Hypothyroidism advances mammary involution in lactating rats through inhibition of PRL signaling and induction of LIF/STAT3 mRNAs

• On lactation day 21, hypothyroid rats showed premature mammary involution.
• PRL signaling pathway was partially blocked, inducing low milk protein expression.
• Involution markers, such as LIF, STAT3 and caspase 3 and 8 were increased.
• PRA, ERa, TRa1 and TRb1 expression and circulating estradiol were increased.
• The dysregulated PRL and LIF pathways may cause the premature mammary involution.

Thyroid diseases have deleterious effects on lactation, litter growth and survival, and hinder the suckling-induced hormone release, leading in the case of hyperthyroidism, to premature mammary involution. To determine the effects of hypothyroidism (HypoT) on late lactation, we analyzed the effect of chronic 6-propyl-2-thiouracil (PTU)-induced HypoT on mammary histology and the expression of members of the JAK/STAT/SOCS signaling pathway, milk proteins, prolactin (PRLR), estrogen (ER), progesterone (PR) and thyroid hormone (TR) receptors, markers of involution (such as stat3, lif, bcl2, BAX and PARP) on lactation (L) day 21. HypoT mothers showed increased histological markers of involution compared with control rats, such as adipose/epithelial ratio, inactive alveoli, picnotic nuclei and numerous detached apoptotic cells within the alveolar lumina. We also found decreased PRLR, β-casein and α-lactoalbumin mRNAs, but increased SOCS1, SOCS3, STAT3 and LIF mRNAs, suggesting a decrease in PRL signaling and induction of involution markers. Furthermore, Caspase-3 and 8 and PARP labeled cells and the expression of structural proteins such as β-Actin, α-Tubulin and Lamin B were increased, indicating the activation of apoptotic pathways and tissue remodelation. HypoT also increased PRA (mRNA and protein) and erβ and decreased erα mRNAs, and increased strongly TRα1, TRβ1, PRA and ERα protein levels. These results show that lactating HypoT rats have premature mammary involution, most probably induced by the inhibition of prolactin signaling along with the activation of the LIF-STAT3 pathway.

Panel A: Mammary cell of the Ctrl group. In the Ctrl group, intracellular signaling and transcriptional activity of the cell are directed towards the synthesis of the components of milk. Panel B: Mammary cell of the HypoT group. In the HypoT group, PRL signaling is inhibited at many levels. The lower prlr synthesis, inhibition of STAT5 transcriptional activity, inhibition of STAT5 nuclear translocation, milk stasis and activation of LIF/STAT3 signaling pathway are key factors that induce premature involution of the mammary gland.Figure optionsDownload high-quality image (435 K)Download as PowerPoint slide