Vascular calcification and osteoporosis—from clinical observation ...

256 Osteoporos Int (2007) 18:251–259
receptor and interference with insulin and IGF-1 signaling
through inhibition of tyrosine phosphorylation, resulting in
increased resistance to insulin and IGF-1 signaling.
Mice with targeted deletion of Klotho display a short life
span and show many features of premature aging, including
infertility, gonadal and thymus atrophy, skin atrophy,
decreased number of Purkinje cells, physical inactivity
and pulmonary emphysema [13]. Of note, they also
displayed severe osteoporosis and progressive atherosclerosis
with associated medial calcification. Both the vascular
and skeletal abnormalities in Klotho-deficient mice were
prevented by transgenic overexpression of Klotho [13].
Inhibition of insulin and IGF-1 signaling by knockout of
insulin receptor substrate (IRS)-1 is able to rescue Klotho
knockout mice and to restore the normal life span [51].
Further studies have indicated that Klotho confers antiapoptotic
activity for endothelial cells [52] and protects
against oxidative stress [53]. Further vascular studies of
Klotho indicated that it is required for proper angiogenesis
and vasculogenesis, since Klotho-deficient mice displayed
reduced tissue capillary density, impaired angiogenesis and
decreased endothelium-derived nitric oxide release after
ischemic challenge [54]. Of note, histomorphometric analysis
revealed low-turnover osteopenia in Klotho-deficient mice
with a reduced number of osteoblast progenitors and a
reduced osteogenic capacity as determined by matrix nodule
formation [55]. Moreover, Klotho-deficient mice showed
decreased osteoclastogenesis and upregulated expression of
osteoprotegerin (OPG), an osteoclastogenesis inhibitor [55].
Thus, the Klotho-deficient mice are a promising murine
model to assess age-related mechanisms in human diseases
[13]. It also shows the essential role of aging and the
protein Klotho in the common pathogenesis of osteoporosis
and vascular disease.
Osteoprotegerin (OPG)
OPG is a glycoprotein that is abundantly expressed by
various tissues, including the skeleton and the vascular
wall. It circulates in serum and serves as a decoy receptor
for the tumor necrosis factor (TNF) ligand superfamily
members RANKL [56] and TNF-related apoptosis-inducing
ligand (TRAIL) [57]. RANKL is an essential cytokine for
osteoclast differentiation and activation, and thus, a
stimulator of bone resorption, while OPG neutralizes
RANKL and prevents bone resorption and bone loss.
OPG knockout mice show severe bone loss and suffer
from multiple osteoporotic fractures at the age of 1 month.
These fractures include the longitudinal bones and the
vertebral bodies and causes progressive crippling [14]. This
phenotype is characterized by an increased number and
activity of osteoclasts and can be completely rescued by an
OPG transgene [58]. By contrast, mice carrying an OPG
transgene or mice treated with an OPG fusion protein [59]
or RANKL knockout mice [60] have no or fewer
osteoclasts and develop osteopetrosis with hepatosplenomegaly
due to extramedullary hematopoiesis.
Surprisingly, the majority of the OPG-deficient mice
developed severe medial calcifications of the renal arteries
and the aorta that led to aneurysm formation and lethal
vessel rupture and hemorrhage [14]. The vascular abnormalities
were completely abolished using an OPG transgene
approach, but not following postnatal administration of
OPG protein, suggesting local production is important in
the inhibition of vascular calcification [58]. The protective
role of OPG in vascular calcification is also underscored by
a rat model of vascular calcification, in which treatment
with warfarin, an inhibitor of vitamin K-dependent γ-
carboxylation, or supraphysiologic doses of vitamin D are
used to induce diffuse vascular calcification [61]. In these
two models, simultaneous administration of OPG fusion
protein with mineralization-inducing agents prevented
arterial calcification [61].
Unifying hypothesis and conclusions
Here we propose a unifying hypothesis of vascular
calcification, that combines both active and passive
mechanisms, aspects of bone metabolism and age-related
changes (Fig. 2). Under appropriate conditions, cells either
residing in the vascular wall (smooth muscle cells) or
precursor cells with mesenchymal differentiation potential
(e.g., calcifying vascular cells) acquire osteogenic properties,
which may involve BMP and cbfa-1 signaling pathways.
This process is physiologically inhibited by factors
such as Smad6 and others. These osteoblast-like cells
deposit bone matrix proteins that subsequently become
mineralized (1). In addition, matrix vesicles and apoptotic
bodies from calcifying SMC form the nidus for passive
calcification, unless physiological inhibitors are present.
These include fetuin-A, MGP and osteopontin (2). In
addition, fetuin-A forms soluble “calciproteins” and serves
as an opsonin, thus facilitating phagocytic removal of
mineral precipitates (3). The major protective effect of OPG
on the vascular system seems to be due to its potent
inhibition of RANKL, thus suppressing osteoclastic release
of calcium and other minerals from bone (4). Whether OPG
has additional beneficial direct effects on vascular wallresident
cells (by inhibition of TRAIL or RANKL) has not
been shown so far. The link between enhanced bone
resorption and vascular calcification is further supported
by the findings that other inhibitors of bone resorption
(bisphosphonates, selective osteoclastic inhibitor V-H+
ATPase) have similar effects in a vitamin D-induced vascular
calcification model [62, 63]. Aging and age-related changes