Biology Reference
In-Depth Information
exclusively associated with pathological phenotypes but it is also observed under
physiological conditions. Bone-resorbing osteoclasts form a sealed and acidified
space between the osteoclast and the bone surface, called the resorption lacuna.
Vacuolar ATPase-driven acidification leads to the demineralization of the bone
matrix and thus to the exposure of the type I collagen scaffold. This acidification
also provides optimal pH conditions in the resorption lacuna for cathepsin K,
the predominant collagenase of osteoclasts (Bromme et al.
1996
; Xia et al.
1999
).
Cathepsin K-dependent bone resorption will be described in more detail in
Sect.
2.5.1
.
Besides the neutral pH, the oxidative environment outside of lysosomes is
thought to be a major predicament for the extracellular activity of cathepsins. It
was thought that the active site cysteine residue of cathepsins is rapidly oxidized
leading to the irreversible inactivation of the proteases. However, recent studies
have shown that cathepsins can retain significant catalytic activities under oxida-
tive stress. Thyroglobulin was degraded by cathepsins B, L, K, and S at pH 7.4 and
under oxidative conditions (Jordans et al.
2009
). It was also shown that H
2
O
2
oxidation of cathepsins is partially reversible. For example, about 30% of cathep-
sin K activity could be restored by dithiothreitol after exposure to H
2
O
2
(Godat
et al.
2008
).
Overexpression of cathepsins is frequently accompanied by the secretion of
procathepsins. A typical example is the massive secretion of the major excreted
protein (MEP) from 3T3 fibroblasts which was subsequently identified as cathepsin L
(Gal et al.
1985
; Mason et al.
1987
). These findings are primarily derived from cell
culture studies and the analysis of the culture media. As discussed above, pro-
cathepsins are catalytically inactive, and thus a significant contribution of secreted
cathepsins to ECM degradation was doubted. However, this argument can be
dismissed when an acidic peri- or extracellular pH is considered. Under acidic pH
conditions, procathepsins are effectively processed into catalytically mature pro-
teases either autocatalytically (Pungercar et al.
2009
; Vasiljeva et al.
2005
)orby
other proteases. Moreover, extracellular components such as polysaccharides can
facilitate the processing of cathepsins as shown for procathepsin L (Mason and
Massey
1992
). Furthermore, the pericellular mobilization of active cathepsins by
macrophages seem to be facilitated by the presence of an elastin-containing ECM
that could act in a positive feedback mechanism to increase the pathophysiological
remodeling of the ECM (Reddy et al.
1995
).
Extralysosomal cathepsin activity is tightly controlled by endogenous inhibitors
such as cystatins (Abrahamson et al.
2003
). Cystatins are small protein inhibitors of
approximately 10-13 kDa. These inhibitors primarily protect against the accidental
release of cathepsins from lysosomes into the intracellular cytosolic or extracellular
environment. Thus cells are protected by various intracellular cystatins such as
cystatins A and B and extracellular cystatins such as cystatin C. Cystatin C is
prevalent in serum and other body fluids. Cystatins are highly selective against
papain-like cathepsins exhibiting inhibitor constants in the picomolar range and
represent a major safeguard against an unwanted extracellular matrix degradation
by cathepsins. However, it has been reported that cystatin can be downregulated in
