Mild heat stress induces transcription of the β-casein gene via unfolded protein response-activated XBP1 signaling in undifferentiated mammary epithelial cells.

It has been reported that the expression of β-casein, a representative milk protein, increases when mammary epithelial cells are exposed to mild heat stress at 39°C. However, the direct effects and detailed molecular mechanisms have not yet been elucidated. In this study, we investigated the relationship between an increase in β-casein expression and the unfolded protein response (UPR) under mild heat stress. After reaching confluence, HC11 cells were incubated at 37°C (control) or 39°C (mild heat stress) without differentiation medium, and the expression levels of β-casein and UPR-related genes were assessed. It was revealed that, even with this mild heat treatment (39°C), β-casein expression in HC11 cells increased at the transcriptional level without differentiation induction. The expression levels of X-box binding protein 1 (XBP1) and activating transcription factor 6 alpha (ATF6α) were significantly higher in cells cultured at 39°C compared to those cultured at 37°C. Moreover, the increase in β-casein mRNA expression levels by mild heat treatment was suppressed in XBP1 or ATF6α knockdown cells generated by siRNA for XBP1 or ATF6α respectively. Thus, these results demonstrate that ATF6α and XBP1 is involved in the increase of β-casein expression following mild heat treatment.