Aging
Alters Endothelial Activation in Vivo:
Endothelial activation was initially defined by Cotran and Pober to be a
program of altered gene expression in vascular endothelial cells induced
by inflammatory cytokines that results in local recruitment of leukocytes
into the tissue. In other words, activating new gene expression in endothelial
cells results in inflammation. In a simple experimental model, such as intradermal
injection of tumor necrosis factor, there is a stereotyped endothelial response,
beginning with increased endothelial E-selectin expression, which correlates
with the onset of neutrophil accumulation in the local tissue, followed
by increased endothelial expression of intercellular adhesion molecule-1
(ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1), which correlates
with the onset of monocyte and T-lymphocyte infiltration. It is now well
appreciated that this pattern may vary with the nature of the inflammatory
stimulus (eg, TNF alone versus TNF in combination IL-4 or IFN-[gamma]),
with the anatomic tissue (eg, skin versus lung), and with the species under
study (eg, rodents, whose neutrophils can interact with VCAM-1, versus humans,
whose neutrophils cannot). In the present issue of the journal, Ashcroft
and colleagues introduce yet another variable that must be considered: the
age of the subject. In a carefully controlled series of biopsies from experimental
wounds in healthy adult human volunteers of varying age, clear differences
emerge between the responses of older adults (60+ years) compared to young
(19 to 39 years) and middle-aged adults (40 to 59 years). Specifically,
older subjects showed augmented E-selectin expression and neutrophil accumulation
at early time-points and delayed ICAM-1 and VCAM-1 expression correlated
with a delay in mononuclear cell accumulation. Because both monocytes (and
their macrophage tissue counterparts) and lymphocytes are important sources
of cytokines and growth factors that influence wound healing, this study
may point to the basis for impaired healing in the elderly. It is important
to etermine in future studies whether the observed differences reflect age-related
changes in endothelial responsiveness or age-related alterations in the
cytokines that are released in response to the stimulus of wounding. This
study also suggests that effective anti-inflammatory therapy in the elderly
may differ from that in younger adults.
Guanylin: A Secretagogue
in the Human Intestine:
Diarrhea caused by the Escherichia coli heat-stable enterotoxin (STa) is
mediated by a receptor in the luminal membranes of intestinal epithelial
cells. This receptor, guanylyl cyclase C (GC-C), activates guanylyl cyclase
and stimulates chloride and bicarbonate secretion via the cystic fibrosis
transmembrane conductance regulator. The natural ligand for the GC-C receptor,
designated guanylin, is a 15-amino acid peptide with approximately 50% amino
acid identity to STa. GC-C receptors are present in the intestine, kidney,
adrenal gland, brain, and airways. The presumed site of synthesis of guanylin
is the intestine; it is interesting, therefore, that guanylin can be detected
not only within the gut lumen, but also within blood. A related peptide
with 60% identity to STa, uroguanylin, is also present in the intestine;
uroguanylin exhibits maximal activity at acidic pH, whereas guanylin is
most potent at alkaline pH. The physiologic function of these two peptides
is not yet known, but it is reasonable to postulate that they function as
endogenous regulators of electrolyte secretion. An important step in defining
their physiologic function is identifying their respective sites of synthesis.
In the present issue of Laboratory Investigation, Cohen and coworkers examine
the localization of guanylin mRNA in the human intestine, using in situ
hybridization (see cover photograph). They demonstrate that guanylin mRNA
is expressed in multiple epithelial cell lineages along the duodenal-to-colonic
axis, with a widespread distribution. Importantly, this expression is not
limited to enteroendocrine cells or Paneth cells, but also includes conventional
crypt and villous cells in the small intestine as well as crypt and surface
epithelial cells in the colon. This study therefore demonstrates that the
molecular machinery is in place for widespread regulation by guanylin of
electrolyte and water secretion in the gut. Evidence of this extensive mRNA
distribution further suggests that guanylin acts not only as an endocrine
peptide as would be expected from synthesis by enteroendocrine cells, but
also as a local autocrine or paracrine mediator through its synthesis in
apically secreting epithelial cells. An additional finding is the virtual
absence of guanylin mRNA expression in the epithelial cells of five moderately
differentiated colorectal adenocarcinomas. Because the guanylin gene, Guca2,
has been mapped to a region of human chromosome 1 that frequently exhibits
loss of heterozygosity in human colon cancer, this provocative finding raises
the possibility that guanylin gene expression may be important in the maintenance
of epithelial cell differentiation.
Protease Cascades in Osteoarthritis:
Although cytokine cascades have been long established in immunology and
enzyme cascades have been recognized in clotting and complement activation,
the roles of and interactions between serine protease and matrix metalloproteinase
cascades in osteoarthritis are not as well recognized or understood. In
the article by Ohta et al, the expression of matrix metalloproteinase-7
(matrilysin, MMP-7) in human osteoarthritic cartilage is demonstrated. Furthermore,
its roles as a cytokine-induced enzyme capable of digesting a number of
cartilage matrix components and as an activator of other matrix metalloproteinases
present in osteoarthritic cartilage are also documented. Thus, MMP-7 may
function as a central modulator of articular cartilage destruction in the
development of osteoarthritis for several reasons: first, MMP-7 is a downstream
responder to cytokine stimulation (IL-1 and TNF[alpha]) and can be activated
by MMP-3, which is, in turn, activated by serine proteases; second, it is
an activator of other cytokine-induced matrix metalloproteinases present
in osteoarthritis, including proMMP-1 and proMMP-9; and, third, MMP-7 is
an enzyme capable of efficient digestion of a wide variety of cartilage
matrix components, including aggrecan. A more complete understanding of
MMP-7 synthesis, activation, and regulation will eventually lead to a better
understanding of the interactions between cytokine and protease cascades
in a variety of degenerative and inflammatory conditions.
Human Imprinted Gene Transcripts
in Archival Tissue:
In recent years, an increasing number of genes have been identified, both
in mice and humans, that are differentially expressed depending on their
parental origin. Such genes, which ``know and remember'' from which parent
they are inherited, are called imprinted genes. The importance of imprinting
an event thought to take place in the germ cell is best appreciated in the
context of development. During mammalian development, the two parental genomes
are not functionally equivalent and play complementary roles involving monoallelic
expression at certain loci. When imprinting is tampered with in the laboratory,
lethality occurs by midgestation. Thus far, approximately 15 imprinted genes
have been identified in the mouse and/or human, and they tend to be clustered
within domains of specific chromosomes. The study of imprinted genes is
of special interest to tumor biologists because a cluster in the human chromosome
region 11p15.5 contains imprinted genes that is associated with the Beckwith-Wiedemann
syndrome and is subject to preferential loss of heterozygosity (LOH) in
embryonal tumors of childhood. In this issue of Laboratory Investigation,
Soejima et al report on the expression pattern of two imprinted genes in
Wilms' tumor: p57KIP2 and IGF2, both of which are located in the 11p15.5
region. The results of their studies, using archival tissue samples, indicate
that the expression of p57KIP2, a cyclin-dependent kinase inhibitor, in
Wilms' tumor is preferentially maternal. However, in the tumors exhibiting
LOH at the KIP2 maternal locus, the paternal allele is expressed. The maintenance
of KIP2 expression suggests that p57KIP2 is an unlikely candidate for a
tumor suppresser gene active in the pathogenesis of Wilms' tumor. Once more,
archival paraffin-embedded tissue has proven to be a rich source of reagents
that can be put to use in illuminating the mechanisms of disease---thus
providing another good argument, if more are needed, against discarding
tissue blocks!
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