Regulatory mechanisms for differentiation of adrenocortical cells

哺乳动物的肾上腺皮质有三个同心带，即球状带(Zg)、束状带(ZF)和网状带(ZR)。我们以前曾报道，在大鼠的ZG和ZF之间存在一个功能未分化的细胞区。大脑皮层的复制细胞主要分布在细胞区及其内外邻近区域，提示细胞区含有干细胞，可产生分化为ZG或ZF/ZR细胞的细胞。为了建立分析分化过程的体外模型系统，我们试图获得处于未分化阶段的肾上腺皮质细胞系，类似于ZG和ZF之间发现的细胞。从含有SV40大T抗原基因温度敏感突变体的转基因小鼠的肾上腺细胞的原代培养中建立了具有这种特性的细胞系。然后，我们尝试通过差异杂交筛选的方法克隆在未分化细胞中表达的蛋白的编码基因。获得了多个克隆。其中一个克隆被发现可能编码一种新的细胞外基质蛋白。另一个基因编码了一个新的蛋白质，其氨基酸序列没有任何已知的同源序列。在ZG到未分化细胞区和未分化细胞区到ZF的区域分别检测到它们的mRNAs。它们的表达部位对应于肾上腺皮质细胞增殖和分化最活跃的区域，这些结果表明它们在肾上腺皮质细胞的生长和分化中起作用。

The adrenal cortex in mammals contains three concentric zones, the zona glomerulosa (zG), zona fasciculata (zF), and zona reticularis (zR). We have previously reported that a functionally undifferentiated cell zone is present between zG and zF in rats. Most of replicating cells in the cortex were observable in the cell zone as well as in the adjacent regions both outside and inside the zone, suggesting that the cell zone contains stem cells which produce cells capable to differentiate into the zG or zF/zR cells. To develop in vitro model systems for analyzing the differentiation processes, we attempted to obtain adrenocortical cell lines at undifferentiated stages similar to that of the cells found between zG and zF. Cell lines with such properties were established from a primary culture of the adrenal cells from transgenic mice harboring a temperature sensitive mutant of SV40 large T antigen gene. Then, we tried to clone cDNAs encoding proteins which were expressed in the undifferentiated cells by using differential hybridization screening. Several cDNA clones were isolated. One of cDNAs clones was found to presumably encode a new extracellular matrix protein. Another cDNA encoded a new protein whose amino acid sequence did not have any known homologous sequence. Their mRNAs were detected, respectively, in the region from ZG to the undifferentiated cell zone and that from the undifferentiated cell zone to ZF. The sites of their expression correspond to the regions where cell proliferation and differentiation of the adrenocortical cells take place most actively, These results suggest that the proteins have a role in growth and differentiation of adrenocortical cells.