UP-REGULATION OF SOMATOSTATIN RECEPTORS BY EPIDERMAL GROWTH-FACTOR AND GASTRIN IN PANCREATIC-CANCER CELLS

Interactions between growth factor receptor systems may be important i n the regulation of cell growth. The proliferation of pancreatic tumor AR42J cells has been shown to be stimulated by Epidermal growth facto r (EGF) and gastrin and inhibited by somatostatin. To analyze the inte raction between these different peptides, we explored the influence of EGF and gastrin on the somatostatin receptors. Treatment of AR42J cel ls with 10 nM EGF or gastrin for 24 hr increased specific binding of [ I-125] Tyr(3)SMS to 131 and 147% of that in control cells, respectivel y. The effect of peptides on [I-125]Tyr(3)SMS binding was time- and do se-dependent, with half-maximal effect at 0.2 +/- 0.03 nM EGF and 0.3 +/- 0.15 nM gastrin. Scatchard plots revealed an increase in somatosta tin receptor number of 27 and 80% after 48 hr of treatment with EGF an d gastrin, respectively, without any change in receptor affinity. The increase in somatostatin receptor density was accompanied by the enhan cement of biological responses to somatostatin. In cells pretreated wi th EGF or gastrin, the potency of somatostatin for inhibiting vasoacti ve intestinal peptide-stimulated cAMP content was increased 2-fold as that of somatostatin analog, SMS, for inhibiting cell proliferation. F urthermore, the efficiency of SMS as antiproliferative agent was great ly increased. Vasoactive intestinal peptide or forskolin did not modif y [I-125]Tyr(3)SMS binding of control or treated cells. The phorbol es ter 12-O-tetradecanoyl phorbol 13-acetate (TPA) did not affect [I-125] Tyr(3)SMS binding. On the other hand, cycloheximide completely blocked the increase in [I-125]Tyr(3)SMS binding induced by EGF and gastrin. Analysis of mRNA expression of the SSTR1, 2, 3 somatostatin receptor s ubtypes demonstrated that in AR42J cells SSTR1 and SSTR3 mRNAs were de tected at very low levels, whereas the steady-state level of SSTR2 mRN A was high. EGF and gastrin enhanced the steady-state level of SSTR2 m RNA. The increase was time dependent and reached 72 and 200% after 24 hr of treatment with EGF and gastrin, respectively. EGF and gastrin al so enhanced the level of SSTR3 mRNA by 300 and 290%, respectively, aft er 24 hr of treatment. In contrast, these agents had no effect on SSTR 1 mRNA levels. We conclude that EGF and gastrin up-regulate functional somatostatin receptors through a protein kinase A-and protein kinase C-independent pathways. This effect requires protein synthesis and is mediated, at least in part, by the increase of SSTR2 mRNA levels and, to a lower extent, by that of SSTR3.