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

Somatostatin modulates several renal tubular cell functions, including gluconeogenesis and proliferation. In this study, we demonstrate that cultured human proximal tubular epithelial cells (PTEC) express somatostatin. We also demonstrate positive and negative regulation of PTEC somatostatin production. We found that PTEC derived from 14 different human donors consistently expressed somatostatin mRNA and/or peptide as detected by RT-PCR and enzyme-linked immunoassay. Furthermore, Northern blot analysis revealed that PTEC express the same size mRNA transcript (750 nucleotides) as human thyroid carcinoma (TT) cells. The PTEC mitogens, epidermal growth factor(EGF) and hydrocortisone, inhibit PTEC somatostatin secretion, whereas forskolin (a direct stimulator of adenylate cyclase) and fetal bovine serum stimulate secretion. These findings raise the possibility that renal-derived somatostatin modulates tubular cell function in an autocrine/paracrine manner. Manipulation of this pathway may lead to novel methods with which to alter tubular cell proliferation and function in vivo.

Characterization of human proximal tubular cells after hypoxic preconditioning: constitutive and hypoxia-induced expression of heat shock proteins HSP70 (A, B, and C), HSC70, and HSP90.

In animal models of cardiac or cerebral ischemic preconditioning, induction of heat shock proteins (HSPs), especially HSP70, correlates with protection from subsequent injury. The extent of HSP70 induction after stress correlates inversely with initial HSP70 levels. Primate cells, unlike nonprimate cells, express high basal levels of HSP70; thus, primate cells may respond differently to preconditioning than nonprimate cells. We have demonstrated that exposing cultured human proximal tubular epithelial cells (PTEC) to 12 h of hypoxia followed by a 24-h recovery period (hypoxic preconditioning) induces resistance to subsequent hypoxic injury. Herein, we characterize the expression of HSP70, HSP90, and heat shock cognate-70 (HSC70) in PTEC under basal conditions and after hypoxic preconditioning. By Northern blot analysis, we demonstrate that hypoxic preconditioning of PTEC increases mRNA for HSP70 > HSP90 > HSC70. With reverse transcription and polymerase chain reaction, mRNA transcripts from three different HSP70 genes (HSP70 A, B, and C) were detected in unstressed PTEC. Transcripts from these genes were also detected in freshly isolated human renal cortex, indicating that all three genes are expressed in vivo. By Western blot analysis, we demonstrate that PTEC express high basal levels of HSP70, HSC70, and HSP90. Hypoxic preconditioning did not lead to a significant increase in protein content of any of these HSPs, despite increased mRNA levels. This suggests that HSP accumulation cannot account for the development of cytoresistance after hypoxic preconditioning in PTEC. However, high basal expression of HSP70 in human PTEC may contribute to their innate resistance for hypoxia.

Somatostatin expression in human renal cortex and mesangial cells.

Somatostatin modulates important physiologic functions of the kidney, including mesangial cell contraction, glomerular prostaglandin synthesis, and phosphate, water and sodium excretion. In diabetic nephropathy, somatostatin inhibits renal hypertrophy. High affinity somatostatin receptors are expressed in the kidney. Circulating somatostatin concentrations, however, are generally well below the affinity constants of known somatostatin receptors. Thus, we hypothesized that somatostatin is produced in the kidney and released locally to act in an autocrine/paracrine manner. Using reverse transcriptase and polymerase chain reaction (RT-PCR) analysis, we found that fresh human renal cortex and cultured human mesangial cells express somatostatin mRNA. Restriction enzyme and Southern blot analysis confirmed that RT-PCR cDNA products were derived from somatostatin mRNA. Radioimmunoassay of mesangial cell culture supernatants demonstrated SS-immunoreactive peptide (87 +/- 30 pg/ml compared to 19 +/- 9 pg/ml in medium not exposed to cells; P < 0.05). In contrast, renal cells did not transcribe detectable levels of vasoactive intestinal peptide (VIP) or neuropeptide Y (NPY) mRNA, nor did they synthesize measurable peptide. Our results demonstrate that renal cells produce somatostatin and suggest that kidney-derived somatostatin may regulate renal function in an autocrine/paracrine manner. Characterization of this pathway may lead to novel methods to alter the course of diabetic nephropathy and other renal diseases.