Thyroid hormone and the D-type cyclins interact in regulating enterocyte gene transcription

Thyroid hormone (T3) is an important regulator of gut mucosal development a nd differentiation, inducing intestinal alkaline phosphatase (IAP) and repr essing lactase gene transcription. In contrast, cyclin D1 (CD1) appears to be a growth promoter in the gut, functioning to maintain the undifferentiat ed state. The present studies were designed to examine the effects of CD 1 on T3 action within intestinal epithelia. Caco-2 cells were maintained ill hypothyroid medium and transiently transfected with either rat lactase (3.0 kb) or human IAP (2.4 kb) luciferase (Luc) reporter plasmids. Cotransfecti ons were carried out using two T3 receptor (TR) isoforms, TR alpha -1 and T R beta -1, as well as plasmids expressing CD1, CD3, CA, or CB1. Cells were then treated +/- 10 nmol/L T3 for 24 hours and luciferase activity was dete rmined. With T3 treatment, IAP-Luc activity was induced (TR alpha -1 = eigh tfold, TR beta -1 = ninefold), but these effects were dramatically inhibite d (>50%) by CD1 and CD3. In contrast, CA and CB1 did not alter T3-mediated IAP gene activation. The ability of CD1 and CD3 to inhibit T3 action was al so tested in tile context of the lactase gene, which is negatively regulate d by T3. As expected, lactase reporter gene activity was repressed by T3 tr eatment in tile case of both receptor isoforms, TR alpha -1 = 30% and TR be ta -1 = 40%. In contrast to its effects on the IAP gene, CD1 did not inhibi t T3-mediated changes in lactase reporter gene activity. The D-type cyclins (CD 1 and CD3), but not CA or CB1, specifically inhibit T3-mediated activa tion of the IAP gene. In contrast, the D-type: cyclins do not inhibit T3-me diated repression of the lactase gene. These studies have identified a nove l molecular interaction that exists between the pathways of growth and diff erentiation within intestinal epithelia.