AN2501 Extended Essay Derek Scott
“Discuss the possible roles of human keratinocyte growth factor in
lung development.”
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Essay target was 1000 ± 100 words
Essay was written by a student at level 2 of undergraduate
degree.
Student did not have access to electronic search engines to
find journal articles.
Several recent scientific studies have shown that keratinocyte growth
factor (hKGF) plays an important role in lung repair and, obviously,
development. Most of these studies have been conducted on continuous
mouse, rabbit, rat or lung keratinocyte cell lines in the hope that, by
learning more about the effects of KGF on the growth and development of
the lungs, we may be able to develop new therapeutic or diagnostic
methods of dealing with lung disease or malfunction.
Keratinocyte growth factor (KGF)
KGF is a member of the fibroblast growth factor (FGF) family, and is also
known as FGF-7 (Finch et al., 1995; Housely et al., 1994). It is only one
of several growth factors that have been identified as acting in lung
growth and repair (see Table 1 and Figure 1), and it does so by affecting
keratinocyte proliferation in a paracrine manner (Perkett, 1995; Marchese
et al., 1990). KGF was first identified as a highly specific keratinocyte
mitogen after its isolation from a rat lung stromal fibroblast line (Housely
et al., 1994; Wilson et al., 1993). It is recognised by KGF receptors on
certain epithelial cells (Marchese et al., 1990; Rubin et al., 1989). KGF is
not only found in the lungs, but influences the proliferation and
differentiation patterns of epithelial cells in the skin, gut and reproductive
tracts also. The gut, in particular, responds to and synthesises KGF
(Housely et al., 1994; Marchese et al., 1990). KGF also has a signal
peptide for secretion (Perkett, 1995).
Table 1. Selection of the growth factors involved in lung repair.
This table depicts the wide variety of potential regulators of lung repair. The wide variety illustrates
the difficulty in working out which growth factors might be the most important in lung growth and
repair (Table taken from Perkett, 1995).
Figure 1. Stages of lung injury and repair, with some of the potential growth factors
involved.
The cascade sequence which occurs in the lung during establishment of normal architecture. FGF –
fibroblast growth factor, HGF – hepatocyte growth factor, IGF – insulin-like growth factor, KGF –
keratinocyte growth factor, PDGF – platelet-derived growth factor, TGF – transforming growth factor,
VEGF – vascular endothelial growth factor. (Image taken from Perkett, 1995).
Role of Keratinocytes
In the lung itself, there are two main kinds of keratinocytes
(pneumocyte) – Type I and Type II. It is the Type II pneumocytes
which are the important cells in lung development and also in
recovery after injury since they proliferate and differentiate into
Type I cells. These Type II cells are mitotically active and cuboidal
in shape, in comparison to the flat, metabolically-inactive Type I
cells which form about 95% of lung architecture. Type II cells are
usually found in the corners of alveoli, and secrete pulmonary
surfactant. This surfactant is a mixture of proteins and lipids that
lowers alveolar surface tension, thus stabilising alveolar structure
(see Figure 2). Type II cells are very prominent in foetal lungs and
are very likely to be responsible for the developmental changes
which occur in them. Without the proper development of Type II
cells, the foetus cannot form functional lungs or surfactant,
preventing its safe transition, preventing its safe transition from an
amniotic to an external environment (Ulich et al., 1994; Panos et
al., 1993 & 1995).
The differentiation of Type II to Type I cells allows the
establishment of normal alveolar parenchymal architecture since
they line alveolar septae as rows of columnar cells (Yi et al., 1995;
Ulich et al., 1994) (see Figure 3). It is thought that not only KGF,
but glucocorticoids, oestrogen, thyroid hormones, bombesin and
EGF promote and enhance foetal lung maturation and surfactant
production in vivo. KGF is also important in the re-establishment of
lung structure after injury since it prevents fibrosis of tissue
(Perkett, 1995). An overdose of KGF, however, may cause fibrosis.
KGF induces an up-regulated expression of the protein v6 integrin
in developing tissues and at wound sites. This affects cell spreading,
migration, and growth during reorganisation of epithelia in
development, repair and neoplasia (Breuss et al., 1995).
Epithelial-mesenchymal interactions mediate aspects of normal lung
development and growth during establishment of normal alveolar
architecture (Panos et al., 1993). It must be remembered that lung
growth and repair requires a coordinated cascade of events with
growth factors being the key regulators (Ulich et al., 1994) (see
Figure 1).
Effects of KGF on Pneumocytes
Ulich et al. (1994) tried to examine the in vivo effects of KGF by
injecting hKGF intratracheally in rats. Three days after injection, the
Type II cells in the lungs exhibited hyperplasia which appeared
histologically similar to healing epithelium in lungs. By day 6, the
epithelium had returned to normal (see Figure 4). Adult rat lungs
expressed mRNA for KGF and its receptor, indicating that KGF is
involved in epithelial maintenance. The effects of KGF were doseand time-dependent. The results of histological sections were
confirmed by immunohistochemical marking of Surfactant protein B
(SP-B). SP-B is produced in hyperplastic alveolar lining cells.
Lamellar inclusions were also seen, features which are characteristic
of surfactant-secreting Type II cells. KGF did not seem to have any
effect on the larger, conducting airways.
The work of Panos et al. (1995) established that intratracheal
administration of recombinant hKGF stimulated Type II cell
proliferation in vivo, and could reduce the effects of hyperoxia-
induced injury in rats. Immunohistochemical analysis was used to
determine intrapulmonary distribution and cellular localisation of
KGF instilled into the rats’ tracheas. Six hours after administration,
KGF was detected in lung parenchyma and along alveolar epithelial
cell membranes. By 18-24 hours, KGF was detected intracellularly
in alveolar epithelial cells and intra-alveolar macrophages. KGF was
not found after 48 hours or in any sections taken from the lung
tissue. Intra-tracheal instillation of 5 mg/kg KGF stimulated a
marked, time-dependent increase in Type II cells that were labelled
(see Figure 5). The increase in cell proliferation was documented by
flow cytometric analysis of isolated Type II cells, which revealed a
5-fold increase in the number of cells which were in the S and G2/M
phases of the cell cycle. Rats were also treated with KGF and placed
in a hyperoxic environment to test if KGF affected lung injury
responses. Animals treated with 1-5 mg/kg showed much lower
mortality rates. Lower doses of KGF gave the same results as those
found with rats that were untreated or who were given heatdenatured KGF (see Figure 6). The treated rats which survived the
hyperoxia had minimal haemorrhage and no exudate in the intraalveolar space.
The work of Rannels & Rannels in 1988 had already given the
background to other more recent studies that Type II cells have a
well-known proliferative response after exposure to insults such as
high concentrations of oxygen or nitrogen dioxide. Typically, the
cells would enlarge, divide and spread to re-epithelialize an alveolar
basement membrane denuded by loss of the Type I cells. In rats, it
only took 45 days after lung bilobectomy for components of
parenchymal architecture (including capillary and alveolar surface
area, volumes and cell populations) to be restored to normal.
Everett et al. (1990) also demonstrated that Type II cells (and
therefore KGF) played an integral role in lung architecture
maintenance by showing that hyperoxia and bacterial infection were
potent stimuli of alveolar epithelial and interstitial hyperplasia. They
saw decreased surfactant production in human lung injury patients
also. Thus, in certain lung injuries that include respiratory failure,
cell cycle kinetics are altered and, possibly, cellular phenotypes are
changed.
KGF is obviously extremely important in lung growth and
development, but whether it will actually be useful in promoting
human lung development or repair when given artificially to humans
is not yet known, since it has not yet undergone clinical trials. Much
is still to be understood about the control of KGF bioactivity (e.g.
receptors, binding proteins, interactions etc.) (Panos et al., 1993;
Finch et al., 1989; Ulich et al., 1994). Directed delivery of KGF into
the lungs may be a form of therapy whereby alveolar epithelium
could be preserved/restored during exposure to hyperoxia or other
injurious agents. (Panos et al., 1995). KGF might facilitate reepithelialisation of the airways or assist in the survival of babies
born with respiratory distress syndrome, where their ability to
produce pulmonary surfactant is compromised. From experimental
evidence (Yi et al., 1995), it is thought that there would be little
chance of permanent, deleterious side-effects due to treatment with
KGF, since its actions are stopped or even reversed after cessation
of treatment. Overdoses could even be reversed by targeting KGF
with monoclonal antibodies (Sugahara et al., 1995).
In summary, a better understanding of KGF and other factors will
allow us to understand our own lung development more clearly, as
well as allowing us to conceive therapeutic interventions that could
prevent some chronic lung diseases (Perkett, 1995).
Reference List
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Expression of the beta 6 integrin subunit in development, neoplasia and tissue
repair suggests a role in epithelial remodeling. Journal of Cell Science, 108(6),
2241-51.
Everett, M.M., King, R.J., Jones, M.B., Martin, H.M. (1990). Lung fibroblasts from
animals breathing 100% oxygen produce growth factors for alveolar type II cells.
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