MEK inhibits secretin release and pancreatic secretion: roles of secretin-releasing peptide and somatostatin

Somatostatin analog therapy effectiveness on the progression of polycystic kidney and liver disease: A systematic review and meta-analysis of randomized clinical trials

Background

Uncertainty underlies the effectiveness of somatostatin analogues for slowing the progression of polycystic kidney or liver disease.

Methods

Eligible studies included randomized controlled trials (RCTs) evaluating somatostatin analog as therapy for patients with polycystic kidney disease (PKD) or polycystic liver disease (PLD) compared to placebo or standard therapy. Two reviewers independently screened studies identified from databases (MEDLINE, EMBASE, Cochrane Database), clinical trial registries, and references from pertinent articles and clinical practice guidelines. Outcome measurements were changes in total liver volume (TLV), total kidney volume (TKV), and estimated glomerular filtration rate (eGFR).

Results

Of 264 nonduplicate studies screened, 10 RCTs met the inclusion criteria. The body of evidence provided estimates warranting moderate confidence. Meta-analysis of 7 RCTs including a total of 652 patients showed that somatostatin analogs are associated with a lower %TLV growth rate compared to control (mean difference, -6.37%; 95% CI -7.90 to -4.84, p<0.00001), and with a lower %TKV growth rate compared to control (mean difference, -3.66%; 95% CI -5.35 to -1.97, p<0.0001). However, it was not associated with a difference in eGFR decline (mean difference, -0.96 mL/min./1.73m²; 95% CI -2.38 to 0.46, p = 0.19).

Conclusions

Current body of evidence suggests that somatostatin analogs therapy slows the increase rate of TLV and TKV in patients with PKD or PLD compared to control within a 3-year follow-up period. It does not seem to have an effect on the change in eGFR. Somatostatin analogs therapy can be a promising treatment for ADPKD or ADPLD, and we need to continue to research its effectiveness for ADPKD or ADPLD.

Somatostatin analogues for treatment of polycystic liver disease

PURPOSE OF REVIEW

The present review summarizes the existing knowledge on polycystic liver disease (PCLD) and highlights the progress made in medical treatment for this condition in the past year.

RECENT FINDINGS

PCLD is associated with autosomal dominant polycystic kidney disease (ADPKD) and autosomal dominant PCLD. Signaling pathways of adenosine 3',5'-cyclic monophosphate (cAMP) and mammalian target of rapamycin (mTOR) are aberrantly regulated in polycystic livers and promote hepatic cystogenesis. Somatostatin analogues reduce intracellular cAMP, and this might prevent fluid accumulation in hepatic cysts. Several clinical trials published over the last year now show that somatostatin analogues when given for 6-12 months in patients with ADPKD and PCLD decrease total liver volume, attenuate polycystic kidney volume, and improve perception of health. In two recent studies mTOR inhibitors failed to halt the progression of ADPKD. It is still too early to recommend to start somatostatin analogues in PCLD and definitive answers should come from future clinical trials.

SUMMARY

Somatostatin analogues are promising new medical drug options in the treatment of PCLD. However, more needs to be elucidated with regard to molecular mechanisms in hepatic cystogenesis, the uncertainty who will respond to therapy and long-term outcomes.

Randomized clinical trial of long-acting somatostatin for autosomal dominant polycystic kidney and liver disease

There are no proven, effective therapies for polycystic kidney disease (PKD) or polycystic liver disease (PLD). We enrolled 42 patients with severe PLD resulting from autosomal dominant PKD (ADPKD) or autosomal dominant PLD (ADPLD) in a randomized, double-blind, placebo-controlled trial of octreotide, a long-acting somatostatin analogue. We randomly assigned 42 patients in a 2:1 ratio to octreotide LAR depot (up to 40 mg every 28+/-5 days) or placebo for 1 year. The primary end point was percent change in liver volume from baseline to 1 year, measured by MRI. Secondary end points were changes in total kidney volume, GFR, quality of life, safety, vital signs, and clinical laboratory tests. Thirty-four patients had ADPKD, and eight had ADPLD. Liver volume decreased by 4.95%+/-6.77% in the octreotide group but remained practically unchanged (+0.92%+/-8.33%) in the placebo group (P=0.048). Among patients with ADPKD, total kidney volume remained practically unchanged (+0.25%+/-7.53%) in the octreotide group but increased by 8.61%+/-10.07% in the placebo group (P=0.045). Changes in GFR were similar in both groups. Octreotide was well tolerated; treated individuals reported an improved perception of bodily pain and physical activity. In summary, octreotide slowed the progressive increase in liver volume and total kidney volume, improved health perception among patients with PLD, and had an acceptable side effect profile.

Role of hepatic opioid receptors in the regulation of bile excretion

Experiments on isolated rat liver perfused with Ringer-Krebs bicarbonate buffer showed that stimulation of delta-opiate receptors in this organ increases bile flow rate and taurocholate secretion. Stimulation of micro-opiate receptors decelerated bile production and inhibited taurocholate secretion. Acceleration of bile production and stimulation of taurocholate secretion under the influence of dalargin is probably related to its interaction with delta-opiate receptors in the liver.

Cholangiocyte anion exchange and biliary bicarbonate excretion

Primary canalicular bile undergoes a process of fluidization and alkalinization along the biliary tract that is influenced by several factors including hormones, innervation/neuropeptides, and biliary constituents. The excretion of bicarbonate at both the canaliculi and the bile ducts is an important contributor to the generation of the so-called bile-salt independent flow. Bicarbonate is secreted from hepatocytes and cholangiocytes through parallel mechanisms which involve chloride efflux through activation of Cl^- channels, and further bicarbonate secretion via AE2/SLC4A2-mediated Cl^-/HCO₃^- exchange. Glucagon and secretin are two relevant hormones which seem to act very similarly in their target cells (hepatocytes for the former and cholangiocytes for the latter). These hormones interact with their specific G protein-coupled receptors, causing increases in intracellular levels of cAMP and activation of cAMP-dependent Cl^- and HCO₃^- secretory mechanisms. Both hepatocytes and cholangiocytes appear to have cAMP-responsive intracellular vesicles in which AE2/SLC4A2 colocalizes with cell specific Cl^- channels (CFTR in cholangiocytes and not yet determined in hepatocytes) and aquaporins (AQP8 in hepatocytes and AQP1 in cholangiocytes). cAMP-induced coordinated trafficking of these vesicles to either canalicular or cholangiocyte lumenal membranes and further exocytosis results in increased osmotic forces and passive movement of water with net bicarbonate-rich hydrocholeresis.

Effect of tetramethylpyrazine on exocrine pancreatic and bile secretion

AIM: To investigate the effect of tetramethylpyrazine (ligustrazine, TMP) on the secretion of exocrine pancreas (and biliary).

METHODS: In in vivo study, we investigated the effect of TMP on the secretion of pancreatic-bile juice (PBJ) in rats. Using human pancreatic duct cell line, CAPAN-1, combined with the short-circuit current (I_SC) technique we further studied the effect of TMP on the pancreatic anion secretion.

RESULTS: Administration of TMP (80 mg/kg, ip) significantly increased the secretion of PBJ (P < 0.05), but the pH of PBJ and the secretion of pancreatic protein were not significantly affected. Basolateral addition of TMP produced a dose-dependent increase in I_SC (EC₅₀ = 1.56 mmol/L), which contained a fast transient I_SC response followed by a slow decay. Apical application of Cl^- channel blockers, DPC (1 mmol/L), decreased the response by about 67.1% (P < 0.001), whereas amiloride (100 μmol/L), a epithelial sodium channel blockers, had no effect. Removal of extracellular HCO₃^- abolished TMP-induced increase in I_SC by about 74.4% (P < 0.001), but the removal of external Cl^- did not. Pretreatment with phosphodiesterase inhibitor, IBMX (0.5 mmol/L), decreased the TMP-induced I_SC by 91% (P < 0.001).

CONCLUSION: TMP could stimulate the secretion of PBJ, especially pancreatic ductal HCO₃^- secretion via cAMP or cGMP-dependent pathway. It need further study to investigate the roles of cAMP or cGMP in the effect of TMP on the secretion of exocrine pancreas.

Neurohormonal control of pancreatic exocrine secretion

The neurohormonal control of pancreatic exocrine secretion is a complex interaction of multiple pathways involving a large number of gut hormones, neurotransmitters, and neuropeptides. Recent studies have elucidated a role for cholecystokinin in the regulation of bicarbonate and fluid secretion from pancreatic duct cells and suggested that cholecystokinin stimulation of human pancreatic acinar cells is likely regulated by an indirect mechanism of stimulation of afferent neurons. Evidence supports the regulation of potassium channels in rat pancreatic acinar cells by the cyclic AMP-mediated agonist secretin. Mechanisms for the regulation of cholecystokinin and secretin release by releasing factors have also been elucidated. The area postrema has been implicated in the mediation of inhibition of pancreatic secretion by the gut hormones peptide YY and pancreatic polypeptide. The neurotransmitter serotonin has been demonstrated to play a role in acid-induced secretin release and in pancreatic secretion stimulated by luminal factors. The regulation of pancreatic exocrine secretion by purines, nitric oxide, and gamma-aminobutyric acid as well as by the neuropeptides pituitary adenylate cyclase-activating peptide, gastrin-releasing peptide, and substance P is reviewed. The role of the central nervous system in modulating pancreatic secretion is also described. This review highlights the recent advances in knowledge of the neurohormonal regulation of pancreatic exocrine secretion.

Neural hormonal regulation of exocrine pancreatic secretion

Exocrine pancreatic secretion is regulated by hormone-hormonal and neural-hormonal interactions involving several regulatory peptides and neurotransmitter from the gut, the pancreas and the vagus nerve. The roles of the gastrointestinal peptides including secretin, CCK, neurotensin, motilin, PYY and pancreatic islet hormones including insulin, pancreatic polypeptide and somatostatin have been established. Interactions among secretin, CCK and neurotensin produce synergistic stimulatory effect. Motilin modulates the cyclic pattern of pancreatic secretion while local insulin provides a permissive role for the action of secretin and CCK at physiological concentration. Somatostatin, PYY and pancreatic polypeptide are inhibitory regulators, acting either on the release of secretin and CCK or on the action of the two stimulatory hormones. The vagal afferent-efferent pathway mediates the actions of many of these regulatory peptides, particularly of secretin and CCK. Acetylcholine and nitric oxide are the neurotransmitters known to mediate the actions of secretin and CCK. Serotonin (5-HT) released from enterochromaffin cells in the intestinal mucosa and nerve terminals of the enteric nervous system and intrapancreatic nerves may be involved in both stimulatory and inhibitory mechanism through its various receptor subtypes. 5-HT also mediates the action of secretin and CCK. The regulatory roles of neuropeptides, PACP and GRP, are now established, whereas those of others are being uncovered. Pancreatic juice provides both positive and negative feedback regulation of pancreatic secretion through mediation of both secretin- and CCK-releasing peptides. Three CCK-releasing peptides have been purified: monitor peptide from pancreatic juice, diazepam-binding inhibitor from porcine intestine, and luminal CCK-releasing factor from rat intestinal secretion. All have been shown to stimulate CCK release and pancreatic enzyme secretion. Pancreatic phospholipase A2 from pancreatic juice and intestinal secretion appears to function as a secretin-releasing peptide. However, the detailed map of neurohormonal regulatory pathways of exocrine pancreatic secretion is yet to be constructed.

Neurohormonal control of exocrine pancreas

The exocrine pancreas is regulated by various hormonal factors derived from the gut through hormone-hormonal and neurohormonal interactions. Physiologic stimuli entering the upper small intestine elicit the release of intestinal hormones and activate sensory reflex mechanisms from the intestinal mucosa to stimulate or inhibit exocrine pancreatic secretion. In addition, the endocrine pancreas, intrapancreatic nerves, and some extrapancreatic neural pathways, with or without mediation by the vagus nerve, are known to participate in regulation of exocrine pancreatic secretion. It has been established that two key intestinal hormones, secretin and cholecystokinin (CCK), in physiologic doses, act through the vagal afferent pathway and interact with each other as well as with other gut hormones. The releases of these two hormones are mediated through the corresponding releasing peptides. In the past few years, the roles of secretin- and CCK-releasing peptides have become more clearly defined. The participation of several neurotransmitters and regulatory peptides in the regulation of exocrine pancreatic secretion has also been established. In addition, neurotransmitters and neuropeptides released from the central nervous system may participate in the regulation of pancreatic secretion. It is conceivable that a few neurotransmitters and neuropeptides are involved in each neural regulatory pathway. However, their roles and sites of action in each pathway remain to be determined.