Protein expression pattern of human MIER1 alpha, a novel estrogen receptor binding protein.

MIER1 is a transcriptional regulator that exists as several isoforms. Of particular interest is the MIER1α isoform, which contains in its unique C-terminus an LXXLL motif for interaction with nuclear hormone receptors. Indeed, MIER1α has been shown to interact with ERα and inhibit estrogen-stimulated growth of breast carcinoma cells. Moreover, the subcellular localization of MIER1α changes dramatically, from nuclear to cytoplasmic, during progression to invasive breast carcinoma. While human MIER1 RNA and protein expression pattern data have been posted on several websites, none of these studies use probes or antibodies that distinguish between the α and ß isoforms. We report here the first immunohistochemical study of the MIER1α protein expression pattern in human tissues. Our analysis revealed intense staining of specific cell types within virtually every endocrine and reproductive tissue except for the thyroid gland. In particular, we detected intense staining of ovarian follicles and germinal epithelium, ductal epithelial cells of the breast, pancreatic islet cells, all areas of the anterior pituitary and all zones of the adrenal cortex; moderate staining of germ cells and Leydig cells within the testis, patches of chromaffin cells in the adrenal medulla and weak staining of the fibromuscular stroma within the prostate. Immunoreactivity was limited to the cytoplasm in all positive cells except for oocytes and germinal epithelial cells in which the nucleus was also stained and in ductal epithelial cells of the breast in which staining was exclusively nuclear. In general, non-endocrine tissues were negative, however a few exceptions were noted. These included hepatocytes, myocardial fibers and neurons in all regions of the brain examined, with the exception of the thalamus. Neuronal staining was restricted to the cell bodies and dendrites, as most axons were negative. These data suggest that human MIER1α functions specifically in endocrine tissues and in a limited number of non-endocrine organs.

Protein expression of the transcriptional regulator MI-ER1 alpha in adult mouse tissues.

MI-ER1 is a novel transcriptional regulator that plays a critical role in embryonic development and is differentially expressed in breast carcinoma. The MI-ER1 protein sequence is highly conserved among species, with 95% identity between mouse and humans and 72% between Xenopus and mouse. There are two major protein isoforms, MI-ER1alpha and MI-ER1beta, which differ in the sequence of their C-terminus. MI-ER1alpha is of particular interest because it contains a consensus LXXLL nuclear receptor interaction motif and the current study was undertaken to determine the expression pattern of MI-ER1alpha protein in adult mouse tissues. Immunohistochemical analysis of paraffin-embedded tissue using an MI-ER1alpha-specific antibody revealed that the majority of mouse adult tissues examined showed very weak or no immunoreactivity; these included tissues of the lung, liver, intestine, uterus, spleen, lymph node, bladder as well as skeletal muscle. Interestingly, a subset of endocrine tissues displayed intense staining for MI-ER1alpha. Specifically, the islets of Langerhans, the zona glomerulosa and medulla of the adrenal gland, the ovary and the hypothalamus were intensely stained. In addition, both anterior and posterior pituitary showed moderate immunoreactivity, as did the parafollicular cells of the thyroid gland and Leydig cells and spermatids in the testes. Negative endocrine tissues included follicular cells of the thyroid gland and the X zone of the adrenal cortex. A few non-endocrine tissues displayed moderate immunoreactivity; these included all tubules and collecting ducts in the kidney, myocardial and endocardial layers of the heart, the hippocampal formation, pyramidal neurons in the cortex and the ductal epithelium of the mammary gland. In all cases, MI-ER1alpha immunoreactivity was cytoplasmic. This study represents the first immunohistochemical analysis of MI-ER1alpha expression in mammals and our data suggest that this transcriptional regulator plays a role in specific endocrine pathways.