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Chemokines and chemokine receptors play diverse roles in homeostasis. The chemokine stromal cell-derived factor 1 (SDF-1) and its receptor CXCR4 have critical functions in the immune, circulatory, and central nervous systems and have also been implicated in tumor biology and metastasis. Here we review the current data regarding the role of the CXCR4/SDF-1 chemokine axis in the development of bone metastases, derived from tumor models of breast or prostate cancers. There is substantial evidence that CXCR4 and SDF-1 directly influence the survival and proliferation of tumor cells. In regards to bone metastases, the CXCR4/SDF-1 axis also appears to facilitate tumor cell recruitment to the bone marrow microenvironment via a homing mechanism. This makes disruption of the chemokine axis an attractive therapeutic target for the prevention of tumor cell spread to bone. However, within the bone microenvironment, SDF-1 and CXCR4 appear to have conflicting roles. While genetic disruption of CXCR4 enhances osteoclast activity and therefore stimulates tumor cell growth in the bone - likely via release of bone-derived growth factors - SDF-1 has been shown to have either a stimulatory effect or no effect on osteoclasts. In short, the effects of the CXCR4/SDF-1 axis on tumor cell growth within the bone are not yet fully defined. Further, there are theoretical risks that blockade of this chemokine axis could impair immune function or mobilize tumor cells leading to other sites of metastasis. As such, caution should be taken when designing therapeutic strategies targeting this chemokine axis.

Osteoclasts are multinucleated hematopoietic cells essential for bone resorption. Macrophage colony-stimulating factor (M-CSF) is critical for osteoclast development and function, although its nuclear targets in osteoclasts are largely unknown. Mitf and TFE3 are two closely related helix-loop-helix (HLH) transcription factors previously implicated in osteoclast development and function. We demonstrate that cultured Mitf(mi/mi) osteoclasts are immature, mononuclear, express low levels of TRAP, and fail to mature upon M-CSF stimulation. In addition, M-CSF induces phosphorylation of Mitf and TFE3 via a conserved MAPK consensus site, thereby triggering their recruitment of the coactivator p300. Furthermore, an unphosphorylatable mutant at the MAPK consensus serine is specifically deficient in formation of multinucleated osteoclasts, mimicking the defect in Mitf(mi/mi) mice. These results identify a signaling pathway that appears to coordinate cytokine signaling with the expression of genes vital to osteoclast development.

Microphthalmia transcription factor (Mitf), a melanocytic nuclear protein critical for the embryonic development and postnatal viability of melanocytes, is a master lineage regulator and modulates extracellular signals. Recently, Mitf expression was shown to be both a sensitive and specific marker of epithelioid melanoma. Because loss of specific melanocytic markers in melanomas with spindle cell morphology is more common compared with those tumors with epithelioid morphology, we investigated the sensitivity of D5, an anti-Mitf antibody, for diagnosis in this diagnostically problematic subset of melanomas. Twenty of 21 (95%) spindle cell and desmoplastic melanomas examined were reactive for S-100 protein. Only 4 of 21 (19%) spindle cell and desmoplastic melanomas were reactive for HMB-45. Six of 21 tumors (29%) were reactive for D5, including one case that was non-reactive for S-100 and HMB-45. Melan-A reactivity was seen in 2 of 13 cases (15%) studied. Eight of 24 (33%) non-melanocytic spindle cell tumors were reactive for D5, including 4 of 6 dermatofibromas, 1 of 6 schwannomas, 1 of 2 leiomyomas, and 2 of 6 leiomyosarcomas. Although D5 was shown in a previous study to be a highly sensitive and specific marker for epithelioid melanomas, the results of this study show it is not a sensitive or specific marker of spindle cell and desmoplastic melanomas. Nevertheless, we believe that diffuse positive staining for D5 when taken in clinical, histologic and immunohistochemical context may be diagnostically useful in selected cases of melanoma.

Microphthalmia transcription factor, a melanocytic nuclear protein critical for the embryonic development and postnatal viability of melanocytes, is a master regulator in modulating extracellular signals. Recently, microphthalmia transcription factor expression was shown to be both a sensitive and specific marker of epithelioid melanoma. We investigated the sensitivity of D5, an anti-microphthalmia transcription factor antibody, for diagnosis of clear cell sarcoma (also known as malignant melanoma of soft parts). Immunoreactivity in a nuclear pattern for D5 was present in 8 of 12 (75%) tumors. D5 staining was strong in three tumors, moderate in two, and weak in three. S-100 protein expression was seen in all 12 cases that had clear cell sarcoma examined. HMB-45 staining was seen in 11 of 12 (92%) tumors. Focal Melan-A positivity was seen in 3 of 7 (43%) tumors. Although D5 was shown in a previous study to be a highly sensitive and specific marker for epithelioid melanomas, the results of this study expand the spectrum of tumors showing immunoreactivity for D5. D5 immunoreactivity in clear cell sarcoma provides further evidence for melanocytic differentiation in this unusual tumor.

The protein encoded by the microphthalmia (mi) gene is a transcription factor essential for the development and survival of melanocytes. Using a monoclonal antibody generated against human Mi transcription factor protein (Mitf) the authors previously demonstrated that Mitf expression is conserved in primary and metastatic malignant melanomas, and appears to be a highly sensitive and specific melanocytic marker. Mitf expression in various cutaneous nevi and cutaneous nonmelanocytic tumors has not been documented systematically. The authors evaluated Mitf immunostaining in 62 benign nevi, 58 primary cutaneous melanomas, and 53 nonmelanocytic tumors. Mitf immunostaining was conserved in all benign nevi, with Spitz nevi and neurotized nevi demonstrating decreased staining intensity. With the exception of desmoplastic melanomas, all primary cutaneous melanomas were immunopositive regardless of the cell type. Only one of 14 desmoplastic melanomas was Mitf positive. None of the nonmelanocytic tumors was immunopositive, including those lesions that may resemble melanoma histologically (spindle cell carcinomas, atypical fibroxanthomas, and leiomyosarcomas). The results demonstrate that Mitf antibody expression is conserved in the majority of benign and malignant melanocytic lesions, and that it may be helpful in the diagnosis of primary melanocytic skin lesions. Its use in desmoplastic melanomas is limited and is reflective of other melanocyte-associated antigens. Mitf discriminates between spindle cell nonmelanocytic tumors and melanomas with a spindle cell morphology, and is useful in a panel with other appropriate antibodies.

Osteopenia and osteoporosis are common complications of bone marrow and peripheral blood stem cell transplantation. Bone loss occurs in 50% to 60% of patients treated with the most common preparatory regimens. The major causes of transplant-related bone loss are primary hypogonadism (low estrogen and testosterone), secondary hyperparathyroidism due to low serum calcium, and posttransplant steroid therapy. Other transplant-related treatments that induce bone loss are discussed. Trabecular bone is particularly vulnerable to transplant-related therapies. The spine and hip contain 50% to 75% trabecular bone and are most at risk for fracture after hematopoietic cell transplantation (HCT). The structure of bone and the bone cells that are involved in maintaining skeletal integrity are discussed, followed by a discussion of the transplant-related therapies that have been shown to cause damage to bone and lead to bone loss. Recommendations for patients undergoing HCT include (1) evaluation of bone mineral density either shortly before or shortly after transplantation and appropriate intervention and monitoring based on the results; and (2) evaluation of estrogen and testosterone levels after HCT and replacement when appropriate; and (3) administration of bisphosphonate therapy to all patients on steroids for >2 months. Early intervention and prevention of bone loss can have a tremendous clinical impact for patients undergoing HCT because once significant bone loss has occurred, it is difficult to reverse.

Malignant melanomas do not uniformly retain expression of melanocytic gene products-an observation associated with diagnostic dilemmas. Microphthalmia transcription factor (Mitf) is a melanocytic nuclear protein critical for the embryonic development and postnatal viability of melanocytes. It serves as a master regulator in modulating extracellular signals, such as those triggered by alpha-MSH and c-Kit ligand. Because of its central role in melanocyte survival and to assess its potential use as a histopathological marker for melanoma, Mitf expression was examined in histologically confirmed human melanoma specimens. Western blot analysis of melanoma cell lines revealed consistent expression of two Mitf protein isoforms differing by MAP kinase-mediated phosphorylation. In a series of 76 consecutive human melanoma surgical specimens, 100% stained positively for Mitf with a nuclear pattern of reactivity. In a side-by-side comparison, Mitf staining was positive in melanomas that failed to stain for either HMB-45 or S-100, the most common currently used melanoma markers. Of 60 non-melanoma tumors, none displayed nuclear Mitf staining and two displayed cytoplasmic staining. Although Mitf does not distinguish benign from malignant melanocytic lesions, for invasive neoplasms it appears to be a highly sensitive and specific histopathological melanocyte marker for melanoma.

Microphthalmia (Mi) is a basic helix-loop-helix-leucine zipper (b-HLH-ZIP) transcription factor implicated in pigmentation, mast cells, and bone development. Two dominant-negative mi alleles (mi/mi and Mior/Mior) in mice cause osteopetrosis. In contrast, osteopetrosis has not been observed in a number of recessive mi alleles, suggesting the existence of Mi protein partners important in osteoclast function. An osteopetrotic rat of unknown genetic defect (mib) has been described whose skeletal sclerosis improves dramatically with age and that is associated with pigmentation defects reminiscent of mouse mi alleles. Here we report that this rat strain harbors a large genomic deletion encompassing the 3' half of mi including most of the b-HLH-ZIP region. Osteoclasts from these animals lack Mi protein in contrast to wild-type rat, mouse, and human osteoclasts. Mi is not detectable in primary osteoblasts. In addition TFE3, a b-HLH-ZIP transcription factor related to Mi, was found to be expressed in osteoclasts, but not osteoblasts, and to coimmunoprecipitate with Mi. These results demonstrate the existence of members of a family of biochemically related transcription factors that may cooperate to play a central role in osteoclast function and possibly in age-related osteoclast homeostasis.