Human proximal tubular epithelial cells express somatostatin: regulation by growth factors and cAMP

Somatostatin in renal physiology and autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by progressive cyst formation, leading to growth in kidney volume and renal function decline. Although therapies have emerged, there is still an important unmet need for slowing the rate of disease progression in ADPKD. High intracellular levels of adenosine 3′,5′-cyclic monophosphate (cAMP) are involved in cell proliferation and fluid secretion, resulting in cyst formation. Somatostatin (SST), a hormone that is involved in many cell processes, has the ability to inhibit intracellular cAMP production. However, SST itself has limited therapeutic potential since it is rapidly eliminated in vivo. Therefore analogues have been synthesized, which have a longer half-life and may be promising agents in the treatment of ADPKD. This review provides an overview of the complex physiological effects of SST, in particular renal, and the potential therapeutic role of SST analogues in ADPKD.

Association of plasma somatostatin with disease severity and progression in patients with autosomal dominant polycystic kidney disease

Background

Somatostatin (SST) inhibits intracellular cyclic adenosine monophosphate (cAMP) production and thus may modify cyst formation in autosomal dominant polycystic kidney disease (ADPKD). We investigated whether endogenous plasma SST concentration is associated with disease severity and progression in patients with ADPKD, and whether plasma SST concentrations change during treatment with a vasopressin V2 receptor antagonist or SST analogue.

Methods

In this observational study, fasting concentrations of SST were measured in 127 ADPKD patients (diagnosed upon the revised Ravine criteria) by ELISA. cAMP was measured in 24 h urine by Radio Immuno Assay. Kidney function was measured (mGFR) as ¹²⁵I-iothalamate clearance, and total kidney volume was measured by MRI volumetry and adjusted for height (htTKV). Disease progression was expressed as annual change in mGFR and htTKV. Additionally, baseline versus follow-up SST concentrations were compared in ADPKD patients during vasopressin V2 receptor antagonist (tolvaptan) (n = 27) or SST analogue (lanreotide) treatment (n = 25).

Results

In 127 ADPKD patients, 41 ± 11 years, 44% female, eGFR 73 ± 32 ml/min/1.73m², mGFR 75 ± 32 ml/min/1.73m² and htTKV 826 (521–1297) ml/m, SST concentration was 48.5 (34.3–77.8) pg/ml. At baseline, SST was associated with urinary cAMP, mGFR and htTKV (p = 0.02, p = 0.004 and p = 0.02, respectively), but these associations lost significance after adjustment for age and sex or protein intake (p = 0.09, p = 0.06 and p = 0.15 respectively). Baseline SST was not associated with annual change in mGFR, or htTKV during follow-up (st. β = − 0.02, p = 0.87 and st. β = − 0.07, p = 0.54 respectively). During treatment with tolvaptan SST levels remained stable 38.2 (23.8–70.7) pg/mL vs. 39.8 (31.2–58.5) pg/mL, p = 0.85), whereas SST levels decreased significantly during treatment with lanreotide (42.5 (33.2–55.0) pg/ml vs. 29.3 (24.8–37.6), p = 0.008).

Conclusions

Fasting plasma SST concentration is not associated with disease severity or progression in patients with ADPKD. Treatment with lanreotide caused a decrease in SST concentration. These data suggest that plasma SST cannot be used as a biomarker to assess prognosis in ADPKD, but leave the possibility open that change in SST concentration during lanreotide treatment may reflect therapy efficacy.

Electronic supplementary material

The online version of this article (10.1186/s12882-018-1176-y) contains supplementary material, which is available to authorized users.

Expression of somatostatin and somatostatin receptor subtypes 1-5 in human normal and diseased kidney

Somatostatin mediates inhibitory functions through five G protein-coupled somatostatin receptors (sst1-5). We used immunohistochemistry, immunofluorescence, and RT-PCR to determine the presence of somatostatin receptors sst1, sst2A, sst2B, sst3, sst4, and sst5 in normal and IgA nephropathy human kidney. All somatostatin receptors were detected in the thin tubules (distal convoluted tubules and loops of Henle) and thick tubules (proximal convoluted tubules) in the tissue sections from nephrectomy and biopsy samples. Immunopositive sst1 and sst4 staining was more condensed in the cytoplasm of tubular epithelial cells. In normal kidney tissue sections, podocytes and mesangial cells in the glomeruli stained for sst1, sst2B, sst4 and sst5, and stained weakly for sst3. In IgA kidney tissue, the expression of somatostatin receptors was significantly increased with particular immmunopositive staining for sst1, sst2B, sst4, and sst5 within glomeruli. In the epithelial cells, the staining for sst2B and sst4 in proximal tubules and sst1, sst2B, and sst5 in distal tubules was increased. The mRNA expression of sst1-5 was also detected by RT-PCR. Somatostatin and all five receptor subtypes were ubiquitously distributed in normal kidney and IgA nephropathy. The increased expression of somatostatin receptors in IgA nephropathy kidney might be the potential pathogenesis of inflammatory renal disease.

EP2 receptor mediates PGE2-induced cystogenesis of human renal epithelial cells

Autosomal-dominant polycystic kidney disease (ADPKD) is characterized by formation of cysts from tubular epithelial cells. Previous studies indicate that secretion of prostaglandin E2 (PGE2) into cyst fluid and production of cAMP underlie cyst expansion. However, the mechanism by which PGE2 directly stimulates cAMP formation and modulates cystogenesis is still unclear, because the particular E-prostanoid (EP) receptor mediating the PGE2 effect has not been characterized. Our goal is to define the PGE2 receptor subtype involved in ADPKD. We used a three-dimensional cell-culture system of human epithelial cells from normal and ADPKD kidneys in primary cultures to demonstrate that PGE2 induces cyst formation. Biochemical evidence gathered by using real-time RT-PCR mRNA analysis and immunodetection indicate the presence of EP2 receptor in cystic epithelial cells in ADPKD kidney. Pharmacological evidence obtained by using PGE2-selective analogs further demonstrates that EP2 mediates cAMP formation and cystogenesis. Functional evidence for a role of EP2 receptor in mediating cAMP signaling was also provided by inhibiting EP2 receptor expression with transfection of small interfering RNA in cystic epithelial cells. Our results indicate that PGE2 produced in cyst fluid binds to adjacent EP2 receptors located on the apical side of cysts and stimulates EP2 receptor expression. PGE2 binding to EP2 receptor leads to cAMP signaling and cystogenesis by a mechanism that involves protection of cystic epithelial cells from apoptosis. The role of EP2 receptor in mediating the PGE2 effect on stimulating cyst formation may have direct pharmacological implications for the treatment of polycystic kidney disease.

Intrarenal octreotide treatment prevents sodium retention in liver cirrhotic rats: evidence for direct effects within the thick ascending limb of Henle's loop

We have previously shown that systemic treatment with the somatostatin analog octreotide has marked beneficial effects on renal function in rats with liver cirrhosis induced by common bile duct ligation (CBL; Jonassen TEN, Christensen S, Sørensen AM, Marcussen N, Flyvbjerg A, Andreasen F, and Petersen JS. Hepatology 29: 1387-1395, 1999). In the present study, we tested the hypothesis that octreotide has a direct effect on renal tubular function. Rats (CBL or Sham-CBL) were intrarenally treated with low-dose octreotide in a long-acting release formulation, which had no systemic actions (100 microg/kg body wt as a single dose). Rats receiving low-dose octreotide (sc) were used as controls. The rats were chronically instrumented, and renal function was examined 4 wk after CBL or Sham-CBL. Intrarenal octreotide administration (IROA) prevented sodium retention in CBL rats without changes in renal plasma flow, glomerular filtration rate, or circulating levels of aldosterone and vasopressin. Renal clearance studies revealed that IROA normalized the increased natriuretic efficacy of furosemide found in CBL rats. Furthermore, IROA protected against the development of hypertrophy of the inner stripe of the outer medulla and thereby the increased the volume of thick ascending limb of Henle's loop (TAL) epithelium found in CBL rats. Finally, Western blot analyses of outer medullary homogenates showed increased abundance of the furosemide-sensitive Na-K-2Cl (NKCC2) cotransporter. IROA did not affect the abundance of NCKK2 within the outer medulla. Together with the histological findings, these results indicate that IROA reduces the total number of NKCC2 within the outer medulla. In conclusion, the results indicate a direct intrarenal effect of octreotide on TAL function and morphology in cirrhotic rats.

Segmental expression of somatostatin receptor subtypes sst(1) and sst(2) in tubules and glomeruli of human kidney

Somatostatin is known to modulate mesangial and tubular cell function and growth, but the somatostatin receptor (sst) subtypes responsible for these effects have not been defined. There are at least five different sst receptor subtypes (sst(1)-sst(5)). We used RT-PCR to demonstrate that normal human kidney consistently expresses mRNA for sst(1) and sst(2) (9 of 9 donors). Some donors expressed sst(4) or sst(5) mRNA, but none expressed sst(3) mRNA. Expression of sst(1) and sst(2) was further assessed by staining serial sections of normal human kidney with sst(1) and sst(2) antisera, Arachis hypogaea (AH) lectin (to define distal tubule/collecting duct cells), Phaseolus vulgaris lectin (proximal tubules), and Tamm-Horsfall protein (THP) antiserum (thick ascending limb of the loop of Henle). Specificity of antisera was demonstrated by transfection and absorption studies. Sst(2), but not sst(1), was expressed in glomeruli. Intense sst(1) and sst(2) staining localized exclusively to AH+ and THP+ tubules. Thus sst(1) and sst(2) subtype-selective analogs may be useful to beneficially modulate renal cell function in pathological conditions.