Hardwick M, Fertikh D, Culty M, Li H, Vidic B, Papadopoulos V
Department of Cell Biology, Georgetown University Medical Center, Washington, DC 20007, USA.
Cancer Res. 1999 Feb 15;59(4):831-42.
Aberrant cell proliferation and increased invasive and metastatic behavior are hallmarks of the advancement of breast cancer. Numerous studies implicate a role for cholesterol in the mechanisms underlying cell proliferation and cancer progression. The peripheral-type benzodiazepine receptor (PBR) is an Mr 18,000 protein primarily localized to the mitochondria. PBR mediates cholesterol transport across the mitochondrial membranes in steroidogenic cells. A role for PBR in the regulation of tumor cell proliferation has also been shown. In this study, we examined the expression, characteristics, localization, and function of PBR in a battery of human breast cancer cell lines differing in their invasive and chemotactic potential as well as in several human tissue biopsies. Expression of PBR ligand binding and mRNA was dramatically increased in the highly aggressive cell lines, such as MDA-231, relative to nonaggressive cell lines, such as MCF-7. PBR was also found to be expressed at high levels in aggressive metastatic human breast tumor biopsies compared with normal breast tissues. Subcellular localization with both antibodies and a fluorescent PBR drug ligand revealed that PBR from the MDA-231 cell line as well as from aggressive metastatic human breast tumor biopsies localized primarily in and around the nucleus. This localization is in direct contrast to the largely cytoplasmic localization seen in MCF-7 cells, normal breast tissue, and to the typical mitochondrial localization seen in mouse tumor Leydig cells. Pharmacological characterization of the receptor and partial nucleotide sequencing of PBR cDNA revealed that the MDA-231 PBR is similar, although not identical, to previously described PBR. Addition of high affinity PBR drug ligands to MDA-231 cells increased the incorporation of bromodeoxyuridine into the cells in a dose-dependent manner, suggesting a role for PBR in the regulation of MDA-231 cell proliferation. Cholesterol uptake into isolated MDA-231 nuclei was found to be 30% greater than into MCF-7 nuclei. High-affinity PBR drug ligands regulated the levels of cholesterol present in MDA-231 nuclei but not in MCF-7. In addition, the PBR-dependent MDA-231 cell proliferation was found to highly correlate (r = -0.99) with the PBR-mediated changes in nuclear membrane cholesterol levels. In conclusion, these data suggest that PBR expression, nuclear localization, and PBR-mediated cholesterol transport into the nucleus are involved in human breast cancer cell proliferation and aggressive phenotype expression, thus participating in the advancement of the disease.
异常的细胞增殖以及侵袭和转移行为增加是乳腺癌进展的标志。众多研究表明胆固醇在细胞增殖和癌症进展的潜在机制中发挥作用。外周型苯二氮䓬受体（PBR）是一种分子量为18,000的蛋白质，主要定位于线粒体。PBR介导胆固醇在类固醇生成细胞的线粒体内膜间转运。PBR在肿瘤细胞增殖调控中的作用也已得到证实。在本研究中，我们检测了一系列侵袭和趋化潜能不同的人乳腺癌细胞系以及多例人体组织活检样本中PBR的表达、特性、定位和功能。与侵袭性较弱的细胞系（如MCF - 7）相比，在侵袭性较强的细胞系（如MDA - 231）中，PBR配体结合和mRNA的表达显著增加。与正常乳腺组织相比，在侵袭性转移性人乳腺肿瘤活检样本中也发现PBR高表达。使用抗体和荧光PBR药物配体进行亚细胞定位显示，MDA - 231细胞系以及侵袭性转移性人乳腺肿瘤活检样本中的PBR主要定位于细胞核内及其周围。这种定位与MCF - 7细胞、正常乳腺组织中主要的细胞质定位以及小鼠肿瘤间质细胞中典型的线粒体定位形成直接对比。对该受体的药理学特性分析以及PBR cDNA的部分核苷酸测序表明，MDA - 231的PBR与先前描述的PBR相似，但并不完全相同。向MDA - 231细胞中添加高亲和力PBR药物配体，可使溴脱氧尿苷以剂量依赖性方式掺入细胞，提示PBR在MDA - 231细胞增殖调控中发挥作用。发现分离的MDA - 231细胞核对胆固醇摄取比MCF - 7细胞核高30%。高亲和力PBR药物配体可调节MDA - 231细胞核中胆固醇水平，但对MCF - 7细胞核无此作用。此外，发现PBR依赖的MDA - 231细胞增殖与PBR介导的核膜胆固醇水平变化高度相关（r = -0.99）。总之，这些数据表明PBR的表达、核定位以及PBR介导的胆固醇转运入核参与了人乳腺癌细胞增殖和侵袭性表型表达，从而参与了疾病的进展。
