Dimeric but not monomeric α-lactalbumin potentiates apoptosis by up regulation of ATF3 and reduction of histone deacetylase activity in primary and immortalised cells

α-lactalbumin is a protein of dual function found in milk of most mammals. α-lactalbumin binds β-1,4-galactosyltransferase to form the regulatory subunit for lactose synthesis and has also been shown to cause cell death. This study shows, for the first time, that α-lactalbumin isolated in a rare 28 kDa dimeric form induces cell death, while 14 kDa monomeric α-lactalbumin is inactive. In contrast to the casein derived and chemically induced α-lactalbumin variants, MAL and HAMLET/BAMLET, the effects of 28 kDa α-lactalbumin are calcium independent and, unlike MAL and HAMLET, 28 kDa α-lactalbumin dimer causes cell death of primary mammary cells and a variety of immortalised cell lines, which are committed to cell death pathways within 1–4 h of exposure. Microarray analysis confirmed that cell death was the result of an apoptotic response. Functional assays determined that the mechanism by which 28 kDa α-lactalbumin kills cells involved inhibition of histone deacetylase activity mediated by NF-kB. We also show that 28 kDa α-lactalbumin occurs naturally in the milk of cows, goats and sheep, is low in concentration during mid-lactation, but accumulates during milk stasis, consistent with a role in involution.