Characterization of somatostatin receptor expression in human pancreatic cancer using real-time RT-PCR

Abstract

Background: Somatostatin inhibits cell proliferation and may act as a tumor suppressor by interacting with five different somatostatin receptors (SSTRs). We hypothesized that SSTR expression is down regulated in human pancreatic cancer. In the present study, we used a powerful real-time RT-PCR technique to examine the mRNA expression levels of all five SSTR subtypes in human pancreatic cancer. Methods: Total RNA was extracted from three pancreatic cancer cell lines (MIA PaCa-2, Panc-1, and HS766T), three surgical specimens of pancreatic cancer, and adjacent pancreatic tissue, and a pancreatic cancer cell line transfected with the sstr2 gene. Specific primers were designed and mRNA levels for the five SSTRs were analyzed with real-time quantitative RT-PCR using a Bio-Rad iCycler system. The presence of SSTR receptor protein was determined by a competitive binding assay. The proliferative response to somatostatin in pancreatic cancer cell lines was analyzed by incorporation of [3H]-thymidine. Results: The pancreatic tumor specimens had a 2.5 and 4.3 fold reduction of SSTR2 and SSTR5 mRNA levels, respectively, as compared to their adjacent pancreatic tissues. SSTR1 and SSTR3 were also detected in both the cancer specimens and adjacent tissues, but SSTR4 was absent. Human pancreatic cancer cell lines also expressed SSTR2 and SSTR5 mRNA, but not SSTR1, SSTR3, and SSTR4. SSTR receptor protein and inhibition of cell proliferation was demonstrated only in wild type MiaPaCa-2 cells. Up regulation of SSTR2 mRNA by 2.2 × 104 fold in Panc-1 cells resulted in receptor expression and growth inhibition. Conclusion: Expression of SSTR2 and SSTR5 could be important in the growth inhibitory effect of somatostatin in human pancreatic cancer. Down regulation of SSTR transcription or SSTR mRNA instability may result in loss of a tumor suppressive effect of SSTRs in human pancreatic cancer.