Inhibition of prostate cancer-meditated osteoblastic bone lesions by the low-calcemic analog 1alpha-hydroxymethyl-16-ene-26,27-bishomo-25-hydroxy vitamin D3

Vitamin D Receptor Signaling and Cancer

The vitamin D receptor (VDR) binds the secosteroid hormone 1,25(OH)₂D₃ with high affinity and regulates gene programs that control a serum calcium levels, as well as cell proliferation and differentiation. A significant focus has been to exploit the VDR in cancer settings. Although preclinical studies have been strongly encouraging, to date clinical trials have delivered equivocal findings that have paused the clinical translation of these compounds. However, it is entirely possible that mining of genomic data will help to refine precisely what are the key anticancer actions of vitamin D compounds and where these can be used most effectively.

Vitamin D deficiency promotes prostate cancer growth in bone

BACKGROUND

Vitamin D is considered as an important determinant of bone turnover as well as cancer growth. Using a murine model of bone metastasis, we investigated the effect of vitamin D deficiency on prostate cancer cell growth in bone.

METHODS

Three-week-old male nude mice were fed either normal chow (control) or a diet deficient in vitamin D. The latter diet resulted in severe hypovitaminosis D within 6 weeks. At this point of time, 5 × 10(4)  cells of the prostate cancer cell line, PC-3, were injected either into the bone marrow (tibia) or subcutaneously into soft tissues. Osteoprotegerin (OPG) was co-administered in subgroups of mice to suppress bone remodeling. Osteolytic lesions were monitored by serial X-ray, while soft tissue tumor growth was measured by caliper. All tissues were analyzed by micro-CT and histology at endpoint.

RESULTS

Bone turnover was significantly accelerated in vitamin D deficient compared to vitamin D sufficient mice from week 6 onwards. Intra-tibially implanted PC-3 cells resulted in mixed osteolytic and osteosclerotic lesion. At endpoint, osteolytic and osteosclerotic lesion areas, total tumor area, and tumor mitotic activity were all significantly increased in vitamin D deficient mice compared to controls. Regardless of diet, OPG reduced bone turnover, total tumor, and osteosclerotic area as well as tumor mitotic activity, while promoting cell apoptosis. In contrast, vitamin D deficiency did not alter tumor growth in soft tissues.

CONCLUSION

Vitamin D deficiency stimulates prostate cancer growth in bone through modulating the bone microenvironment.

A new insight into the role of rat cytochrome P450 24A1 in metabolism of selective analogs of 1α,25-dihydroxyvitamin D₃

We examined the metabolism of two synthetic analogs of 1α,25-dihydroxyvitamin D₃ (1), namely 1α,25-dihydroxy-16-ene-23-yne-vitamin D₃ (2) and 1α,25-dihydroxy-16-ene-23-yne-26,27-dimethyl-vitamin D₃ (4) using rat cytochrome P450 24A1 (CYP24A1) in a reconstituted system. We noted that 2 is metabolized into a single metabolite identified as C26-hydroxy-2 while 4 is metabolized into two metabolites, identified as C26-hydroxy-4 and C26a-hydroxy-4. The structural modification of adding methyl groups to the side chain of 1 as in 4 is also featured in another analog, 1α,25-dihydroxy-22,24-diene-24,26,27-trihomo-vitamin D₃ (6). In a previous study, 6 was shown to be metabolized exactly like 4, however, the enzyme responsible for its metabolism was found to be not CYP24A1. To gain a better insight into the structural determinants for substrate recognition of different analogs, we performed an in silico docking analysis using the crystal structure of rat CYP24A1 that had been solved for the substrate-free open form. Whereas analogs 2 and 4 docked similar to 1, 6 showed altered interactions for both the A-ring and side chain, despite prototypical recognition of the CD-ring. These findings hint that CYP24A1 metabolizes selectively different analogs of 1, based on their ability to generate discrete recognition cues required to close the enzyme and trigger the catalytic mechanism.

EB1089 inhibits the parathyroid hormone-related protein-enhanced bone metastasis and xenograft growth of human prostate cancer cells

Parathyroid hormone-related protein (PTHrP) plays a major role in prostate carcinoma progression and bone metastasis. Once prostate cancers become androgen-independent, treatment options become limited. Vitamin D analogues represent a potentially valuable class of agents in this clinical context. Using the prostate cancer cell line C4-2 as a model, we studied the effects of PTHrP and the noncalcemic vitamin D analogue EB1089 on markers of prostate cancer cell progression in vitro and in vivo. C4-2 is a second-generation androgen-independent LNCaP subline that metastasizes to the lymph nodes and bone when injected into nude mice and produces mixed lytic/blastic lesions, mimicking the in vivo situation. We report that PTHrP increases cell migration and invasion, and that a pathway via which EB1089 inhibits these processes is through down-regulation of PTHrP expression. PTHrP also increases anchorage-independent cell growth in vitro and xenograft growth in vivo; EB1089 reverses these effects. The in vivo PTHrP effects are accompanied by increased tumor cell proliferation and survival. Treatment with EB1089 reverses the proliferative but not the antiapoptotic effects of PTHrP. PTHrP also increases intratumor vessel density and vascular endothelial growth factor expression; EB1089 reverses these effects. Intracardially injected C4-2 cells produce predominantly osteoblastic lesions; PTHrP overexpression decreases the latency, increases the severity and alters the bone lesion profile to predominantly osteolytic. EB1089 largely reverses these PTHrP effects. A direct correlation between PTHrP immunoreactivity and increasing tumor grade is observed in human prostate cancer specimens. Thus, decreasing PTHrP production by treatment with vitamin D analogues may prove therapeutically beneficial for prostate cancer.

Vitamin d and its role in cancer and immunity: a prescription for sunlight

Vitamin D has been recognized for more than a century as essential for the normal development and mineralization of a healthy skeleton. More extensive roles for vitamin D were suggested by the discovery of the vitamin D receptor (VDR) in tissues that are not involved in calcium and phosphate metabolism. VDR has been discovered in most tissues and cells in the body and is able to elicit a wide variety of biologic responses. These observations have been the impetus for a reevaluation of the physiologic and pharmacologic actions of vitamin D. Here, we review the role of vitamin D in regulation of the immune system and its possible role in the prevention and treatment of cancer and immune-mediated diseases.

Vitamin D and cancer

1,25-dihydroxy Vitamin D [1,25-(OH)(2)D] exerts its effects via the vitamin D receptor (VDR) that belongs to the steroid/thyroid hormone receptor superfamily leading to gene regulation which results in various biological responses. Within the last two decades, the receptor has been shown to be present not only in classical target tissues such as bone, kidney and intestine but also in many other non-classical tissues. Besides the almost universal presence of VDRs, some cell types (e.g. keratinocytes, monocytes, bone, placenta) are capable of metabolizing 25-hydroxyvitamin D to 1,25(OH)(2)D by the enzyme 1alpha-hydroxylase (CYP27B1). The combined presence of 25(OH)D-1alpha-hydroxylase as well as the specific receptor in several tissues introduced the idea of a paracrine role for 1,25(OH)(2)D. Moreover, it has been demonstrated that 1,25(OH)(2)D can induce differentiation and inhibit proliferation of a wide variety of cell types. The molecular mechanisms behind this antiproliferative action is thoroughly explored but the whole picture is still difficult to understand. 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. However the precise hierarchical structure of this wide diversity of actions of 1,25(OH)(2)D on genes influencing cell cycle progression is not firmly established nor do we understand which pathways are essential and which redundant. The antiproliferative action makes 1,25-(OH)(2)D and its analogs a possible therapeutic tool to treat hyperproliferative disorders, among which different types of cancer. This review focuses on the effects of 1,25(OH)(2)D and its analogs on cell proliferation, the results in in vivo experiments in Vitamin D deficient or resistant animals to cancer and the current epidemiological and intervention studies linking Vitamin D status or treatment and human cancer.

Promise of vitamin D analogues in the treatment of hyperproliferative conditions

1Alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2D3; calcitriol] is best known as a hormone involved in calcium homeostasis but is also a potent antiproliferative agent in many cell types, particularly epithelial cells. 1Alpha,25(OH)2D3 mediates its actions through a classic steroid hormone-like transcriptional mechanism by influencing the expression of hundreds of genes. Effects of 1alpha,25(OH)2D3 have been observed on expression of cell cycle regulators, growth factors and their receptors, apoptotic machinery, metastatic potential, and angiogenesis; all of which have some effect on hyperproliferative conditions. This minireview focuses on the anticancer potential of 1alpha,25(OH)2D3 and its analogues by summarizing the promising data from animal and human trials of 1alpha,25(OH)2D3 and some of the more interesting synthetic vitamin D analogues in the treatment of a variety of different animal cancer models and in human patients with advanced cancer. Optimal administration of vitamin D analogues is only just being achieved with high-dose intermittent administration overcoming bioavailability and hypercalcemia problems and combination therapy with cytotoxic agents (taxols and cisplatins), antiresorptive agents (bisphosphonates), or cytochrome P450 inhibitors being attempted. Although the potential of vitamin D as an antiproliferative drug has been realized in the treatment of psoriasis and in parathyroid cell hyperplasia associated with secondary hyperparathyroidism, the search for an anticancer treatment incorporating a vitamin D analogue remains elusive.