Nuclear Localization of STAT5A Modified with O-Linked N-Acetylglucosamine and Early Involution in the Mammary Gland of Hirosaki Hairless Rat

The study investigated the mammary gland development in Hirosaki hairless rats (HHR), which exhibited early involution during lactation. Morphological examinations showed underdeveloped mammary glands in virgin HHRs and distended alveoli on day 1 of lactation, followed by involution. Apoptosis was induced early in lactation, and milk secretion was observed. STAT5A levels were similar in HHR and Sprague-Dawley rats (SDR), but Tyr-phosphorylated STAT5A was absent in HHR. Instead, STAT5A in HHR was modified with O-linked N-acetylglucosamine (O-GlcNAc), which was not seen in SDR or heterozygous rats. This modification was suggested to be involved in the transient activation of STAT5A, contributing to the short period of terminal differentiation followed by involution in HHR mammary glands.