EXPRESSION AND REGULATION OF THE MYC PROTOONCOGENE IN THE PITUITARY-GLAND OF RAINBOW-TROUT

We have isolated two c-myc cDNA clones from a trout pituitary gland (P G). One of these clones (tmyc2) is expressed as a 1.9-2.0 kb transcrip t in the PG, brain and heart, but not in the liver and codes for a pro tein of 398 amino acids. The second clone (tmyc3) is expressed in the PG (1.6 kb), but not in brain, liver or heart and codes for a protein of 401 amino acids. The expression level of tmyc3 in the PG is 10-fold greater than that of tmyc2. This profile is markedly different from a previously isolated genomic c-myc clone (tmyc1) that is expressed pri marily in the liver (2.4 kb) and codes for a larger protein consisting of 417 amino acids. In situ hybridization and Northern blot analysis showed that tmyc3 is expressed primarily in somatolactotrophs of the i ntermediate lobe (IL) of the pc. Tmyc2 is expressed at similar levels in both the IL and the pars distalis (PD). In vitro stimulation experi ments show that releasing factors, known to control cells in the PD, f ailed to stimulate tmyc2 or tmyc3. However, dopamine and norepinephrin e (two neurotransmitters known to control hormone release from the IL) increased tmyc3 expression two to five times. Two other neurotransmit ters (GABA and serotonin) failed to stimulate tmyc3 expression. None o f the neurotransmitters tested affected tmyc2 expression. These result s suggest that tmyc3 is involved in regulating hypertrophy of somatola ctotrophs and may have a role in stimulating production of somatolacti n, the only hormone known to be synthesised by these cells. (C) 1997 E lsevier Science Ireland Ltd.