Expression and regulation of the myc proto-oncogene in the pituitary gland of rainbow trout

We have isolated two c- myc cDNA clones from a trout pituitary gland (PG). One of these clones (t myc2) is expressed as a 1.9–2.0 kb transcript in the PG, brain and heart, but not in the liver and codes for a protein of 398 amino acids. The second clone (t myc3) 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 t myc3 in the PG is 10-fold greater than that of t myc2. This profile is markedly different from a previously isolated genomic c- myc clone (t myc1) that is expressed primarily 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 t myc3 is expressed primarily in somatolactotrophs of the intermediate lobe (IL) of the PG. T myc2 is expressed at similar levels in both the IL and the pars distalis (PD). In vitro stimulation experiments show that releasing factors, known to control cells in the PD, failed to stimulate t myc2 or t myc3. However, dopamine and norepinephrine (two neurotransmitters known to control hormone release from the IL) increased t myc3 expression two to five times. Two other neurotransmitters (GABA and serotonin) failed to stimulate t myc3 expression. None of the neurotransmitters tested affected t myc2 expression. These results suggest that t myc3 is involved in regulating hypertrophy of somatolactotrophs and may have a role in stimulating production of somatolactin, the only hormone known to be synthesised by these cells.