Low levels of raf kinase inhibitory protein in growth hormone-secreting pituitary adenomas correlate with poor response to octreotide treatment.
BACKGROUND
Excessive GH production by pituitary tumors causes acromegaly. Medical treatment of acromegaly with somatostatin analogs (SMSs), like octreotide, is well established, but the clinical effect is variable. One mechanism for octreotide effect is inhibition of the MAPK signaling pathway after binding to the G protein-coupled somatostatin receptor. Nonphosphorylated Raf kinase inhibitory protein (RKIP) binds to and inhibits Raf1 kinase, and thereby attenuates MAPK signaling, whereas phosphorylated RKIP inhibits G protein receptor internalization and degradation due to inhibition of G protein receptor kinase 2.


OBJECTIVE
Our objective was to study RKIP levels in pituitary somatotroph adenomas, and relate them to clinical characteristics and response to octreotide treatment in patients with acromegaly.


METHODS
RKIP level was analyzed by Western blot of proteins extracted from somatotroph tumors frozen a short time after surgery in 51 patients with active acromegaly. An acute somatostatin test was performed in 46 of the patients, and in 21 the IGF-I level before and 6 months after SMS treatment was available.


RESULTS
The adenoma RKIP level correlated significantly to both the acute and the long-term octreotide responses on serum levels of GH and IGF-I, respectively. In multiple regression analyses, the RKIP level was a significant determinant for both the GH reduction in the acute test and the IGF-I reduction after approximately 6 months.


CONCLUSIONS
The RKIP level in somatotroph adenomas seems to be important for the clinical effect of SMS treatment, in which low levels of RKIP correlate to poor clinical response to SMSs.
Fougner
Stine Lyngvi
SL
Research Institute for Internal Medicine, Section of Endocrinology, University of Oslo, Oslo N-0027, Norway. s.l.fougner@medisin.uio.no
Bollerslev
Jens
J
Latif
Fahim
F
Hald
John K
JK
Lund
Terje
T
Ramm-Pettersen
Jon
J
Berg
Jens Petter
JP
D000236:Adenoma / Q000737:chemistry* / Q000188:drug therapy; D000328:Adult; D018931:Antineoplastic Agents, Hormonal / Q000627:therapeutic use*; D005260:Female; D049912:Growth Hormone-Secreting Pituitary Adenoma / Q000737:chemistry* / Q000188:drug therapy; D006801:Humans; D008297:Male; D008875:Middle Aged; D015282:Octreotide / Q000627:therapeutic use*; D049488:Phosphatidylethanolamine Binding Protein / Q000032:analysis*; D017481:Receptors, Somatostatin / Q000032:analysis
D018931:Antineoplastic Agents, Hormonal; C511482:PEBP1 protein, human; D049488:Phosphatidylethanolamine Binding Protein; D017481:Receptors, Somatostatin; C093733:somatostatin receptor 2; D015282:Octreotide