Hormonal Regulation of Gene Expression 
Thyroid Hormone Action 
at the Post-transcriptional Level 
In addition to the well-known transcriptional 
effect of thyroid hormone on TSH synthesis, we 
have recently examined the effect of thyroid hor- 
mone on the half-lives of TSH subunit mRNAs in 
vitro and in vivo. The turnover rate of the TSH|8 
mRNA was greater in the presence of thyroid hor- 
mone. This phenomenon was associated with a 
shortening of the poly(A) tail of the mRNA. These 
events occurred in a dose- and time-dependent 
fashion. Thus thyroid hormone reduces the half- 
life of the TSH/3 mRNA and causes a concomitant 
decrease in its poly (A) size. These data indicate 
that thyroid hormones may also exert their effects 
on gene expression at the post-transcriptional 
level. 
Summary 
Our work has focused on the molecular mecha- 
nisms of thyroid hormone action, with a focus on 
the regulation of TSH gene expression by thyroid 
hormones, including the cis-elements and trans- 
factors that are involved in these processes. The 
effects of these hormones are manifest at both the 
transcriptional and post-transcriptional levels. 
We hope that our efforts will provide insight into 
the hormonal regulation of gene expression and 
cellular function in normal and pathological en- 
docrine states and in cancer. 
76 
