ALBERT

Notes: Abstract. Protein tyrosyl phosphorylation is a key determinant of cell proliferation and differentiation. The aim of this study was to test the hypothesis that the signal transduction pathway(s) responsible for human bone cell proliferation may involve different groups of protein tyrosine kinase (PTKs) as compared with that for differentiation. To achieve this, we investigated the effects of two structurally different PTK inhibitors, viz, tyrphostin A51 and genistein, on the proliferation ([3H]thymidine incorporation) and differentiation [alkaline phosphatase (ALP) specific activity and collagen synthesis] of two normal human bone cell types: mandible-derived and vertebra-derived bone cells. Tyrphostin A51 and genistein each markedly reduced cellular tyrosyl phosphorylation level (assessed by Western analysis using a commercial anti-phosphotyrosine antibody and the enhanced chemiluminescence detection assay), confirming that these two effectors are potent PTK inhibitors in human bone cells. Regarding bone cell proliferation, tyrphostin A51 (5–30 μM) caused, a dose-dependent inhibition of basal [3H]thymidine incorporation of both human bone cell types. In contrast, genistein (5–20 μM), not only did not inhibit, but significantly stimulated [3H]thymidine incorporation of these same cell types in a dose-dependent, biphasic manner, with the optimal stimulatory dose between 10 and 20 μM. These effects on cell proliferation were confirmed by cell number counting. In addition, whereas the mitogenic activity of 10 ng/ml epidermal growth factor (EGF) on human mandible-derived bone cells was completely abolished by 5–30 μM tyrphostin A51, genistein at 5–30 μM enhanced the EGF-induced bone cell proliferation in an additive manner. With respect to bone cell differentiation, tyrphostin A51 and genistein each significantly increased basal ALP specific activity and collagen synthesis in human bone cells. In summary, (1) PTKs are involved in human bone cell proliferation and differentiation; (2) tyrphostin A51 inhibited both basal and EGF-induced cell proliferation, thus tyrphostin-sensitive PTKs are involved in basal and EGF-induced human bone cell proliferation; (3) genistein stimulated basal proliferation and enhanced EGF-mediated cell proliferation, suggesting that genistein-sensitive PTKs may play an inhibitory role in human bone cell proliferation; and (4) these differential effects of PTK inhibitors on human bone cell proliferation and differentiation are independent of basal differentiation status of the cells.

Notes: Abstract. Normal intestinal calcium (Ca) absorption is an essential feature of bone homeostasis. As with many other organ systems, intestinal Ca absorption declines with aging, and this is one pathological factor that has been identified as a cause of senile osteoporosis in the elderly. This abnormality leads to secondary hyperparathyroidism, which is characterized by high serum parathyroid hormone (PTH) and an increase in bone resorption. Secondary hyperparathyroidism due to poor intestinal Ca absorption has been implicated not only in senile osteoporosis but also in age-related bone loss. Accordingly, in population-based studies, there is a gradual increase in serum PTH from about 20 years of age onward, which constitutes a maximum increase at 80 years of age of 50% of the basal value seen at 30 years of age. The cause of the increase in PTH is thought to be partly due to impaired intestinal Ca absorption that is associated with aging, a cause that is not entirely clear but at least in some instances is related to some form of vitamin D deficiency. There are three types of vitamin D deficiency: (1) primary vitamin D deficiency, which is due to a deficiency of vitamin D, the parent compound; (2) a deficiency of 1,25(OH)2D3 resulting from decreased renal production of 1,25(OH)2D3; and (3) resistance to 1,25(OH)2D3 action owing to decreased responsiveness to 1,25(OH)2D3 of target tissues. The cause for the resistance to 1,25(OH)2D3 could be related to the finding that the vitamin D receptor level in the intestine tends to decrease with age. All three types of deficiencies can occur with aging, and each has been implicated as a potential cause of intestinal Ca malabsorption, secondary hyperparathyroidism, and senile osteoporosis. There are two forms of vitamin D replacement therapies: plain vitamin D therapy and active vitamin D analog (or D-hormone) therapy. Primary vitamin D deficiency can be corrected by vitamin supplements of 1000 U a day of plain vitamin D whereas 1,25(OH)2D3 deficiency/resistance requires active vitamin D analog therapy [1,25(OH)2D3 or 1α(OH)D3] to correct the high serum PTH and the Ca malabsorption. In addition, in the elderly, there are patients with decreased intestinal Ca absorption but with apparently normal vitamin D metabolism. Although the cause of poor intestinal Ca absorption in these patients is unclear, these patients, as well as all other patients with secondary hyperparathyroidism (not due to decreased renal function), show a decrease in serum PTH and an increase in Ca absorption in response to therapy with 1,25(OH)2D3 or 1α(OH)D3. In short, it is clear that some form of vitamin D therapy, either plain vitamin D or 1,25(OH)2D3 or 1α(OH)D3, can be used to correct all types of age-dependent impairments in intestinal Ca absorption and secondary hyperparathyroidism during aging. However, from a clinical standpoint, it is important to recognize the type of vitamin D deficiency in patients with senile osteoporosis so that primary vitamin D deficiency can be appropriately treated with plain vitamin D therapy, whereas 1,25(OH)2D3 deficiency/resistance will be properly treated with 1,25(OH)2D3 or 1α(OH)D3 therapy. With respect to postmenopausal osteoporosis, there is strong evidence that active vitamin D analogs (but not plain vitamin D) may have bone-sparing actions. However, these effects appear to be results of their pharmacologic actions on bone formation and resorption rather than through replenishing a deficiency.

Notes: Abstract. Long-term use of hydrochlorothiazide (HCTZ), a common diuretic agent for hypertension, has been associated with increased bone density and reduced hip fracture rates in patients. In this study, we sought to examine whether HCTZ has an anabolic effect on the proliferation of human osteoblasts (derived from either vertebrate or rib bone samples) in vitro. Cell proliferation was determined by [3H]thymidine incorporation and cell number counting. In medium supplemented with 1% bovine calf serum, HCTZ significantly and reproducibly increased [3H]thymidine incorporation and cell number. The stimulatory effect was dose dependent in a biphasic manner, with the maximal stimulation (approximately 60% above control, P 〈 0.001) seen at 1 μM HCTZ. In fresh serum-free medium, HCTZ was ineffective as a bone cell mitogen, indicating that the bone cell mitogenic activity of HCTZ required a serum growth factor (GF). HCTZ at doses greater than 10 μM was inhibitory in the presence or the absence of serum, presumably because of the cytotoxic effects. The serum requirement for the bone cell mitogenic activity of HCTZ could be replaced with a conditioned medium (conditioned with normal human osteoblasts for 24 hours), or with a mitogenic dose (1 ng/ml) of PDGF. The GF requirement was specific for PDGF, because other bone cell-derived growth factors (i.e., TGFβ, IGF-I, IGF-II, and bFGF) were unable to replace serum for the bone cell mitogenic activity of HCTZ. In summary, this study shows that (1) HCTZ stimulated the proliferation of normal, untransformed, human osteoblasts in vitro; (2) the bone cell mitogenic effect of HCTZ required the presence of a serum GF; (3) the serum requirement could be replaced with a bone cell GF in conditioned medium; (4) the GF requirement was specific for PDGF. In conclusion, we have demonstrated for the first time that HCTZ has a direct anabolic effect on human osteoblasts in vitro, and that the mitogenic activity is dependent on the presence of PDGF. Because increased bone cell proliferation is a key determinant of bone formation, these observations raise the interesting possibility that HCTZ could act directly on bone cells to stimulate bone formation in patients.

Notes: Abstract Long-term use of hydrochlorothizide (HCTZ), a common diuretic agent for hypertension, has been associated with increased bone density and reduced hip fracture rates in patients. In this study, we sought to examine whether HCTZ has an anabolic effect on the proliferation of human osteoblasts (derived from either vertebrate or rib bone samples) in vitro. Cell proliferation was determined by [3H]thymidine incorporation and cell number counting. In medium supplemented with 1% bovine calf serum, HCTZ significantly and reproducibly increased [3H]thymidine incorporation and cell number. The stimulatory effect was dose dependent in a biphasic manner, with the maximal stimulation (approximately 60% above control, P〈0.001) seen at 1 μM HCTZ. In fresh serum-free medium, HCTZ was ineffective as a bone cell mitogen, indicating that the bone cell mitogenic activity of HCTZ required a serum growth factor (GF). HCTZ at doses greater than 10 μM was inhibitory in the presence or the absence of serum, presumably because of the cytotoxic effects. The serum requirement for the bone cell mitogenic activity of HCTZ could be replaced with a conditioned medium (conditioned with normal human osteoblasts for 24 hours), or with a mitogenic dose (1 ng/ml) of PDGF. The GF requirement was specific for PDGF, because other bone cell-derived growth factors (i.e., TGFβ, IGF-I, IGF-II, and bFGF) were unable to replace serum for the bone cell mitogenic activity of HCTZ. In summary, this study shows that (1) HCTZ stimulated the proliferation of normal, untrasformed, human osteoblasts in vitro; (2) the bone cell mitogenic effect of HCTZ required the presence of a serum GF; (3) the serum requirement could be replaced with a bone cell GF in conditioned medium; (4) the GF requirement was specific for PDGF. In conclusion, we have demonstrated for the first time that HCTZ has a direct anabolic effect on human osteoblasts in vitro, and that the mitogenic activity is dependent on the presence of PDGF. Because increased bone cell proliferation is a key determinant of bone formation, these observations raise the interesting possibility that HCTZ could act directly on bone cells to stimulate bone formation in patients.

Notes: Summary Regulation of the dephosphorylation of glycogen synthase in extracts from rat heart has been studied by adding exogenous phosphatase to the extract. These experiments were possible only because the endogenous protein phosphatase activity of the extract could be inhibited by KF under conditions where alkaline phosphatase activity was not. The concentration of substrate (glycogen synthase from the heart extract) and catalyst (purified E. coli alkaline phosphatase) could be varied independently, by adding known amounts of alkaline phosphatase to the KF-containing heart extracts. Alkaline phosphatase could completely dephosphorylate glycogen synthase while phosphorylase was unchanged. The rate of dephosphorylation was proportional to both the concentration of alkaline phosphatase added to the tissue extract and the amount of glycogen synthase in the extract. The Km for glycogen synthase was close to the concentration found in heart tissue. The Km and the maximum rate of dephosphorylation were both dependent on the phosphorylation state of the glycogen synthase. Less phosphorylated enzyme forms were dephosphorylated faster. These results indicate the necessity for precise control of many variables in studying the rate of glycogen synthase dephosphorylation. Alkaline phosphatase-catalyzed dephosphorylation could be inhibited by physiological concentrations of glycogen. Glycogen synthase dephosphorylation in extracts from fasted-refed rats was less sensitive to glycogen inhibition than in extracts from normal animals. The phosphorylation state of the glycogen synthase in these animals was assessed by kinetic studies to show that differences in phosphorylation state probably could not account for the observations. Fasting led to a decreased rate of dephosphorylation of glycogen synthase due to both an apparent change in kinetic properties of glycogen synthase as a substrate for alkaline phosphatase, and an increased inhibitory effect of glycogen. Stable modifications of glycogen synthase caused by altered nutritional states in the animals are thought to produce these effects.