{at}columbia.edu

(Received in original form January 29, 2000).

	References

1. Kaner, R. J., J. V. Ladetto, R. Singh, N. Fukuda, M. A. Mathay, and R. G. Crystal. 2000. Lung overexpression of the vascular endothelial growth factor gene induces pulmonary edema. Am. J. Respir. Crit. Care Med. 22: 657-664 .

2. Bates, D. O., D. Lodwick, and B. Williams. 1999. Vascular endothelial growth factor and microvascular permeability. Microcirc. 6: 83-96 [Medline].

3. Poliakova, L., I. Kovesdi, X. Wang, M. C. Capogrossi, and M. Talan. 1999. Vascular permeability effect of adrenovirus-mediated vascular endothelial growth factor gene transfer to the rabbit and rat skeletal muscle. J. Thorac. Cardiovasc. Surg. 118: 339-347 [Abstract/Free Full Text].

4. Wong, V., and B. M. Gumbiner. 1997. A synthetic peptide corresponding to the extracellular domain of occludin perturbs the tight junction permeability barrier. J. Cell Biol. 136: 399-409 [Abstract/Free Full Text].

5. Antonetti, D. A., A. J. Barber, L. A. Hollinger, E. B. Wolpert, and T. W. Gardner. 1999. Vascular endothelial growth factor induces rapid phosphorylation of tight junction proteins occludin and zonula occludin 1: a potential mechanism for vascular permeability in diabetic retinopathy and tumors. J. Biol. Chem. 274: 23464-23467 .

6. Morita, K., H. Sasaki, M. Furuse, and S. Tsukita. 1999. Endothelial claudin: claudin-5/TMVCF constitutes tight junction strands in endothelial cells. J. Cell Biol. 147: 185-194 [Abstract/Free Full Text].

7. Tsopanoglou, N. E., and M. E. Maragoudakis. 1999. On the mechanism of thrombin-induced angiogenesis: potentiation of vascular endothelial growth factor activity on endothelial cells by up-regulation of its receptors. J. Biol. Chem. 274: 23969-23976 [Abstract/Free Full Text].

8. Tuder, R. M., B. E. Flook, and N. F. Voelkel. 1995. Increased gene expression for VEGF and the VEGF receptors KDR/Flk and Flt in lungs exposed to acute or to chronic hypoxia: modulation of gene expression by nitric oxide. J. Clin. Invest. 95: 1798-1807 .

9. Watkins, R. H., C. T. D'Angio, R. M. Ryan, A. Patel, and W. M. Maniscalco. 1999. Differential expression of VEGF mRNA splice variants in newborn and adult hyperoxic lung injury. Am. J. Physiol. 276: L858-L867 [Abstract/Free Full Text].

10. Grad, S., W. Ertel, M. Keel, M. Infanger, D. J. Vonderschmitt, and F. E. Maly. 1998. Strongly enhanced serum levels of vascular endothelial growth factor (VEGF) after polytrauma and burn. Clin. Chem. Lab. Med. 36: 379-383 [Medline].

11. The Acute Respiratory Distress Syndrome Network. 2000. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute lung injury syndrome. N. Engl. J. Med. 342: 1301-1308 [Abstract/Free Full Text].