REGION-SPECIFIC REGULATION OF TRANSFORMING GROWTH-FACTOR-ALPHA (TGF-ALPHA) GENE-EXPRESSION IN ASTROCYTES OF THE NEUROENDOCRINE BRAIN

Certain glial cells of the hypothalamus have been implicated in the ne uroendocrine control of reproductive development. Hypothalamic astrocy tes appear to exert this function via a cell-cell interactive mechanis m that involves the production of transforming growth factor alpha (TG F alpha), a polypeptide able to affect both glial and neuronal functio ns in the CNS. In the hypothalamus, TGF alpha stimulates neuronal secr etion of luteinizing hormone-releasing hormone (LHRH), the neuropeptid e controlling sexual development, via activation of epidermal growth f actor receptors (EGFR). Since astrocytes but not LHRH neurons express EGFR, it has been postulated that the stimulatory effect of TGF alpha on LHRH release is not exerted directly on LHRH neurons, but rather vi a glial intermediacy. The present experiments were undertaken to defin e whether TGF alpha is able to exert paracrine/autocrine effects on is olated hypothalamic astrocytes, and to determine if estradiol-previous ly shown to increase TGF alpha mRNA levels in the hypothalamus of imma ture animals-can act directly on hypothalamic astrocytes to upregulate TGF alpha gene expression. Treatment with either TGF alpha or its str uctural homolog, epidermal growth factor(EGF), increased TGF alpha mRN A levels within 8 hr of exposure; the phorbol ester 12-O-tetradecanoyl -phorbol-13-acetate (TPA) was similarly effective. Blockade of EGFR wi th either tyrphostin RG-50864, an inhibitor of tyrosine kinase activit y, or a monoclonal antibody that prevents ligand binding abolished the upregulatory effect of TGF alpha on TGF alpha mRNA levels. In contras t to hypothalamic astrocytes, cerebellar astrocytes did not respond to either TGF alpha or EGF with changes in TGF alpha mRNA abundance. Thi s lack of response was not due to an absence of EGFR in cerebellar gli a, as astrocytes from both brain regions were found to have similar le vels of EGFR mRNA, and respond to TGF alpha or EGF with a similar chan ge in EGFR autophosphorylation. Hypothalamic, but not cerebellar, astr ocytes express the estrogen receptor (ER) gene and respond to estradio l with increased TGF alpha mRNA levels. This effect of estradiol was p revented by blockade of EGFR. The results provide evidence for the con cept that TGF alpha can act in a paracrine/autocrine manner to upregul ate its own gene expression in hypothalamic astrocytes, and indicate t hat such regulatory mechanism does not operate in the cerebellum, a re gion irrelevant to neuroendocrine control. Expression of the ER gene i n hypothalamic astrocytes and the ability of estradiol to upregulate T GF alpha mRNA abundance in these cells, but not in cerebellar astrocyt es, further suggest that some glial cells of the hypothalamus are mole cularly and functionally specialized to subserve neuroendocrine reprod uctive functions.