Biomolecular advances in gastrointestinal hormones

Cholecystokinin and gastrin receptors targeting in gastrointestinal cancer

Cholecystokinin and Gastrin are amongst the first gastrointestinal hormone discovered. In addition to classical actions (contraction of gallbladder, growth and secretion in the stomach and pancreas), these also act as growth stimulants for gastrointestinal malignancies and cell lines. Growth of these tumours is inhibited by antagonists of the cholecystokinin and gastrin receptors. These receptors provides most promising approach in clinical oncology and several specific radiolabelled ligands have been synthesized for specific tumour targeting and therapy of tumours overexpressing these receptors. Therefore, definition of the molecular structure of the receptor involved in the autocrine/paracrine loop may contribute to novel therapies for gastrointestinal cancer. Hence, this review tries to focus on the role and distribution of these hormones and their receptors in gastrointestinal cancer with a brief talk about the clinical trial using available agonist and antagonist in gastrointestinal cancers.

Lactational changes in the rat exocrine pancreas somatostatin receptors and modulation of guanylate cyclase

Hyperplasia of the pancreatic tissue during late lactation (third week) and lasting for at least the first two weeks after weaning has been observed by several authors. Since the tetradecapeptide somatostatin (SS) inhibits pancreatic growth and its plasma levels are elevated during these periods, the aim of the present study was to determine the possible implication of the somatostatinergic system in the pancreatic changes cited above. Thus, the present study investigated 125I-Tyr(11)-somatostatin (125I-Tyr(11)-SS) binding and the effects of SS on guanylate cyclase activity as well as pancreatic somatostatin-like immunoreactivity (SSLI) levels in pancreatic acinar membranes from control, lactating and weaning rats. SS receptors were identified using 125I-Tyr(11)-SS and isolated pancreatic acinar membranes in vitro. There was an increase in the number of SS receptors after the third week of lactation (244 +/- 6 vs. 155 +/- 12 fmol/mg protein, P < 0.01) and the first two weeks after weaning (327 +/- 8 vs. 164 +/-10 fmol/mg protein, P < 0.001). No change in the affinity of the receptor site was detected at either study time. In addition, SS-stimulated guanylate cyclase activity was markedly increased at the third week of lactation (119%) and at the second week after weaning (158%) when compared with the control group. In contrast, basal guanylate cyclase activity was not modified at either study period. Thus, SS-stimulated guanylate cyclase activity is increased in pancreatic acinar membranes at late lactation and at the second week after beginning weaning probably due to an increase in the number of SS receptors. Significant decreases in SSLI content were observed at the third week of lactation (69%) and the second week after weaning (37%) when compared with the respective controls. The present results suggest that pancreatic acinar cell growth observed at the third week of lactation and the second week after weaning is associated with up-regulation of SS receptors which would represent a mechanism promoted by the cell that would negatively regulate the mitogenic activity of the increased number of pancreatic growth factors observed during both periods.

Progress toward hormonal therapy of gastrointestinal cancer

OBJECTIVE

The authors discuss ongoing research to determine the mechanisms by which peptide hormones regulate growth of gastrointestinal cancer and ways in which this information might be used to develop noncytotoxic therapy.

SUMMARY BACKGROUND

Approximately 100,000 people die of cancer of the gastrointestinal (GI) tract each year. Surgery is curative only when the tumor is localized. Chemotherapy and radiotherapy have limited efficacy for locally advanced or widespread disease. Various peptide hormones regulate the growth of these tumors. As for breast and prostate cancer, the growth regulatory effect of these hormones is being studied as a potential mechanism of hormonal therapy.

CONCLUSION

Gut peptide hormones regulate the growth of GI malignancies through all-specific receptors and signal transduction pathways. Interruption or modification of these growth regulatory mechanisms may lead to specific noncytotoxic therapy. In combination with surgical extirpation, such hormonally based treatment may be curative of advanced GI cancer.

Somatostatin binding capacity, guanylate cyclase and tyrosine phosphatase activities during pancreatic proliferation in the rat induced by gastrectomy

Gastrectomy increased pancreatic growth and this effect was associated with an increase in the number of somatostatin-14 (SS) receptors (146% of control) without altering their affinity. SS increased guanylate cyclase activity twofold in pancreatic acinar membranes from gastrectomized rats. The gastrectomy decreased pancreatic SS-like immunoreactivity (SS-LI) content (55% of control levels) and tyrosine phosphatase activity (74% of control levels). Administration of proglumide (20 mg/kg, IP), a gastrin/cholecystokinin (CCK) receptor antagonist, suppressed the inhibitory effect of gastrectomy on basal tyrosine phosphatase activity and SS-LI content, which returned to control levels. Furthermore, proglumide suppressed the increase of the number of SS receptors and of SS-stimulated guanylate cyclase activity induced by gastrectomy. All this suggests that pancreatic acinar cell growth is associated with upregulation of SS receptors, which could represent a mechanism promoted by the cell to negatively regulate the mitogenic activity of pancreatic growth factors such as CCK. In addition, the results also suggest that the negative regulation of tyrosine phosphatase activity may be important in the events involved in the pancreatic hyperplasia observed after gastrectomy.

The effect of pentagastrin on the somatostatin receptor/effector system in rat pancreatic acinar membranes

An intraperitoneal (i.p.) injection of pentagastrin (250 micrograms/kg, three times daily) for 1 week increased somatostatin like-immunoreactivity (SSLI) content in the pancreas and the number of somatostatin (SS) receptors in pancreatic acinar membranes without influencing their apparent affinity as compared with control animals. No significant differences were seen in basal or forskolin (FK)-stimulated adenylate cyclase (AC) enzyme activities in the control and pentagastrin treated rats. In spite of the increase in the number of SS receptors, SS caused a significantly lower inhibition in AC activity in these membranes. This finding is related to the fact that the stable GTP analogue, 5'-guanylylimidodiphosphate (Gpp[NH]p) was a much less potent inhibitor of binding in the pancreatic acinar cell membranes from pentagastrin-treated animals than in those from controls. In addition the ability of Gpp(NH)p to inhibit FK-stimulated AC activity was also decreased in pancreatic acinar cell membranes from pentagastrin-treated rats. Pretreatment with proglumide, (20 mg/kg i.p.) a gastrin/cholecystokinin (CCK) receptor antagonist, prevented the pentagastrin-induced changes in SS level and binding as well as the inhibitory effect of SS on AC activity in pancreatic acinar cell membranes. Proglumide alone had no observable effect on the somatostatinergic system. These data suggest a SS receptor/G protein uncoupling as a result of binding of pentagastrin to gastrin receptors present in pancreatic acinar cell membranes.

Gastrointestinal hormones and cell proliferation

There is no question that gut peptides are trophic for normal gut mucosa. Gut peptides can function in an endocrine, paracrine or autocrine fashion. We examined the effects of gut peptides on the growth of animal and human cancers of the gastrointestinal (GI) tract and pancreas in vivo and in vitro. We also examined the role of growth factors and bioamines in the regulation of growth of human endocrine tumors. Our studies have shown that gut peptides (gastrin, VIP, neurotensin, and bombesin) regulate growth of some cancers of the GI tract and pancreas. We have found that peptide action is mediated through specific receptors and that cell-specific differences in receptor expression occur. We have also begun to examine the intracellular signal-transduction pathways involved in endocrine and autocrine actions of these peptides on growth of GI cancers. We have found that cell-type-specific differences exist among the various signal-transduction pathways (cyclic AMP, phosphatidylinositol hydrolysis (PI), intracellular calcium ([Ca2+]i) mobilization, and tyrosine phosphorylation) and that different receptors for the same hormone may be linked to different signal-transduction pathways depending upon cell type. We have also found that autocrine growth regulation of human pancreatic carcinoid occurs through specific receptor-mediated signal-transduction pathways. We will discuss the mechanisms of action and potential therapeutic uses of manipulation of gut hormone levels or hormone antagonists to inhibit the growth of GI tract cancers.