Alterations in retinoid X receptor-alpha expression contribute to cell-type dependent differences in thyroid hormone...

Triiodothyronine (T3) stimulates a marked increase (> 40-fold) in transcription of the malic enzyme gene in chick embryo hepatocytes (CEH), but has no effect on malic enzyme transcription in chick embryo fibroblasts (CEF) that express nuclear T3 receptors (TR) at levels which are similar to those of CEH. Heterodimerization of the TR with other nuclear proteins is a potential mechanism for the regulation of T3 action. For example, heterodimers of retinoid X receptors (RXR) and TR bind to T3 response elements (T3RE) with higher affinity and modulate transcription more effectively than TR homodimers. In the present report, we investigated the role of RXR in mediating differences in T3 responsiveness of the malic enzyme gene between CEH and CEF. Data from gel mobility shift analyses demonstrated that endogenous TRs from CEH and CEF bind to the major T3RE of the malic enzyme gene primarily as heterodimers with RXR alpha or a protein highly related to RXR alpha. The total binding activity of RXR alpha/TR complexes in CEF was decreased relative to that observed in CEH. Cell-type dependent differences in RXR alpha/TR complex formation were greater in cells incubated in the presence of T3 because T3 treatment increased RXR alpha/TR binding activity in CEH but had no effect on RXR alpha/TR binding activity in CEF. Decreased RXR alpha/TR complex formation in CEF relative to CEH was associated with a reduction in the abundance of RXR alpha protein and RXR alpha mRNA in the former cell-type. Expression of exogenous RXR alpha in CEF increased the T3 responsiveness of the malic enzyme promoter by about 4-fold. In contrast, expression of exogenous RXR alpha in CEH had no effect on the regulation of malic enzyme transcription by T3. These observations support the hypothesis that alterations in RXR alpha expression contribute to cell-type dependent differences in T3 responsiveness of the malic enzyme gene.