Mechanism and potential of the growth-inhibitory actions of vitamin D and ana-logs.

1alpha,25-Dihydroxyvitamin D(3) [1,25-(OH) (2)D(3)] can exert its biological actions through binding with the nuclear vitamin D receptor (VDR), a ligand-activated transcription factor. Next to control of bone and mineral homeostasis, these actions include an immunomodulatory effect and a potent growth-inhibitory, antiproliferative or prodifferentiating action on a wide variety of cell types. The molecular mechanisms underlying this antiproliferative action form an intriguing research topic and they remain, although thoroughly studied, not completely understood. Important cell cycle regulators are involved such as cyclins, cyclin dependent kinases and their corresponding inhibitors as well as E2F transcription factors and accompanying pocket proteins. Whether 1,25-(OH)(2)D(3) influences the expression of all these proteins directly through the nuclear VDR or rather in an indirect manner is not always clear. The antiproliferative action makes 1,25-(OH) (2)D(3) a possible therapeutic tool to treat hyperproliferative disorders, among which different types of cancer. Clinical application, however, is severely hampered by calcemic effects such as hypercalcemia, hypercalciuria and increased bone resorption. Rational design of chemically modified 1,25-(OH) (2)D(3)-analogs tries to overcome this problem. As such, several thousands of analogs have been synthesized and evaluated, some of which display the desired dissociation between beneficial antiproliferative and unwanted calcemic effects. A number of those analogs are 'superagonistic' and have a several-fold stronger antiproliferative action than the parent compound. This review focuses on recent findings about the complex mechanisms behind the antiproliferative and prodifferentiating effect of 1,25-(OH) (2)D(3). Furthermore, the mode of action and possible clinical application of chemically modified 1,25-(OH) (2)D(3)-analogs will be discussed.

Two novel 14-Epi-analogues of 1,25-dihydroxyvitamin D3 inhibit the growth of human breast cancer cells in vitro and in vivo.

The biological activity of two novel 14-epi-analogues of 1,25(OH)2D3, 19-nor-14-epi-23-yne-1,25(OH)2D3 (TX 522) and 19-nor-14,20-bisepi-23-yne-1,25(OH)2D3 (TX 527), is described. Both analogues were at least 10 times more potent than 1,25(OH)2D3 in inhibiting in vitro cell proliferation and had much lower in vivo calcemic effects than 1,25(OH)2D3. Treatment with 1,25(OH)2D3, TX 522, or TX 527 in vitro was accompanied by an accumulation of cells in the G1 phase of the cell cycle. Protein levels of cyclin C and cyclin D1 in in vitro cultures of MCF-7 cells were down-regulated to 50 and 30%, respectively, of control levels at 72 and 120 h after stimulation. Protein levels of p21 and p27 at 72 h were significantly enhanced by 1,25(OH)2D3 and TX 522 but surprisingly not by TX 527. The inability of TX 527 to up-regulate p21 seemed to be cell type specific because p21 was induced in other cell types. Diminished phosphorylation of the retinoblastoma protein after treatment with 1,25(OH)2D3, TX 522, or TX 527 may ultimately contribute to the growth inhibition caused by these compounds. According to the data presented, the induction of apoptosis seemed not to be a major mechanism responsible for the growth-inhibitory effect of 1,25(OH)2D3 and analogues. Both 14-epianalogues significantly retarded tumor progression (40% reduced compared with control mice) in an in vivo model of MCF-7 breast cancer cells established in nude mice. In conclusion, these novel analogues have the eligible profile to be tested as therapeutic agents for the treatment of hyperproliferative diseases such as breast cancer.

Forkhead box O transcription factors in chondrocytes regulate endochondral bone formation.

The differentiation of embryonic mesenchymal cells into chondrocytes and the subsequent formation of a cartilaginous scaffold that enables the formation of long bones are hallmarks of endochondral ossification. During this process, chondrocytes undergo a remarkable sequence of events involving proliferation, differentiation, hypertrophy and eventually apoptosis. Forkhead Box O (FoxO) transcription factors (TFs) are well-known regulators of such cellular processes. Although FoxO3a was previously shown to be regulated by 1,25-dihydroxyvitamin D3 in osteoblasts, a possible role for this family of TFs in chondrocytes during endochondral ossification remains largely unstudied. By crossing Collagen2-Cre mice with FoxO1lox/lox;FoxO3alox/lox;FoxO4lox/lox mice, we generated mice in which the three main FoxO isoforms were deleted in growth plate chondrocytes (chondrocyte triple knock-out; CTKO). Intriguingly, CTKO neonates showed a distinct elongation of the hypertrophic zone of the growth plate. CTKO mice had increased overall body and tail length at eight weeks of age and suffered from severe skeletal deformities at older ages. CTKO chondrocytes displayed decreased expression of genes involved in redox homeostasis. These observations illustrate the importance of FoxO signaling in chondrocytes during endochondral ossification.

Visceral yolk sac-derived tumors.

Externalization of the visceral yolk sac, after fetectomy, induces the development of extra-embryonal fetal tumors in rodents. These tumors are either benign teratomas that appear 3 to 4 weeks after the displacement of the yolk sac or malignant tumors, i.e. yolk sac carcinomas. The latter appear 4 to 8 months after the surgery. If however, Mouse Sarcoma Virus (MSV) is injected in the placentas at the time of fetectomy (day 12 of pregnancy) the malignant tumors develop much earlier (2 to 3 months after surgery) and some display characteristics of embryonal carcinoma. Whether virus induced or not, the yolk sac carcinomas that develop from the displaced visceral yolk sac possess the same morphological and biological characteristics. They are composed of both parietal and visceral yolk sac structures and sometimes trophoblast. The tumors metastasize, grow in ascites form and kill their host. They are readily transplantable in syngeneic rats and grow in tissue culture as an epithelial-like sheet of cells. On the other hand, the benign teratomas are composed of various well differentiated adult tissues. In these tissues, derivatives of all three germ layers are observed. Numerous experiments prove that the stem cells for these various adult tissues are not germ cells. Instead the stem cells are multipotential cells that arise in the displaced yolk sac by a process of dedifferentiation. These poorly differentiated cells originate from the endoderm of the displaced visceral yolk sac. By redifferentiation they give rise to the various adult tissues characteristic for benign teratomas. The multipotential poorly differentiated cells are also likely to be the target cells for malignant transformation. Malignant transformation of these cells, whether induced by a virus or spontaneously occurring in the displaced yolk sac, leads not only to the development of yolk sac carcinomas and eventually embryonal carcinoma but also, although rarely, to choriocarcinoma. The latter tumor is transplantable in allogeneic hosts. It is hormonally active since it secretes lactogen and progesterone. The extra-embryonal fetal tumors and in particular the rat yolk sac carcinomas and choriocarcinoma proved to be a good source for the detection of oncofetal antigens. At least two different oncofetal endodermal antigens were detected with monoclonal antibodies (mab) made after immunization with yolk sac carcinoma. Another mab, made against choriocarcinoma, was found to react specifically with the cytotrophoblast both in the normal placenta and in the tumor. No other placental cells showed a positive reaction.

Mice lacking the plasminogen activator inhibitor 1 are protected from trabecular bone loss induced by estrogen deficiency.

Bone turnover requires the interaction of several proteases during the resorption phase. Indirect evidence suggests that the plasminogen activator/plasmin pathway is involved in bone resorption and turnover, and recently we have shown that this cascade plays a role in the degradation of nonmineralized bone matrix in vitro. To elucidate the role of the plasminogen activator inhibitor 1 (PAI-1) in bone turnover in vivo, bone metabolism was analyzed in mice deficient in the expression of PAI-1 gene (PAI-1-/-) at baseline (8-week-old mice) and 4 weeks after ovariectomy (OVX) or sham operation (Sham) and compared with wild-type (WT) mice. PAI-1 inactivation was without any effect on bone metabolism at baseline or in Sham mice. However, significant differences were observed in the response of WT and PAI-1-/- mice to ovariectomy. The OVX WT mice showed, as expected, decreased trabecular bone volume (BV/TV) and increased osteoid surface (OS/BS) and bone formation rate (BFR), as assessed by histomorphometric analysis of the proximal tibial metaphysis. In contrast, no significant change in any of the histomorphometric variables studied was detected in PAI-1-/- mice after ovariectomy. As a result, the OVX PAI-1-/- had a significantly higher BV/TV, lower OS/BS, lower mineral apposition rate (MAR) and BFR when compared with the OVX WT mice. However, a comparable decrease in the cortical thickness was observed in OVX PAI-1-/- and WT mice. In addition, the cortical mineral content and density assessed in the distal femoral metaphysis by peripheral quantitative computed tomography (pQCT), decreased significantly after ovariectomy, without difference between PAI-1-/- mice and WT mice. In conclusion, basal bone turnover and bone mass are only minimally affected by PAI-1 inactivation. In conditions of estrogen deficiency, PAI-1 inactivation protects against trabecular bone loss but does not affect cortical bone loss, suggesting a site-specific role for PAI-1 in bone turnover.

Interaction of two novel 14-epivitamin D3 analogs with vitamin D3 receptor-retinoid X receptor heterodimers on vitamin D3 responsive elements.

This study provides a detailed and exact evaluation of the interactions between vitamin D3 receptor (VDR), retinoid X receptor (RXR), and vitamin D3 responsive elements (VDREs) mediated by two novel 14-epianalogs of 1,25-dihydroxyvitamin D [1,25(OH)2D3], 19-nor-14-epi-23-yne-1,25(OH)2D3 (TX 522) and 19-nor-14,20-bisepi-23-yne-1,25(OH)2D3 (TX 527). Both analogs were more potent (14- and 75-fold, respectively) than 1,25(OH)2D3 in inhibiting cell proliferation and inducing cell differentiation. However, DNA-independent experiments indicated that both analogs had a lower affinity to VDR and that the stability of the induced VDR conformation, as measured by limited protease digestion assays, was similar (TX 527) or even weaker (TX 522) than that induced by the parent compound. However, DNA-dependent assays such as gel shift experiments revealed that those analogs were slightly more potent (3-7 times) than 1,25(OH)2D3 in enhancing binding of VDR-RXR heterodimers to a direct repeat spaced by three nucleotides (DR3) type VDRE. The functional consequences of the ligand-VDR-RXR-VDRE interactions observed in vitro were subsequently evaluated in transfection experiments. Both 14-epianalogs enhanced transcription of VDRE containing reporter constructs more efficiently than 1,25(OH)2D3 in COS-1 and MCF-7 cells regardless of the presence of ketoconazole. Transactivation activity is suggested to be a cell-specific process because maximal transcriptional induction and the half-maximal transactivation concentration for each reporter construct were different in both cell lines. The superagonistic transactivation activity closely resembled the biological potency of these analogs on the inhibition of MCF-7 cell proliferation. These data clearly indicate that superagonistic activity starts beyond the binding of the ligand-heterodimer (VDR-RXR) complex to VDRE and thus probably involves coactivator/corepressor molecules.

In vitro and in vivo analysis of the immune system of vitamin D receptor knockout mice.

Immune cells carry receptors for 1,25-dihydroxyvitamin D3 [1,25(OH)2D3; vitamin D receptor (VDR)] and individuals with severe vitamin D deficiency have immune abnormalities. The aim of this study was to investigate the role of vitamin D in the immune system by studying VDR-knockout (VDR-KO) mice. VDR-KO mice had the same metabolic phenotype as rachitic animals with severe hypocalcemia. Leukocytosis, lymphocyte subset composition in different immune organs, and splenocyte proliferation to several stimuli were normal, except for a lower response to anti-CD3 stimulation (simulation index [SI] of 13 +/- 4 vs. 24 +/- 9 in wild-type mice; p < 0.01). Macrophage chemotaxis was impaired (41 +/- 19% vs. 60 +/- 18% in wild-type mice; p < 0.01) but phagocytosis and killing were normal. In vivo rejection of allogeneic (31 +/- 12 days vs. 45 +/- 26 days of survival in wild-type mice, NS) or xenogeneic (10 +/- 2 days vs. 16 +/- 9 days of survival in wild-type mice, NS) islet grafts was comparable with wild-type mice. Surprisingly, VDR-KO mice were protected from low-dose streptozotocin-induced diabetes mellitus (LDSDM; 5% vs. 65% in wild-type mice; p < 0.001). Correcting hypocalcemia by use of lactose-rich or polyunsaturated fat-rich diets fully restored the immune abnormalities in vitro and the sensitivity to diabetes in vivo. On the other hand, treatment with 1,25(OH)2D3 protected wild-type mice against diabetes but did not protect normocalcemic VDR-KO mice. We conclude that immune defects observed in VDR-KO mice are an indirect consequence of VDR disruption because they can be restored by calcium homeostasis normalization. This study proves that although 1,25(OH)2D3 is a pharmacologic and probably a physiological immunomodulator, its immune function is redundant. Moreover, we confirm the essential role of calcium in the immune system.

The biological activity of nonsteroidal vitamin D hormone analogs lacking both the C- and D-rings.

1alpha,25-dihydroxyvitamin D is a key calcium-regulating hormone but also displays potent differentiating and antiproliferative activities on many cell types. The structural requirements of this secosteroid hormone have been extensively studied for the A-ring and side chain, whereas relatively little is known about the requirements of the natural CD-ring structure for the vitamin D-like biological activity. We have embarked on a vast program in which derivatives were synthesized and evaluated characterized by profound structural changes in the central C/D-region. This first series of nonsteroidal analogs consists of (1R,3S)-5-((Z,2E)-4-((1S,3S)-3-(4-hydroxy-4-methylpentyl)-1,2,2-++ +trimethylcyclopentyl)-2-butenylidene)-4-methylenecyclohexan e-1,3-diol (KS 176) and derivatives thereof. These analogs are characterized by the absence of normal C- and D-rings and by the presence of an unnatural five-membered ring which we call the E-ring. KS 176 with the otherwise natural side chain structure of 1alpha,25(OH)2D3 has between 10 and 30% of the biological activity of 1alpha,25(OH)2D3 when tested in vitro (prodifferentiating effects on HL-60 and MG-63; antiproliferating activity on MCF-7 and keratinocytes) but has minimal in vivo calcemic effects. Introduction of several side chain modifications created analogs with increased intrinsic noncalcemic biological properties, whereas their calcemic potency remains very low. These data demonstrate that the full CD-rings are not mandatory for the biological activity of 1alpha,25(OH)2D3 since they can be replaced by a new ring structure which generates an appropriate spacing of the A-seco B-rings in relation to the side chain. The biological activity of these nonsteroidal analogs probably involves a classical genomic activation since they are also active in transfection assays using an osteocalcin vitamin D responsive element coupled to a human growth hormone reporter gene.

Biological activity of CD-ring modified 1alpha,25-dihydroxyvitamin D analogues: C-ring and five-membered D-ring analogues.

Nonsteroidal analogues of 1alpha,25(OH)2D3, lacking either the full five-membered D ring (C-ring analogues) or the full six-membered C ring (D-ring analogues) are more potent inhibitors of cell proliferation or inducers of cell differentiation than is 1alpha,25(OH)2D3. Maximal superagonistic activity was seen for the C-ring analogue with a 24(R)-hydroxyl group in the side chain [30- to 60-fold the activity of 1alpha,25(OH)2D3]. The 19-nor-16-ene-26,27-bishomo C-ring analogue showed the best ratio of antiproliferative to calcemic effects (1275-fold better than 1alpha,25(OH)2D3 and severalfold better than all vitamin D analogues so far described). The analogues are able to stimulate specific vitamin D-dependent genes and are active in transfection assays using an osteocalcin promoter VDRE. Low binding affinity to the vitamin D binding protein, differences in metabolism, or affinity for the vitamin D receptor (VDR) are not the most important explanations for the enhanced intrinsic activity. However, the analogues are able to induce conformational changes in the VDR, which makes the VDR-ligand complex more resistant against protease digestion than is 1alpha,25(OH)2D3. In contrast to 20-epimer steroidal vitamin D analogues, 20-epimer C-ring analogues were less potent than analogues with a natural C-20 configuration. In conclusion, several nonsteroidal vitamin D analogues are superagonists of 1alpha,25(OH)2D3 despite lower receptor affinity and, for the C-ring analogues, higher flexibility of the side chain; moreover, they have a better selectivity profile than all analogues yet published.

Bone resorption induced by 1 alpha,25 dihydroxyvitamin D(3) in vivo is not altered by inactivation of the plasminogen activator inhibitor 1.

One of the proteolytic systems produced by bone cells is the plasminogen activator/plasmin pathway, which involves the two plasminogen activators and the type 1 plasminogen activator inhibitor (PAI-1) and results in plasmin generation. We have recently demonstrated that this pathway plays a specific role in the degradation of the nonmineralized matrix of bone in vitro. To evaluate whether PAI-1 is required during bone resorption in vivo, we studied the effects of PAI-1 inactivation on bone metabolism using systemic administration of 1alpha,25 dihydroxyvitamin D(3) [1, 25(OH)(2)D(3)] as model. PAI-1-deficient (PAI-1-/-) and wild-type (WT) mice were injected intraperitoneally with 1,25(OH)(2)D(3) (2 microg/kg) or vehicle every other day during 4 weeks and analyzed using biochemical parameters of bone turnover, histomorphometric analysis of the proximal tibial metaphysis, and pQCT analysis of the distal femoral metaphysis. PAI-1 inactivation did not affect bone metabolism in vehicle-treated mice. Treatment with 1,25(OH)(2)D(3) induced bone resorption similarly in PAI-1-/- and WT mice, as assessed by the increase in the urinary excretion of calcium (2. 2-fold and 2.3-fold, respectively) and of pyridinoline crosslinks (by 24% and 22%, respectively). In addition, a comparable reduction in bone mass was observed in PAI-1-/- and WT mice after treatment with 1,25(OH)(2)D(3), as evidenced by the decrease in the femoral calcium content (by 25% and 32%, respectively), in the trabecular bone volume (by 50% and 40%, respectively), in the trabecular mineral content (by 17% and 15%, respectively), and in the cortical mineral content (by 45% and 52%, respectively). The parameters of bone turnover also increased after 1,25(OH)(2)D(3) treatment. Serum osteocalcin was, respectively, 25% and 28% higher in PAI-1-/- and WT mice treated with 1,25(OH)(2)D(3) compared with the mice injected with vehicle. Similarly, the osteoid surface increased in 1, 25(OH)(2)D(3)-treated PAI-1-/- and WT mice by 40% and 51%, respectively, the mineral apposition rate increased by 15% and 8%, respectively, and the bone formation rate by 54% and 48%, respectively. These data indicate that PAI-1 is not critical during bone resorption induced by 1,25(OH)(2)D(3) in vivo.

Combination of a 1,25-dihydroxyvitamin D3 analog and a bisphosphonate prevents experimental autoimmune encephalomyelitis and preserves bone.

The vitamin D analog TX527 (19-nor-14,20-bis epi-23-yne-1,25(OH)(2)D(3)), decreased disease severity (P < 0.001) and postponed disease onset (P < 0.0001) in SJL mice in which experimental autoimmune encephalomyelitis was induced. Levels of IFN-gamma and IL-2 mRNA were decreased in spinal cord and spleen in the analog-treated mice, suggesting a Th(1)-targeted effect. Adding the bisphosphonate pamidronate did not affect analog-protective efficacy, but completely prevented TX527-caused acceleration of bone turnover and increased total bone mineral content as well as femoral mineral and calcium content (P < 0.01). Less calcemic analogs of 1,25-dihydroxyvitamin D(3), in combination with bone sparing products such as bisphosphonates allow immune modulation in vivo without affecting bone.

Skeletal effects of estrogen deficiency as induced by an aromatase inhibitor in an aged male rat model.

Aromatization of androgens into estrogens may be important for maintenance of the male skeleton. To address this hypothesis, we evaluated the skeletal effects of selective estrogen deficiency as induced by the aromatase inhibitor vorozole (Vor), with or without 17beta-estradiol (E(2)) administration (1.35 microg/day), in aged (12-month-old) male rats. A baseline group was killed at the start of the experiment (Base). The control group (Control), the group treated with vorozole alone (Vor), the group treated with E(2) alone (E(2)), or the group with a combination of both (Vor + E(2)) were killed 15 weeks later. Vorozole significantly increased serum testosterone (T) and reduced serum E(2) compared with Control. Body weight gain and serum insulin-like growth factor-I (IGF-I) were also lower in Vor, whereas significant weight loss and decrease of serum IGF-I occurred as a result of E(2) administration. Bone formation as assessed by serum osteocalcin was unaffected but osteoid surface in the proximal metaphysis of the tibia was increased in Vor-treated rats. Bone resorption as evaluated by urinary deoxypyridinoline excretion was increased in Vor. Biochemical parameters of bone turnover were reduced significantly in all E(2) treated rats. Premature closure of the growth plates and decreased osteoid and mineralizing surfaces were also observed in E(2) and Vor + E(2). Apparent bone density of lumbar vertebrae and femur, as measured by dual-energy X-ray absorptiometry (DXA), was significantly reduced in Vor. Vorozole decreased femoral bone density mainly in the distal femur (trabecular and cortical region). This decrease of bone density was not present in E(2) and Vor + E(2). Similar findings were observed when bone density was assessed by peripheral quantitative computed tomography (pQCT); that is, trabecular density of the distal femur, the proximal tibia, and the distal lumbar vertebra were all lower in Vor. This decrease in density was not observed in all E(2)-treated animals. In conclusion, administration of the aromatase inhibitor, vorozole, to aged male rats induces net trabecular bone loss in both the appendicular and axial skeleton, despite a concomitant increase in serum testosterone. E(2) administration is able to prevent this trabecular bone loss in vorozole-treated animals.

Synthesis, biological activity, and conformational analysis of four seco-D-15,19-bisnor-1alpha,25-dihydroxyvitamin D analogues, diastereomeric at C17 and C20.

The synthesis of four CD-ring-modified 19-nor-1alpha, 25-dihydroxyvitamin D(3) derivatives lacking C15, referred to as 6C analogues, and diastereomeric at C17 and C20 is described. The synthesis involves an Ireland-Claisen rearrangement of a 3-methyl-substituted ester of (1R)-3-methyl-2-cyclohexen-1-ol as the key step, followed by elaboration of the side chain, transformation into a C8 cyclohexanone derivative, and final Wittig-Horner coupling with the 19-nor A-ring phosphine oxide. Despite possessing a more flexible side chain than the parent hormone, biological evaluation showed an unexpected superagonistic antiproliferative and prodifferentiating activity (10-50 times higher as compared to that of 1alpha,25(OH)(2)D(3)) for the diastereomer with the "natural" configuration at C17 and C20. The other diastereomers exhibit a 25-90% decrease in activity. All four analogues show a decreased binding affinity (45% or less), and their calcemic activity is 4-400 times less than that of 1alpha,25(OH)(2)D(3). The conformational behavior of their side chain was studied using molecular mechanics calculations, and the result is presented as volume maps. A relative activity volume was determined by subtraction of the volume map of the least active analogue from the volume map of the most active one. This shows three regions corresponding to preferred orientations in space of the side chain of the active analogue. One of these regions was found to overlap with the region that is preferentially occupied by the most active of the four diastereomeric 22-methyl-substituted 1alpha,25(OH)(2)D(3) analogues.

Analogs of 1,25-dihydroxyvitamin D3 as dose-reducing agents for classical immunosuppressants.

BACKGROUND: Most immunosuppressants have a narrow margin between efficacy and side effects. A major goal in the development of immunomodulatory strategies is the discovery of combinations of drugs exerting synergistic immunomodulatory effects. The active form of vitamin D, 1,25(OH)2D3, is an immunomodulator that interacts with T cells but mainly targets antigen-presenting cells. We have demonstrated synergism between 1,25(OH)2D3 and cyclosporine, rapamycin, and FK506. The aim of this study was to investigate whether this synergism could be observed with other immunosuppressants (mycophenolate mofetil, leflunomide, and the methylxanthine A802715) and whether analogs of 1,25(OH)2D3 share this synergistic capacity in vivo. METHODS: In vitro, the median effect analysis was applied to the inhibition of phytohemagglutinin A-induced lymphocyte proliferation. In vivo, synergism between analogs of 1,25(OH)2D3 and cyclosporine or mycophenolate mofetil was evaluated in experimental autoimmune encephalomyelitis. RESULTS: In vitro, all combinations with 1,25(OH)2D3 were synergistic. The strongest synergism was seen with the inhibitors of interleukin 2 secretion, cyclosporine and FK506 (indexes 0.16 and 0.27, respectively). The weakest synergism was observed in combinations using A802715, a second-signal inhibitor (index 0.52), or the nucleotide synthesis inhibitor mycophenolate mofetil (index 0.43). In vivo, analogs of 1,25(OH)2D3 share the in vitro-observed synergism with 1,25(OH)2D3. Moreover, the differences in synergism with different immunomodulators were also present in vivo, where the best synergism was again seen in combination with cyclosporine (up to 100% paralysis protection). CONCLUSIONS: These data confirm that 1,25(OH)2D3 and its analogs are potent dose-reducing drugs for other immunomodulators, making them potentially interesting for clinical use in autoimmunity and transplantation.

Histochemical differences in expression of X-linked glucose-6-phosphate dehydrogenase between ectoderm- and endoderm-derived embryonic and extra-embryonic tissues.

We examined the activity of X-linked glucose-6-phosphate dehydrogenase (G6PD) in concepti of the enzyme-deficient mutant and wild-type C3H mice. By using different crosses between the G6PD-deficient homozygous, heterozygous, or wild-type females and hemizygous or wild-type males, we confirmed the inactivation of one of the two X chromosomes in female concepti by a histochemical method. With this technique, a dual (G6PD + or -) cell population could be observed in the tissue sections. We demonstrate that the paternal X chromosome is inactivated in the endoderm of parietal and visceral yolk sac and in the trophoblast, whereas in the embryo and in the yolk sac mesoderm this inactivation is random. Our results confirm biochemical observations showing that only the maternal X chromosome is expressed in all derivatives of trophectoderm and primitive endoderm, whereas derivatives of the primitive ectoderm show random X chromosome expression.

Recent developments in the use of vitamin D analogues.

The non-classical effects of 1alpha,25-dihydroxyvitamin D(3) (1alpha, 25(OH)(2)D(3)) create possible therapeutic applications for immune modulation (e.g., autoimmune diseases and graft rejection), inhibition of cell proliferation (e.g., psoriasis, cancer) and induction of cell differentiation (e.g., cancer). The major drawback related to the use of 1alpha,25(OH)(2)D(3) is its calcaemic effect, which prevents the application of pharmacological concentrations. Intensive research has led to the development of analogues of 1(2)D(3) characterised by a clear dissociation of the antiproliferative and prodifferentiating capacity from the calcaemic effects. Due to this dissociation, these analogues can be used not only for the treatment of bone disorders but also for non-classical applications. In the present review, a summary is given on the use of the 1alpha,25(OH)(2)D(3) analogues for the treatment of cancer, skin and immune disorders and for the prevention of graft rejection. Moreover a brief overview is given on the use of analogues for secondary hyperparathyroidism.

Action of 1,25(OH)2D3 on the cell cycle genes, cyclin D1, p21 and p27 in MCF-7 cells.

1,25(OH)2D3 is a known growth inhibitor and differentiation inducer of several cancer cell lines. To establish the molecular mechanism of 1,25(OH)2D3 as an antiproliferating agent, its effect on proliferation and gene regulation was studied in human breast cancer MCF-7 cells. 1,25(OH)2D3 inhibited cell proliferation dose dependently through G1 arrest. Cyclin D1 transcription levels decreased rapidly in 1,25(OH)2D3-treated cells while protein levels only decreased after 72 h of treatment. Transcription levels of p21 and p27 were upregulated with chronologically consistent changes in cell cycle distribution. Experiments with TGF-beta neutralising antibodies revealed that the largest effect of 1,25(OH)2D3 on cell proliferation is likely due to a TGF-beta independent mechanism of action. The cell cycle regulatory genes, cyclin D1 and p27, are probably involved herein as their expression was not affected by the presence of neutralising antibodies. However, upregulation of p21 was completely abrogated. Therefore, the TGF-beta signalling pathway is thought to be responsible for p21 upregulation.

Rat prolactin-like protein A partial gene and promoter structure: promoter activity in placental and pituitary cells.

Rat prolactin-like protein A (rPLP-A) is a member of a rapidly expanding family of prolactin-related proteins that are expressed during pregnancy by the rat placenta according to specific developmental patterns. Although the factors involved in the pituitary-specific expression of the prolactin and growth hormone genes themselves have been extensively studied, essentially nothing is known of the factors responsible for the placental expression of these new family members. In this paper we describe the isolation of rPLP-A genomic clones, analyze a portion of the 5' flanking sequence of this gene and use the recently described rat choriocarcinoma cell line, Rcho, in transient transfection studies to show that a 975 base-pair (bp) fragment of 5' flanking sequence is sufficient to specify placental expression of the rPLP-A gene.

Differentiation induction of HL60 cells by 1,25(OH)2D3, all trans retinoic acid, rTGF-beta2 and their combinations.

1,25 Dihydroxyvitamin D3 (D3), all trans retinoic acid (atRA) and the cytokine rTGF-beta2 are growth and differentiation modulators of promyelocytic leukemia. D3 gives rise to a functional monocytic cell population whereas single atRA therapy induces granulocytic cell features. rTGF-beta2 reduces HL60 cell proliferation but has no differentiating capacity. Combination treatment demonstrates additive effects between either D3 and atRA or D3 and rTGF-beta2, resulting in a cell population with mixed characteristics since individual cells exhibit both monocytic as granulocytic cell features. The capacity of single and combined treatments to induce a permanent differentiation was investigated. Therefore, cells were preincubated with the drugs during six days, test drugs were removed and cell number was monitored. The total cell count of populations treated with single agents remains constant for only a few days and then increases rapidly. rTGF-beta2 cooperated with D3 in inducing a long-lasting differentiation state (3 weeks). Addition of atRA to this combination did not significantly alter proliferation or differentiation, but some cells underwent apoptosis. Therefore, a total and permanent differentiation of leukemic cells may be achieved by repeated exposure to a combination of differentiation inducing agents.

Enhancement of antiproliferative activity of 1alpha,25-dihydroxyvitamin D3 (analogs) by cytochrome P450 enzyme inhibitors is compound- and cell-type specific.

Ketoconazole (keto) or liarozole (liaro), inhibitors of the cytochrome P450 enzymes that mediate vitamin D and A hydroxylations, could potentiate the antiproliferative effects of 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] and its analogs. Proliferation of MCF-7 and T47-D human breast cancer cells, MG-63 human osteosarcoma cells and HL-60 human promyeloid leukemia cells was concentration dependently inhibited by 1alpha,25(OH)2D3. The vitamin D analogs KH 1060 [20-epi-22-oxa-24,26,27-trihomo-1alpha,25(OH)2D3], RO 23-6010 [16-ene-23-yne-26-trifluoro-1,25(OH)2D2D3], ZXY 835 [20-epi-23-yne-25,26-epoxy-1alpha(OH)D3], and CD 99 [11alpha-methyl-1alpha,25(OH)2D3] were 150-,58-,16- and 7-fold more potent than 1alpha,25(OH)2D3 in inhibiting the proliferation of MCF-7 cells, respectively. A similar rank order of potency was observed in other cell lines. The antiproliferative effects of the vitamin D hormone and analogs was enhanced in MCF-7 cells when coincubated with 1 microM keto (7-, 10-, 5-, 25- and 1.3-fold more potent than in the absence of keto), respectively. The antiproliferative effect was less enhanced when 1alpha,25(OH)2D3 or its analogs KH 1060, ZXY 835 and RO 23-6010 were combined with liaro (3-, 7-, 2- and 3-fold, respectively). Keto and liaro did not markedly potentiate the activity of 1alpha,25(OH)2D3 or its analogs in MG-63 or HL-60 cells. These results suggest that differences in cellular metabolism can at least partially explain the different potency of vitamin D analogs. Moreover, the metabolism of vitamin D analogs is cell-type specific.