Region-specific regulation of transforming growth factor α (TGFα) gene expression in astrocytes of the neuroendocrine brain

Certain glial cells of the hypothalamus have been implicated in the neuroendocrine control of reproductive development. Hypothalamic astrocytes appear to exert this function via a cell-cell interactive mechanism that involves the production of transforming growth factor α (TGFα), a polypeptide able to affect both glial and neuronal functions in the CNS. In the hypothalamus, TGFα stimulates neuronal secretion of luteinizing hormone- releasing hormone (LHRH), the neuropeptide controlling sexual development, via activation of epidermal growth factor receptors (EGFR). Since astrocytes but not LHRH neurons express EGFR, it has been postulated that the stimulatory effect of TGFα on LHRH release is not exerted directly on LHRH neurons, but rather via glial intermediacy. The present experiments were undertaken to define whether TGFα is able to exert paracrine/autocrine effects on isolated hypothalamic astrocytes, and to determine if estradiol- previously shown to increase TGFα mRNA levels in the hypothalamus of immature animals-can act directly on hypothalamic astrocytes to upregulate TGFα gene expression. Treatment with either TGFα or its structural homolog, epidermal growth factor (EGF), increased TGFα mRNA levels within 8 hr of exposure; the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) was similarly effective. Blockade of EGFR with either tyrphostin RG-50864, an inhibitor of tyrosine kinase activity, or a monoclonal antibody that prevents ligand binding abolished the upregulatory effect of TGFα on TGFα mRNA levels. In contrast to hypothalamic astrocytes, cerebellar astrocytes did not respond to either TGFα or EGF with changes in TGFα mRNA abundance. This lack of response was not due to an absence of EGFR in cerebellar glia, as astrocytes from both brain regions were found to have similar levels of EGFR mRNA, and respond to TGFα or EGF with a similar change in EGFR autophosphorylation. Hypothalamic, but not cerebellar, astrocytes express the estrogen receptor (ER) gene and respond to estradiol with increased TGFα mRNA levels. This effect of estradiol was prevented by blockade of EGFR. The results provide evidence for the concept that TGFα can act in a paracrine/autocrine manner to upregulate its own gene expression in hypothalamic astrocytes, and indicate that such regulatory mechanism does not operate in the cerebellum, a region irrelevant to neuroendocrine control. Expression of the ER gene in hypothalamic astrocytes and the ability of estradiol to upregulate TGFα mRNA abundance in these cells, but not in cerebellar astrocytes, further suggest that some glial cells of the hypothalamus are molecularly and functionally specialized to subserve neuroendocrine reproductive functions.