---start---- dr zadi Q:describe cellular and hormonal influences on resorptive ability of osteoclasts. bone remodeling: osteoporosis is a disease of this. remodeling is two things: resorption of bone by osteoclasts, and building of bone by osteoblasts. osteoclastic resorption should be followed by osteoblastic building, and no net loss of bone. but in older people or postmenopausal women, osteoclastic resorption can exceed osteoblastic activity and result in net loss. bone remodeling cycle is as follows: a phase of ostseoclastic resorption of bone creates holes in the bone. The holes are then filled up with new bone by osteoblasts. slide: osteoporotic 80 yr old woman with dowager hump - loss of height in old age is a main clinical indicator. the hump is due to major loss of bone in vertebral column- occurs rapidly after menopause. takes about 5 yrs. the bone that's lost is cancellous spongy bone, that's the estrogen sensitive type. older people lose cortical bone. slide: older woman's spinal column. less bone, and different architecture. loss of total bone volume, and loss of architecture. Bone trabeculae - struts that give bone mechanical strength. WIth osteoporosis there are areas of resorption, and the struts become disorganized and destroyed. microscopically - bone biopsy of osteoporotic bone shows loss of struts, replaced by cellular tissue. note that the struts also have microstruts only seen on SEM. slide: SEM - shows trabecular meshwork of bone. SEM of osteoporotic bone shows struts are much thinner, and further apart - bigger holes b/w them - and microfractures of struts is present. it is at this point when osteoporosis is totally irreversible. once the trabeculae are thin you can make them thicker, but once they microfracture, they're gone. need to tx early before it gets really bad. we don't have a clinical way to detect the microarchitectural damage. definition of osteoporosis: disease characterized by low bone mass and by microarchitectural deterioration of bone tissue leading to increased fragility and increased fracture risk. 1.5 million bone fx annually - vertebral, wrist, and hip are major ones. consequence of hip fx - 50,000 deaths, 60,000 admissions to nursing homes per yr types of osteoporosis: primary: cause not known. type I: hormone deficiency after menopause or artificial menopause; type II: senile or involutional osteoporosis. one of these is high turnover, other is low turnover secondary: due to disease or medication difference in bone loss b/w men and women: why are men not as predisposed to osteoporosis? men start at higher level of bone mass- their peak bone mass in their 30s is higher than in women. if rates of decline were the same it would still take longer for men to reach fracture threshold. but i women, there is accelerated loss right after menopause too. so there's age related bone loss plus accelerated loss just after menopause. postmenopausal bone loss: both resorption and formation of bone increase as estrogen is withdrawn and cytokines are upregulated. however, resorption exceeds formation. so there is net bone loss. old people: resorption and formation both decline but formation declines more secondary osteoporosis: thyrotoxicosis type I diabetes cushings dz GI dz w/malabsorption of calcium immobilization rheumatoid arthritis other medications: corticosteroids cyclosporine lithium gonadotrophin releasing hormone diuretics dx of osteoporosis: measure bone density via xray method bnoe densitometric procedures biopsy and histomorhpometric quantitation markers of bone resorption measured in serum and urine. diagnostic categories: he went too fast markers: plasma tartrate resistant acid phosphatase and plasma and urinary pyridinoline crosslinnks. the osteoclasts make this PTRAP and it spills into plasma. The UPC - when osteoclasts resorb bone they degrade collagen as well as mineral, and the spillover of the degradation of collagen is this crosslink stuff, found in plasma and urine. basic biology: osteoclasts osteoblasts both form in bone marrow but have different origins. clast - from granulocyte/mphage precursor - CFU-GM blast - from fibroblastic lineage - CFU-F clasts localize to endosteum, forming tight seal b/w cell and bone, and they secrete acid to dissolve hydroxyapatite and proteolytic enzymes to dissolve collagen and other proteins. forms a lacuna. then blasts are signalled to invade the site - these cells then secrete osteoid into the lacuna and cavity fills up. you want each cavity filled up completely so there is no net loss of bone. slide: osteoblast - plump cells secreting osteoid, laying down bone. slide: osteoclast - large, multinucleated cells, eating bone, carving out hole if you culture osteoclasts - you can see it is basically creating a lacuna, an excavation, underneath itself, on the bone. when it is done it retracts. it doesn't like all that calcium hitting its receptors and it contracts. then it is sort of rounded up and smaller. it moves to another spot and starts over. osteoblasts and clasts have different cells of origin - as stated. the CFU-GM can make monocyte, mphage, giant cell or osteoclast. the CFU-F is a distant cousin- can make fibroblasts or osteoblasts. at various stages of growth, cytokines influence production of these cells. we want to prevent osteoclast formation in osteoporotic people. we should be able to do that with cytokines. IL6, IL11, PTH, TNF and others influence production of osteoclasts. directly acting osteoclast inhibitors - endogenous: calcitonin from thyroid C cells is a potent osteoclast inhibitor. therapy of osteoporosis: to prevent bone loss decrease osteoclastic activity or increase bone formation to decrease osteoclast activity: estrogens calcitonin bisphosphonates ipriflavone androgens and anabolic steroids to increase bone formation: fluoride vitamin d metabolites exercise -one of the best electrical stimulation PTH estrogen: how it influences osteoclast formation and bone loss it reduces bone loss and fx risk causes decrease in LDL cholesterol (bad cholesterol) reverses urogenital atrophy increases risk of uterine cancer unless given with progesterone possible action on breast tissues thoughts on mode of action of estrogen: 1. most accepted - can inhibit secretion of the cytokines that promote osteoclast formation - esp IL-6 2. marked effects on synthesis of procollagen and other growth factors in osteoblasts 3. in animal models has anabolic action on bone 4. direct receptor mediated action on mature osteoclasts (inhibits them) postmenopause: estrogen withdrawal upregulates IL6 etc which increases osteoclast production causing increased bone resorption. one piece of evidence implicating IL6 here is from a study in IL6 deficient mice. ovariectomy and orchiectomy were studied in these mice. we saw increases in osteoclast formation and decreases in cancellous bone volume. they used mice which were IL6 knockouts. the respose was abolished - this suggested a major role of IL6. can also look at expression of IL6 gene in isolated osteoclast. this is being used to look at overexpression of IL6 in response to estrogen withdrawal. estrogen protects bone and heart however, it also has adverse effects on breast and uterus so we want to make a drug that is a mixed estrogen agonist antagonist that protects bone and heart but spares breast and uterus. one: raloxifene - spares uterine and breast receptors but conserves lipid and bone activity.droloxifene also does this. calcitonin - 32AA polypeptide secreted by thyroid C cells. circulates in blood. used in prevention and tx of osteoporosis in women who can't have estrogen, also in secondary osteoporosis - steroid induced, immobilization osteoporosis, and male osteoporosis. reverses bone loss. calcitonin acts on osteoclasts directly - on high affinity receptors - there are like a million receptors on each osteoclast. low concentrations can potently inhibit osteoclast action. we saw the slides showing osteoclasts in action - can quantitate that action - as calcitonin concentration increases, osteoclastic bone resorption decreases. calcitonin from fish is more potent than from mammals so we use salmon calcitonin to treat disease. how does calcitonin act? imagine it to be thrown onto an osteoclast and binding the receptors. osteoclast does not like it, scrinches into itself, instead of being nice and big it is all retracted in and stuff. will return to normal if you withdraw the osteoclast. you can quantitate the quiescence and retraction responses of the osteoclasts. several calcitonin receptors have been cloned and sequenced. G protein coupled receptors. rats, humans. differences are in lengths of loops. we've also found critical sites in receptors for signal transduction. these receptors act by triggering action of GTP binding proteins, and we found the region of receptor that allows that interaction to occur. different isoforms couple different G proteins. one activates Gs protein, leading to cAMP, another activates Gp and activates a calcium pathway. still looking at calcitonin receptor also looking at orally absorbable analogues and vehicles since calcitonin must be injected some nasal formulation in use bisphosphonates: used to tx like chalk. high affinity for bone. backbone is P-C-P similar to P-O-P found in bone. used many yrs to tx tumor induced osteolysis, other. recently anti-osteoporotic tx. positive effects in animal models. a number of these exist with various potencies.some are 10,000 times more potent than others. Fosamax is one clinically used. pharmacokinetics of bisphosphanates - can preferentially localize in bone and act as poison to osteoclastic bone resorption. 20-50% of it deposits on osteoclasts and prevents activity. sits there forever. bone forms on top of it so you then need to give more. main action - can be relased from bone and act directly to inhibit mature osteoclast activity. main way most act is by preventing the resorption. they accumulate under the osteoclasts and prevent the resorbing. more recent studies show that action of bisphosphanates can also be exerted through osteoblasts - which are stimulated to release osteoclast inhibitory factor in presence of some bisphosphanates. those prevent resorption these promote formation: fluoride: bone forming agent increases bone density up to 13% per year but there is no change in fx frequency, and there are big side effects. so FDA is reevaluating approval for this use. PTH: an anabolic agent intermittent PTH will increase trabecular bone volume. normally PTH acts on osteoblasts to release soluble factor to promote osteoclastic bone resorption. BUT can also be anabolic on bone, and when low doses of PTH are given to humans/animals in intermittent way, it paradoxically increases bone volume via osteoclast. how? well, normally when bone is resorbed a hole is created and then filled up by new bone -this is at a given activation frequency of events. a faster activation frequency occurs in osteoporosis. now a same amount of bone is excavated, but blasts can't keep up. so holes don't getfully filled. what does PTH do then? well, it gives a push to the osteoblasts, stimulates them and allows them, at the same activation frequency, to put down more bone. this is the anabolic effect of PTH which is in use in some clinical trials. problem with PTH administration is same as calcitonin -both are charged peptides requiring injection. so, we're trying to develop a calcium receptor antagonist to promote secretion of PTH from PTH gland future: anti resorptive agents: target is osteoclast - Ca++ receptor, calcitonin receptor, prostaglandin and endothelin receptors, NO synthase other targets - can also inhibit osteoclast formation which is promoted by IL6 can inhibit osteoclast attachment to bone (integrins promote this) proton secretion (acid from clast dissolves bone, uses atpase, can inhibit it) secreted proteases bone forming agents: osteoblast is target - PTH analogues, growth hormone, bone morphogenetic proteins, ILGFs, TGFb, prostenoids, EMF esp over hip. ---end----