Here we review and extend a new unitary model for the pathophysiology of involutional osteoporosis that identifies estrogen (E) as the key hormone for maintaining bone mass and E deficiency as the major cause of age-related bone loss in both sexes. Also, both E and testosterone (T) are key regulators of skeletal growth and maturation, and E, together with GH and IGF-I, initiate a 3- to 4-yr pubertal growth spurt that doubles skeletal mass. Although E is required for the attainment of maximal peak bone mass in both sexes, the additional action of T on stimulating periosteal apposition accounts for the larger size and thicker cortices of the adult male skeleton. Aging women undergo two phases of bone loss, whereas aging men undergo only one. In women, the menopause initiates an accelerated phase of predominantly cancellous bone loss that declines rapidly over 4-8 yr to become asymptotic with a subsequent slow phase that continues indefinitely. The accelerated phase results from the loss of the direct restraining effects of E on bone turnover, an action mediated by E receptors in both osteoblasts and osteoclasts. In the ensuing slow phase, the rate of cancellous bone loss is reduced, but the rate of cortical bone loss is unchanged or increased. This phase is mediated largely by secondary hyperparathyroidism that results from the loss of E actions on extraskeletal calcium metabolism. The resultant external calcium losses increase the level of dietary calcium intake that is required to maintain bone balance. Impaired osteoblast function due to E deficiency, aging, or both also contributes to the slow phase of bone loss. Although both serum bioavailable (Bio) E and Bio T decline in aging men, Bio E is the major predictor of their bone loss. Thus, both sex steroids are important for developing peak bone mass, but E deficiency is the major determinant of age-related bone loss in both sexes.

Sex steroids and the construction and conservation of the adult skeleton.

The insulin-like growth factors (IGFs) are mitogens that play a pivotal role in regulating cell proliferation, differentiation, and apoptosis. The effects of IGFs are mediated through the IGF-I receptor, which is also involved in cell transformation induced by tumor virus proteins and oncogene products. Six IGF-binding proteins (IGFBPs) can inhibit or enhance the actions of IGFs. These opposing effects are determined by the structures of the binding proteins. The effects of IGFBPs on IGFs are regulated in part by IGFBP proteases. Laboratory studies have shown that IGFs exert strong mitogenic and antiapoptotic actions on various cancer cells. IGFs also act synergistically with other mitogenic growth factors and steroids and antagonize the effect of antiproliferative molecules on cancer growth. The role of IGFs in cancer is supported by epidemiologic studies, which have found that high levels of circulating IGF-I and low levels of IGFBP-3 are associated with increased risk of several common cancers, including those of the prostate, breast, colorectum, and lung. Evidence further suggests that certain lifestyles, such as one involving a high-energy diet, may increase IGF-I levels, a finding that is supported by animal experiments indicating that IGFs may abolish the inhibitory effect of energy restriction on cancer growth. Further investigation of the role of IGFs in linking high energy intake, increased cell proliferation, suppression of apoptosis, and increased cancer risk may provide new insights into the etiology of cancer and lead to new strategies for cancer prevention.

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Role of the insulin-like growth factor family in cancer development and progression

I. Introduction II. Characteristics of the IGFBPs III. Target Cell Actions of the IGFBPs A. To modulate IGF actions B. To facilitate storage of IGFs in extracellular matrices C. To exert IGF-independent effects IV. IGF-IGFBP Complexes in Biological Fluids A. Serum B. Milk C. Urine D. Cerebrospinal fluid (CSF) E. Follicular fluid F. Amniotic fluid G. Lymph H. Seminal fluid I. Other biological fluids V. Assays for Circulating Levels of IGFBP A. Western ligand blotting B. Western immunoblotting C. RIA D. Immunoradiometric assay (IRMA) VI. Relative Distribution of IGFBPs in Serum VII. Regulation of Serum IGFBPs A. Physiological conditions B. Development and aging C. Hormonal effects: mechanisms D. Pathological conditions VIII. IGFBP Proteases in Circulation A. Proteases under normal conditions B. Pregnancy-associated proteases C. Proteases under catabolic and disease states IX. Endocrine Functions of IGFBPs in Serum A. To prevent insulin-like effects B. To increase the half-lives of IGFs C. To control the tra...

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Insulin-Like Growth Factor-Binding Proteins in Serum and Other Biological Fluids: Regulation and Functions

Bone grafts and biomaterials substitutes for bone defect repair: A review.

We propose here a new unitary model for the pathophysiology of involutional osteoporosis that identifies estrogen (E) deficiency as the cause of both the early, accelerated and the late, slow phases of bone loss in postmenopausal women and as a contributing cause of the continuous phase of bone loss in aging men. The accelerated phase in women is most apparent during the first decade after menopause, involves disproportionate loss of cancellous bone, and is mediated mainly by loss of the direct restraining effects of E on bone cell function. The ensuing slow phase continues throughout life in women, involves proportionate losses of cancellous and cortical bone, and is associated with progressive secondary hyperparathyroidism. This phase is mediated mainly by loss of E action on extraskeletal calcium homeostasis which results in net calcium wasting and increases in the level of dietary calcium intake required to maintain bone balance. Because elderly men have low circulating levels of both bioavailable E and bioavailable testosterone (T) and because recent data suggest that E is at least as important as T in determining bone mass in aging men, E deficiency may also contribute substantially to the continuous bone loss of aging men. In both genders, E deficiency increases bone resorption and may also impair a compensatory increase in bone formation. For the most part, this unitary model is well supported by observational and experimental data and provides plausible explanations to traditional objections to a unitary hypothesis.

A Unitary Model for Involutional Osteoporosis: Estrogen Deficiency Causes Both Type I and Type II Osteoporosis in Postmenopausal Women and Contributes to Bone Loss in Aging Men

Fluoride is one of the most potent but least well understood stimulators of bone formation in vivo. Bone formation was shown to arise from direct effects on bone cells. Treatment with sodium fluoride increased proliferation and alkaline phosphatase activity of bone cells in vitro and increased bone formation in embryonic calvaria at concentrations that stimulate bone formation in vivo.

https://science.sciencemag.org/content/sci/222/4621/330.full.pdf

Fluoride directly stimulates proliferation and alkaline phosphatase activity of bone-forming cells.

This report describes the first observation of a direct mitogenic effect of androgens on isolated osteoblastic cells in serum-free culture. [3H]thymidine incorporation into DNA and cell counts were used as measures of cell proliferation. The percentage of cells that stained for alkaline phosphatase was used as a measure of differentiation. Dihydrotestosterone (DHT) enhanced mouse osteoblastic cell proliferation in a dose dependent manner over a wide range of doses (10(-8) to 10(-11) molar), and was maximally active at 10(-9) M. DHT also stimulated proliferation in human osteoblast cell cultures and in cultures of the human osteosarcoma cell line, TE89. Testosterone, fluoxymesterone (a synthetic androgenic steroid) and methenolone (an anabolic steroid) were also mitogenic in the mouse bone cell system. The mitogenic effect of DHT on bone cells was inhibited by antiandrogens (hydroxyflutamide and cyproterone acetate) which compete for binding to the androgen receptor. In addition to effects on cell proliferation, DHT increased the percentage of alkaline phosphatase (ALP) positive cells in all three bone cell systems tested, and this effect was inhibited by antiandrogens. We conclude that androgens can stimulate human and murine osteoblastic cell proliferation in vitro, and induce expression of the osteoblast-line differentiation marker ALP, presumably by an androgen receptor mediated mechanism.

Androgens directly stimulate proliferation of bone cells in vitro

We determined the skeletal content of insulin-like growth factor-I (IGF-I) and transforming growth factor-beta (TGF beta) in human bone as a function of age, using 66 samples of femoral cortical bone obtained from 46 men and 20 women between the ages of 20-64 yr. We found a linear decline in the skeletal content of IGF-I (nanograms per mg protein) with donor age (r = -0.43; P < 0.001) in the total population. The skeletal content of TGF beta also decreased with age (i.e. 1/TGF beta vs. age; r = 0.28; P < 0.02) for the total population. We did not observe any difference in the skeletal growth factor content between male and female donors. IGF-I content, when analyzed by decade divisions of age, showed a reduction between the 20- to 29-yr-old and the 50- to 59-yr-old subjects (P < 0.02). The loss rate of IGF-I was 1.56 ng/mg protein.yr, corresponding to a net loss of 60% of skeletal IGF-I between the ages of 20-60 yr. The loss rate of TGF beta was 0.03 ng/mg protein.yr, corresponding to a net loss of 25% of the skeletal TGF beta between the ages of 20-60 yr.

Age-related decreases in insulin-like growth factor-I and transforming growth factor-beta in femoral cortical bone from both men and women: implications for bone loss with aging.

Circulating and skeletal insulin-like growth factor-I (IGF-I) concentrations in two inbred strains of mice with different bone mineral densities.

These studies were intended to examine the relationship between skeletal collagen formation and skeletal alkaline phosphatase (ALP) activity in vitro. Embryonic chick calvaria were exposed to skeletal effectors (including high and low pH, a range of [p i ] and [Ca], PTH, NaF, etc), and collagen formation was assessed by the incorporation of 3 [H]-proline as 3 [H]-hydroxyproline ( 3 [H]-hyp). ALP activity was measured in the serum-free conditioned medium and in 20% butanol extracts of the bones. We found that ALP activity and 3 [H]-hyp incorporation were coordinately increased from pH 5.5 to pH 7.2 ( r = .99, P i ], but low [Ca] increased ALP and coordinately decreased collagen formation ( r = −.81, P 3 [H]-hyp incorporation were coordinately increased by NaF, vanadate, PGE 2 , calcitonin, and insulin, the slopes of the correlations were not the same for all effectors (eg, NaF: r = .97, P r = .95, P 3 [H]-hyp incorporation. When a variety of effectors, including low [Ca], were used to treat different groups of calvaria, ALP activity was not correlated with 3 [H]-hyp incorporation ( r = .35), but when the exposure to effectors was limited to a preincubation, or when the low [Ca] data were excluded, a correlation was observed ( r = .87, P r = .64, P r = .60, P r = .89, P 3 [H]-hyp incorporation and (2) even under conditions in which that correlation is disrupted (ie, in the presence of specific skeletal effectors), skeletal ALP is still useful as a predictive index of the capacity for in vitro calvarial collagen synthesis (ie, the predicted rate of 3 [H]-hyp incorporation if the effectors were removed).

John R. Farley

Papers

Fluoride directly stimulates proliferation and alkaline phosphatase activity of bone-forming cells.

Androgens directly stimulate proliferation of bone cells in vitro

Age-related decreases in insulin-like growth factor-I and transforming growth factor-beta in femoral cortical bone from both men and women: implications for bone loss with aging.

Circulating and skeletal insulin-like growth factor-I (IGF-I) concentrations in two inbred strains of mice with different bone mineral densities.

Skeletal alkaline phosphatase activity as a bone formation index in vitro.