Unexpected maturation of PI3K and MAPK-ERK signaling in fetal ovine cardiomyocytes

In the first two-thirds of gestation, ovine fetal cardiomyocytes undergo mitosis to increase cardiac mass and accommodate fetal growth. Thereafter, some myocytes continue to proliferate while others mature and terminally differentiate into binucleated cells. At term (145 days gestational age; dGA) about 60% of cardiomyocytes become binucleated and exit the cell cycle under hormonal control. Rising thyroid hormone (T₃) levels near term (135 dGA) inhibit proliferation and stimulate maturation. However, the degree to which intracellular signaling patterns change with age in response to T₃ is unknown. We hypothesized that in vitro activation of ERK, Akt, and p70^S6Kby two regulators of cardiomyocyte cell cycle activity, T₃ and insulin like growth factor-1 (IGF-1), would be similar in cardiomyocytes at gestational ages 100 and 135 dGA. IGF-1 and T₃ each independently stimulated phosphorylation of ERK, Akt, and p70^S6K in cells at both ages. In the younger mononucleated myocytes, the phosphorylation of ERK and Akt was reduced in the presence of IGF-1 and T_3. However, the same hormone combination led to a dramatic twofold increase in the phosphorylation of these signaling proteins in the 135 dGA cardiomyocytes—even in cells that were not proliferating. In the older cells, both mono- and binucleated cells were affected. In conclusion, fetal ovine cardiomyocytes undergo profound maturation-related changes in signaling in response to T₃ and IGF-1, but not to either factor alone. Differences in age-related response are likely to be related to milestones in fetal cardiac development as the myocardium prepares for ex utero life.