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Impact of cyclical heat stress during follicular development on ovarian TLR4, PI3K and steroidogenic proteins in synchronized post-pubertal gilts
Heat stress (HS) is caused by the sustained elevation of core body temperature due to high ambient temperatures. HS is associated with seasonal infertility, which results in economic losses for the swine industry. Hyperinsulinemia and metabolic endotoxemia are physiological hallmarks of HS, both of which potentially modulate ovarian function via the toll-like receptor 4 (TLR4), the receptor for LPS, and/or the phosphotidylinositol-3 kinase (PI3K) pathways. Our previous findings demonstrated that HS enhanced phosphorylation of ovarian AKT (pAKT), increased TLR4, steroidogenic acute regulatory protein (StAR), and aromatase (CYP19A) protein abundance in pre pubertal gilts exposed to 7 or 35 d of HS. The current study investigated whether HS also altered TLR4, PI3K and enzymes involved in steroid hormone production in heat-stressed, post-pubertal gilts. The estrous cycles of 12 post-pubertal gilts were synchronized using Matrix®, administered orally for 14 d, followed by exposure to thermal neutral conditions (TN; 20.3oC ± 0.1°C) or cyclical HS conditions (26-32°C) during follicular development (5 d) preceding ovulation. Both TN and HS gilts were limit-fed 2.7 kg/d for the duration of the study. HS gilts had increased (P = 0.01) average rectal temperatures (39.8°C ± 0.2°) compared to the TN controls (38.8°C ± 0.2°) demonstrating hyperthermia in response to elevated ambient temperatures. Gilts were euthanized and ovaries collected for protein isolation and analysis. The abundance of ovarian pAKT, StAR, CYP19A and TLR4 were determined using western blotting. No impact (P > 0.05) of HS on protein abundance of CYP19A or StAR was observed. TLR4 was increased (P < 0.05) in ovaries from HS gilts relative to the TN controls. Additionally, HS decreased (P < 0.01) phosphorylation of ovarian AKT, relative to TN gilts. These findings demonstrate that ovarian signaling is altered by HS: activation of TLR4 indicates an ovarian response to elevated, systemic LPS, while decreased pAKT may reflect reduced altered PI3K activity. These data provide mechanistic insight into ovarian physiological alterations that could contribute to seasonal infertility in post-pubertal swine. This work was supported by the Iowa Pork Producers Association.
Keywords: Heat Stress, TLR4, PI3K, AKT, Endotoxemia