Low beta-arrestin expression correlates with the responsiveness to long-term somatostatin analog treatment in acromegaly

Objective: The high expression of somatostatin receptor subtype 2 (SSTR2 also known as sst(2)) usually present in growth hormone (GH)-secreting adenomas is the rationale for therapy with somatostatin analogs (SSAs) in acromegaly. Although SSTR2 expression is a good predictor for biochemical response to SSA treatment, we still face tumors resistant to SSAs despite high SSTR2 expression. Recently, beta-arrestins (beta-arrestins) have been highlighted as key players in the regulation of SSTR2 function. Design: To investigate whether beta-arrestins might be useful predictors of responsiveness to long-term SSA treatment in acromegaly, we retrospectively evaluated 35 patients with acromegaly who underwent adenomectomy in two referral centers in The Netherlands. Methods: beta-arrestin mRNA levels were evaluated in adenoma samples, together with SSTR2 (and SSTR5) mRNA and protein expression. Biochemical response to long-term SSA treatment (median 12 months) was assessed in 32 patients. Results: beta-arrestin 1 and 2 mRNA was significantly lower in adenoma tissues from patients who achieved insulin-like growth factor 1 normalization (P = 0.024 and P = 0.047) and complete biochemical control (P = 0.047 and P = 0.039). The SSTR2 mRNA was higher in SSA responder patients compared with the resistant ones (P = 0.026). This difference was more evident when analyzing the SSTR2/beta-arrestin 1 and SSTR2/beta-arrestin 2 ratio (P = 0.011 and P = 0.010). beta-arrestin 1 and 2 expression showed a significant trend of higher median values from full responders, partial responders to resistant patients (P = 0.045 and P = 0.021, respectively). Interestingly, SSTR2 protein expression showed a strong inverse correlation with both beta-arrestin 1 and 2 mRNA (rho = -0.69, P = 0.0011 and rho = -0.67, P = 0.0016). Conclusions: Low beta-arrestin expression and high SSTR2/beta-arrestin ratio correlate with the responsiveness to long-term treatment with SSAs in patients with acromegaly.