Paralemmin-1 is expressed in numerous tissues but is most highly expressed in the brain, where it is thought to affect plasma membrane dynamics, cell shape
, and ultimately the development and plasticity of the nervous system. It may do so by serving as an adaptor protein that connects membrane proteins with each other, with the cytoskeleton, or with motor proteins
. These properties of paralemmin-1 might implicate this protein not only in normal morphogenesis but also abnormal development and cancer. Here we show for the first time that paralemmin-1 is expressed in breast cancer cell lines and human breast cancers.
Our discovery of paralemmin-1 overexpression in breast cancer cell lines came from a cDNA array comparison between the ER-positive breast cancer cell line, MCF-7, and its tamoxifen-selected, ER-negative derivative, TMX2-28.
. We proposed that the overexpression of paralemmin-1 may contribute to the invasive nature of the TMX2-28 cells, and possibly to the greater metastatic potential of ER-negative breast cancers. Therefore, we examined paralemmin-1 RNA and protein expression in a larger sample of 10 breast cell lines. Contrary to our expectation, paralemmin-1 expression was not inversely correlated with ER status among the tumorigenic and non-tumorigenic cell lines we examined. Furthermore, when we examined RNA levels in 26 primary breast tumors we found paralemmin-1 expression to be significantly higher in ER-positive as compared to ER-negative tumors. This finding was confirmed when we examined tissue samples by immunohistochemistry; a greater percentage of ER-positive tumors were scored as having ‘high’ levels of paralemmin-1 protein, suggesting that paralemmin-1 expression may still play a role in the differences observed between ER-positive and ER-negative breast tumors. Although it is conceivable that hormonal status may influence the variability of paralemmin-1 expression in benign breast epithelium, we did not evaluate this possible association which is beyond the scope of this study.
Kutzleb and colleagues
 investigated the cellular and subcellular localization of paralemmin-1 in rat brain and kidneys. Their study revealed that paralemmin-1 in the brain was widely distributed in most neuron cell bodies, axons, dendrites and glial processes, while in the kidney paralemmin-1 showed differential expression based on the cell type with a mosaic of paralemmin-1-positive and -negative cells in the proximal and distal tubules, parietal epithelium of Bowman’s capsule and the endothelium of many blood vessels. At a subcellular level paralemmin-1 has been shown to concentrate at the apical membranes of adrenal chromaffin cells, but at the basolateral membranes of proximal and distal tubule cells in the kidney. Paralemmin-1 detected in the cytoplasm was usually associated with endomembranes.
The present study provides the first histological characterization of paralemmin-1 immunolocalization in normal and cancerous mammary tissue. In the breast paralemmin-1 is constitutively expressed in vascular endothelial cells (lymphatic and small blood vessel), and variably expressed in stromal cells (not further defined). Paralemmin-1 is focally expressed in breast ducts and lobules, showing variable expression in epithelial cells within each case and between cases, from no staining to strong focal staining. The subcellular localization of paralemmin-1 in breast epithelium is consistent with other tissue types: paralemmin-1 was primarily detected at the cell membrane and to a much lesser extent in the cytoplasm. The major difference between paralemmin-1 staining in normal breast tissue (RM) and breast cancer (DCIS and IDC) was the greater frequency of strong staining in cancer tissue.
There have been only a few reports of paralemmin-1 expression in cancer tissue. In two reports paralemmin-1 was identified through mRNA microarray analyses as being upregulated in cancer cells. Paralemmin-1 was upregulated in androgen-independent relative to both androgen-dependent tumors and to normal controls in a mouse prostate model
, and high levels of paralemmin-1 and paralemmin-2-AKAP2 expression were correlated with an invasive morphological phenotype of breast cancer cell lines
Duncavage and colleagues
 investigated the expression of paralemmin-2 in non-small cell lung carcinomas because it is a potential target of the microRNA-221, which they showed to be down regulated in nonrecurrent tumors. However, paralemmin-2 was equally expressed in both recurrent and nonrecurrent non-small cell lung carcinomas and it was not determined whether paralemmin-2 levels were higher in tumors than in normal tissue. In the present report we not only found that paralemmin-1 expression was greater in a subset of breast tumors, but also that paralemmin-1 expression was greater in tumor tissues than in RM tissues. We have not examined the expression of benign epithelium adjacent to tumor, however our observations in RM tissue suggest that this protein is expressed at low levels in normal breast tissue.
Phosphorylation and mRNA splicing of paralemmin-1 is tissue-specific, developmentally regulated and contributes to the electrophoretic heterogeneity frequently seen on Western Blots. Amino acids 154–230 of paralemmin-1, which correspond to exon 8 of the mRNA sequence, have been shown to interact with the third intracellular loop of the D3 dopamine receptor in the hippocampus and cerebellum in rat brain, and in glial and neuronal cell cultures
. Thus variants in which exon 8 is spliced out of the RNA likely result in specific changes in paralemmin-1 function in different tissue types. In the present study we detected significant expression of paralemmin-1 RNA missing exon 8 in both ER-positive and ER-negative breast cancers. In contrast, there was very little expression of the Δ exon 8 splice variant in the RM tissue. At present we do not know whether normal tissue adjacent to the cancer expresses paralemmin-1 or the Δ exon 8 splice variant. Use of laser-capture microdissection to isolate RNA from tumor and adjacent benign tissue may be valuable in defining the expression and role of paralemmin-1 in breast carcinogenesis.