Casein α s1 Is Expressed by Human Monocytes and Upregulates the Production of GM-CSF via p38 MAPK

Abstract

Caseins are major constituents of mammalian milks that are thought to be exclusively expressed in mammary glands and to function primarily as a protein source, as well as to ameliorate intestinal calcium uptake. In addition, proinflammatory and immunomodulatory properties have been reported for bovine caseins. Our aim was to investigate whether human casein α s1 (CSN1S1) is expressed outside the mammary gland and possesses immunomodulatory functions in humans as well. For this purpose, CSN1S1 mRNA was detected in primary human monocytes and CD4+ and CD8+ T cells, but not in CD19+ B cells. CSN1S1 protein was traceable in supernatants of cultured primary human CD14+ monocytes by ELISA. Similarly, CSN1S1 mRNA and protein were detected in the human monocytic cell lines HL60, U937, and THP1 but not in Mono Mac 6 cells. Moreover, permeabilized human monocytes and HL60 cells could be stained by immunofluorescence, indicating intracellular expression. Recombinant human CSN1S1 was bound to the surface of Mono Mac 6 cells and upregulated the expression of GM-CSF mRNA in primary human monocytes and Mono Mac 6 cells in a time- and concentration-dependent manner. A similar increase in GM-CSF protein was found in the culture supernatants. CSN1S1-dependent upregulation of GM-CSF was specifically blocked by the addition of the p38 MAPK inhibitor ML3403. Our results indicated that human CSN1S1 may possess an immunomodulatory role beyond its nutritional function in milk. It is expressed in human monocytes and stimulates the expression of the proinflammatory cytokine GM-CSF.

Exome-chip association analysis reveals an Asian-specific missense variant in PAX4 associated with type 2 diabetes in Chinese individuals

“…Our finding that a variant of FGFR1 is associated with type 2 diabetes is supportive of such a possibility. Casein α S1 (CSN1S1) is a member of the casein family that has been shown to possess proinflammatory properties, such as the upregulation of IL-1β [45]. Data from animal studies and clinical trials is suggestive of a causative role for IL-1β in the loss of beta cell mass in type 2 diabetes [46].…”

Autoantibodies to αS1-Casein Are Induced by Breast-Feeding

“…This might suggest a more recent antigenic stimulus. It has recently been shown, that human CSN1S1 is expressed in human monocytes and induces pro-inflammatory cytokines [15]. Such ectopic expression of CSN1S1 could potentially be an explanation for the IgM mediated reaction on human CSN1S1 observed in the corresponding SD ELISA for a few sera.…”

“…Human CSN1S1 is expressed in monocytes and stimulates the expression of proinflammatory cytokines e.g. GM-CSF [15].…”

Human casein alpha s1 (CSN1S1) skews in vitro differentiation of monocytes towards macrophages

“…30 μg/ml polymyxin (Sigma-Aldrich, Munich, Germany) was added to experiments to exclude any LPS effects (Sigma-Aldrich) [13], except for the assessment of cellular morphology of living cells, where 200 ng/ml LPS was added in a control experiment. Recombinant human casein alpha S1 (CSN1S1) (Calbiochem, Darmstadt, Germany) was added to cultured cells in indicated concentrations for 24 or 120 h. The following compounds were used to induce in vitro differentiation of monocytes as control experiments: M-CSF (R&D Systems, Wiesbaden, Germany) 50 ng/ml, GM-CSF 50 ng/ml, IL-4 20 ng/ml, IFNγ 10 ng/ml (all CellGenix, Freiburg, Germany).…”

“…Furthermore, independent studies reported overexpression of CSN1S1 in synovial tissue of patients with osteoarthritis and rheumatoid arthritis [10-12]. Consequently, a potential function of CSN1S1 was further characterized by the finding of proinflammatory effects on monocytic cells, like for instance increased expression of IL-1β [13]. Thus, the concept of CSN1S1 as a multifunctional protein with both nutritional and immunomodulatory functions is evolving.…”