SRIF mRNA Research Articles

Hypothalamic preprosomatostatin messenger ribonucleic acid expression in mice transgenic for excess or deficient endogenous growth hormone.

The influence of altered endogenous GH status on somatostatin (somatotropin release-inhibiting hormone; SRIF) gene expression was studied in two transgenic mouse models. Transgenic dwarf mice carried the rat GH gene promoter fused to the diphtheria toxin A-chain gene, placing toxin expression under GH promoter control. As a result, the toxic product of the transgene ablated all GH-expressing cells, resulting in undetectable circulating GH, reduced weight (10.6 +/- 1.0 g for transgenic dwarfs vs. 29.5 +/- 1.7 g for controls; P less than 0.001), and no detectable somatotrophs. Transgenic giant mice contained a construction combining a widely expressed metallothionein promoter and the human GH-releasing hormone (hGHRF) structural gene. Transgene expression of hGHRF resulted in overproduction of endogenous mouse GH in the anterior pituitary and weight increases (42.7 +/- 2.7 g for giants vs. 29.5 +/- 1.7 g for controls; P less than 0.005). Using in situ hybridization, control mice, transgenic dwarfs, and transgenic giants were compared for levels of prepro-SRIF mRNA. Hybridization signal intensities for prepro-SRIF mRNA were similar in transgenic dwarfs to those in littermate nontransgenic mice in non-GH-regulating regions of the brain, such as cortex (control, 31 +/- 2 U; dwarf, 27 +/- 2) and reticulothalamic nucleus (control, 41 +/- 2 U; dwarfs, 39 +/- 3). Transgenic giant mice had hybridization intensity of SRIF mRNA similar to that of normals in cortex (controls, 31 +/- 2 U; giant, 27 +/- 1) and reticulothalamic nucleus (controls, 41 +/- 2 U; giant, 40 +/- 4). In the GH-regulating neurons of the anterior periventricular hypothalamus (PeN), prepro-SRIF mRNA signal in transgenic dwarf mice decreased to 60% of that in controls (88 +/- 13 U for dwarfs vs. 147 +/- 17 U for controls; P less than 0.01), although the numbers of mRNA-expressing cells in the PeN were not different between the transgenic dwarfs and controls (dwarfs, 69 +/- 6 cells; controls, 72 +/- 4 cells). The transgenic giant mice had 230% higher prepro-SRIF mRNA signal than control mice in the PeN (343 +/- 30 U in giants vs. 147 +/- 17 U in controls; P less than 0.001). Again, the numbers of mRNA-expressing cells were not different in giants (57 +/- 9) and normals (72 +/- 4). These results suggest that while the lack of endogenous GH is accompanied by a slight decrease in transcriptional expression of SRIF in the PeN, the overproduction of endogenous GH greatly stimulates hypothalamic SRIF steady state mRNA levels.

Somatostatin gene expression in the developing monkey frontal and cerebellar cortices

Somatostatin (SRIF) mRNA was determined in the developing monkey frontal and cerebellar cortices by the dot blot and the northern blot analyses at embryonic day 120 (E120), embryonic day 140 (E140), newborn stage (Nb), postnatal day 60 (P60) and adult stage (Ad.) At E120, at which time the migration of the cortical neurons had already been completed, SRIF mRNA was detectable with 50% of the maximal value at E140 in the cerebral frontal cortex (von Bonin and Bailey's area FD). After E140, the level of mRNA gradually declined to the adult level by P60 with 25% of the maximal value. In the cerebellum, SRIF mRNA was highly expressed at E120. The level decreased to 18% of the maximum at E140. Between the newborn and adult stages, there existed no positive signal of the mRNA. In contrast, both fetal and adult liver tissues contained no amounts of SRIF mRNA. We discussed the physiological meanings of the enhanced SRIF gene expression in the developing monkey cerebral and cerebellar cortices

The Influence of Thyroid Hormone Status on the Hypothalamo-Hypophyseal Growth Hormone Axis*

Growth hormone (GH) synthesis is known to be impaired by either abnormally high or low levels of thyroid hormone. To determine the effects of these conditions on the central regulation of GH secretion, we have examined their effects on the hypothalamic regulatory peptides GH-releasing hormone (GH-RH) and somatostatin (SRIF). In thyroidectomized rat hypothalamus, a dramatic increase in GH-RH mRNA occurred in parallel with a decrease in peptide content. The significance of this phenomenon is uncertain and might possibly reflect some posttranscriptional derangement of GH-RH synthesis or an increased rate of GH-RH synthesis and release. In the hyperthyroid group, GH-RH showed significant decreases in both peptide and mRNA levels that might possibly reflect a decrease in GH-RH synthesis and secretion. No change was observed in SRIF peptide or mRNA levels in either thyroidectomized or T4-treated animals. As expected, GH mRNA levels in the anterior pituitary were dramatically decreased by thyroidectomy and unaffected by T4 treatment. In addition, in thyroidectomized pituitaries, the mature GH mRNA was observed to alter its structure, increasing in size by approximately 100 nucleotides. This increase in size was found to result from an increase in poly(A) tail length, the significance of which is as yet unclear.

In situ hybridization analysis of the somatostatin-containing neuron system in developing cerebellum of rats

We used in situ hybridization histochemistry to examine the postnatal development of the somatostatin (SRIF) synthesizing system in the cerebellum of rats. There are numerous hybridizing neurons from 1 to 9 days after birth. These occur throughout the cerebellum including the developing medulla and cortex except in the external granular cell layer. The lateral cerebellar nucleus also contains SRIF gene-containing cells. The intensity of the signals for SRIF mRNA in the cerebellum decreases with age. There is a drastic decrease in SRIF mRNA in the lateral cerebellar nucleus. SRIF cells cannot be detected in the lateral cerebellar nucleus of adult rats, whereas a small, yet significant number of SRIF cells are scattered in the cerebellar medulla. However, the cerebellum of adult rats still contains a significant number of labeled cells in the granular cell layer, although the intensity for SRIF mRNA decreases from 14 days after birth to adulthood. SRIF gene-expressing cells in the cerebellar cortex are located primarily in the granular cell layer and appear to correspond to Golgi cells judging from their characteristic features. These results are consistent with our previous immunohistochemical study on the decrease of SRIF immunoreactivity in the cerebellum of adult rats. These findings, together with a recent study of transient SRIF receptor-expressing cells in the developing cerebellum suggest that SRIF acts during cerebellar development.

1,25-Dihydroxyvitamin D3Silences 3′,5′-Cyclic Adenosine Monophosphate Enhancement of Somatostatin Gene Transcription in Human Thyroid C Cells

Treatment of the somatostatin (SRIF)-producing TT cell line with 1,25 dihydroxyvitamin D3 (1,25 D3) lowered intracellular SRIF mRNA and peptide concentration. In separate experiments, the cAMP analog 8-(4-chlorophenylthio)-cAMP stimulated a rapid increase in SRIF mRNA content of the TT cells and SRIF peptide secretion. To determine whether 1,25 D3 could inhibit either the transcriptional or secretory effects of cAMP, TT cells were pretreated with 1,25 D3 for 4 days followed by treatment with the cAMP analog. Pretreatment with 1,25 D3 inhibited the cAMP-mediated increase of SRIF mRNA, but had no effect on the secretory response. We conclude that the ability of 1,25 D3 to silence SRIF gene expression is more potent than cAMP enhancer activity but that 1,25 D3 has no effect on that portion of the cAMP-dependent pathway which regulates peptide secretion.

Developmental regulation of somatostatin gene expression in the brain is region specific.

Developmental regulation of somatostatin (SRIF) gene expression was studied in five regions of rat brain and in rat stomach. Total RNA was isolated from hypothalamus, cortex, brainstem, cerebellum, and olfactory bulb, as well as stomach at eight stages of development from prenatal day 16 to postnatal day 82. Hybridization of a 32P-labeled rat SRIF cDNA probe to Northern blots of total RNA from the above tissues during development demonstrated a single hybridizing band approximately 670 base pairs in length. When SRIF mRNA levels from each stage of development were quantified and normalized by the amount of poly (A)+ RNA present at that stage of development, a unique pattern of SRIF gene expression was seen in each region. In brainstem and cerebellum, SRIF mRNA levels peaked early in development between prenatal day 21 and postnatal day 8 and then declined until postnatal day 82. Hypothalamus and cortex, on the other hand, showed a progressive increase during development with peak levels occurring between postnatal days 13 and 82. In contrast, stomach and olfactory bulb showed SRIF mRNA levels which were low during early development and which rose late in development (postnatal days 13 to 82). Marked differences in the amount of SRIF mRNA within each region were present as well. These data suggest that there is differential expression of the SRIF gene in different regions of the brain and in the stomach during development. Further study of this phenomenon may provide insight into the in vivo control of SRIF gene expression and the role of SRIF in the developing brain.

Sodium butyrate stimulates somatostatin production by cultured cells.

Expression of the neuropeptide SRIF can be enhanced by sodium butyrate, a known modulator of gene transcription. Two different cell lines, RIN cells (derived from a rat pancreatic islet tumor) as well as HeLa cells, showed basal secretion of SRIF immunoreactive material that was induced between 5- and 15-fold by sodium butyrate. The induction was dose and time dependent, fully reversible, and specific for sodium butyrate. Analysis of the induced mRNAs by Northern blot hybridization using a SRIF specific cDNA probe revealed a 12-fold increase in the level of SRIF mRNA caused by butyrate treatment. These results suggest that sodium butyrate may be a useful tool for studying the regulation of SRIF gene expression.