Osteoporosis is a universal major public health problem which is characterized by low bone mass, deterioration of bone tissues and increased risk of fracture. The incidence of osteoporosis varies from country to country. Cummings and Melton noted that the risk of osteoporotic fracture in the USA was higher in urban than in rural areas [1
]. The prevalence of osteoporosis based on bone density at the femoral neck was found to be 18~28% in women and 6~22% in men over the age of 50 years due to a reduction in endogenous oestrogen production [2
]. Estrogen plays crucial roles in maintaining bone restoration, in particular for females. And estrogen hormone replacement therapy (HRT) has been suggested as the main therapeutic measure for prevention and treatment of post-menopausal osteoporosis for decades [3
]. However, prolonged HRT therapy is not well accepted due to its potential to increase the incidences of breast cancer and cardiovascular events [4
]. Therefore, it is urgent for the clinicians to develop alternative therapy with less undesirable side effects that can immensely reduce the need for drugs usage.
Although there are few studies indicate negative outcomes, phytoestrogens still raise great interests in recent years due to their clinical benefits in a variety of estrogen-dependent disorders [5
]. There has been considerable interest towards the consumption dietary phytoestrogens and their potential benefits to alleviate menopausal symptoms, enhance post-menopausal bone health and reduce cancer risk [6
]. Among phytoestrogens, isoflavones exerts the greatest advantage because of its abundance in food sources, notably soy products, and to their wide commercial availability as nutritional supplements. As for osteoporosis, the incidence of fracture has been analyzed among 24,403 post-menopausal Chinese women who underwent a soy protein or isoflavone diet for a 4.5-year period. The result showed a significant linear negative association between soy protein or isoflavone consumption (≥21 mg daily) and fracture risk [7
]. Genistein (GEN) is the main soy isoflavone and it is freely absorbed from the intestine, and a large fraction is converted to the 7β-O
-glucuronide as it crosses the brush border and ultimately enters the portal vein. Genistein has been extensively studied for its important hormonal properties [8
]. Indeed, as suggested by the structural resemblance with the endogenous ligand estradiol, genistein shows potent estrogen-like properties both in vivo
and in vitro
. Abundant studies have demonstrated the bone-sparing effects of genistein in experimental animal models. Our previous studies have suggested genistein revealed bone sparing effects in ovariectomized rats [9
]. Bitto et al.
also showed that genistein restored better quality bone than alendronate, raloxifene, and estradiol [11
]. Further studies demonstrated that genistein prevented and restored bone in animal models of secondary osteoporosis induced by steroids as well [12
]. Recently, a well-controlled clinical trial revealed that 3 years of consecutive genistein administration at the dose of 54 mg/day significantly improves bone markers at a rate comparable with other standard therapies for osteoporosis in post-menopausal women [14
]. An estimated average intake of genistein at 0.01–12 mg/day in Asian countries significantly reduces the incidence of post-menopausal osteoporosis, whereas genistein rich diet is rare in Western countries, which partly explain the lower prevalence of osteoporosis in Eastern countries [15
However, besides mimicking estrogen properties, the underlying mechanisms of genistein-mediated bone-sparing effects are not fully understood. Parathyroid hormone (PTH) is a major regulator of ionized calcium and phosphate concentrations in the blood and extracellular fluids. Parathyroid hormone receptor 1 (PTHR1) is a specific receptor for PTH and it belongs to the G-protein coupled receptor family [19
]. Upon activation in the presence of PTH, PTHR1 triggers calcium and phosphorus mobilization, which leads to osteogenesis and bone turnover. The primary target organs for PTH/PTHR1 are kidney and bone. In the kidney, PTH/PTHR1 stimulate reabsorption of calcium from renal tubules, stimulate 1,25-dihydroxycholecalciferol (1,25-(OH)2
-VitD) synthesis, and prevent reabsorption of phosphate. In bone, PTH/PTHR1 mediate bone resorption by osteoclasts and reduce osteoblast proliferation, resulting in calcium liberation and decreased bone mass [20
]. Interestingly, intermittent PTH administration has been found to increase bone density, improve skeletal architecture, enhance biomechanical strength, and reduce fracture risk [21
]. Therefore, PTH or analogue and PTHR1 are potential novel targets for the therapeutic strategy for osteoporosis. But whether PTH/PTHR1 participates in the bone-sparing effect in response to genistein therapy remains unclear. In this regard, the aim of the present study is to explore the PTH/PTHR1 mechanisms of genistein-mediated bone protection in ovariectomized rats and to compare clinical beneficial effects between HRT and genistein.