Institutionen för kvinnors och barns hälsa / Department of Women's and Children's Health
Abstract
More than 300 years ago Dr. Stephen Hales drilled holes in the shaft of
chicken bones and noted that as the animal grew, the distance between
these holes remained constant. This led him to conclude that longitudinal
bone growth occurs at the end of the long bones, rather than in the
middle. During the subsequent centuries, employing increasingly
sophisticated approaches, we have learned that bone elongation involves
cells located in the cartilage of the epiphyseal growth plate at the end
of the long bones.
The transient epiphyseal growth plate consists of cartilage present only
during the growth period. Cell proliferation and differentiation and
subsequent bone formation in this cartilage are controlled by various
endocrine, autocrine and paracrine factors which finally eliminate the
cartilaginous tissue and promote epiphyseal fusion. It is well known that
sex steroids in particular estrogens, play an important role in
longitudinal bone growth during puberty. High doses of estrogen therapy
can reduce the final height of an individual, but such treatment is also
associated with severe side-effects. At the same time, attenuation of
estrogen production by aromatase inhibitors increases this final height,
inhibiting bone turnover, which influences bone architecture and may
increase the risk for vertebrae fracture.
Selective estrogen receptor modulators (SERMs), which display either
estrogenic and or anti-estrogenic effects, bind to estrogen receptors
ER(s) with different affinities and subsequently recruit co-modulators of
transcription in a tissue specific manner. Therefore, our hypothesis is
that SERMs may prove to be valuable tools for modulating longitudinal
bone growth.
First, we examined the effect of tamoxifen, a first generation SERM, on
the longitudinal growth of fetal rat metatarsal bones, in culture. We
found that this drug retards such growth in a dose-dependent manner, as a
result of specific elimination of chondrocytes, primarily in the resting
zone of the growth plate, by apoptosis (Paper I).
To extend thesefindings to the in vivo situation and at the same time
evaluate the long-term effects of tamoxifen on bone growth and
mineralization, we used young male rats. At a clinically relevant dose
tamoxifen causes persistent retardation of longitudinal and cortical
radial bone growth in these animals (Paper II).
Next in attempt to improve clinical approaches to altering growth plate
cartilage and longitudinal bone growth by reducing side effects, we
investigated Trans-resveratrol (3, 5, 4΄-trihydroxystilbene), a phytoSERM
with a polyphenolic structure that is produced by a variety of plants in
response to infection. We found that in ovariectomized rabbits,
resveratrol improves both axial and appendicular bone growth, an effect
associated with an increased number and size of hypertrophic chondrocytes
and attenuation of the expression of VEGF by these same cells. At the
same time, the serum level of IGF-I was unaltered by treatment with this
phytoSERM (Paper III).
Finally, we developed new culturing conditions that allow long-term study
of the growth of postnatal rat metatarsal bones ex vivo. This model can
be employed to characterize persistent long-term growth in culture under
serum-free conditions, and responses to known suppressors and stimulators
of bone growth, thereby offering the possibility to study the phenomenon
of catch-up growth in vitro. This system also facilitates the screening
of the effects of various SERMs at different concentrations on postnatal
bones, the growth of which is regulated in a different manner than that
of fetal bones (Paper IV).
The studies described here demonstrate that SERMs have the potential to
influence growth plate cartilage in such a manner as to affect the
longitudinal bone growth