Milk is a highly specialized, complex nutrient system developed by mammalian evolution to promote postnatal growth. In contrast to feeding artificial infant formula, only human milk allows appropriate metabolic programming and protects against diseases of civilization in later life [
1]. However, continued consumption of cow´s milk and dairy products during adolescence and adulthood is an evolutionarily novel behavior that may have long-term adverse effects on human health [
2]. It is the intention of this review article to unravel milk´s functionality as a signaling system of evolution. The mechanisms of milk signaling presented here have been elucidated by translational research of the endocrine effects of cow´s milk consumption as well as individual protein components of bovine milk (whey protein and casein) on human subjects. The crucial function of milk of all mammals is to promote postnatal growth and to assure appropriate species-specific postnatal metabolic programming. On the molecular level, cell growth, cell proliferation, protein- and lipid synthesis, anabolic metabolic processes, and inhibition of autophagy are mediated by the nutrient-sensitive kinase
mechanistic target of rapamycin complex 1 (mTORC1) [
3‐
5]. mTORC1 is activated by branched-chain amino acids, especially leucine, the most abundant amino acid of whey proteins, growth factors like insulin and insulin-like growth factor-1 (IGF-1), and sufficient cellular energy sensed by AMP-activated kinase (AMPK) [
3,
5,
6]. Cow´s milk (subsequently termed “milk”) appears to promote mTORC1 signaling by providing amino acids that function as endocrine messengers to increase IGF-1 and insulin secretion as well as by milk-derived exosomal regulatory microRNAs (miRs), especially miR-21, which attenuates the inhibitory effects of various tumor suppressor proteins like phosphatase and tensin homolog (PTEN), Sprouty 1 and 2 and programmed cell death 4 (PDCD4) on mTORC1-signaling.
Potential role of milk-derived miRs in metabolic regulation
There is accumulating evidence that miRs are highly connected nodes in regulatory networks underlying adipogenesis and adipose dysfunction in obesity [
100]. Kim et al. [
101] demonstrated that miR-21 regulates adipogenic differentiation in mesenchymal stem cells derived from human adipose tissue. The same group observed a correlation between miR-21 levels and adipocyte number in white adipose tissue (WAT) of high fat-diet induced obese mice [
102]. The later study suggests that miR-21 may control the proliferation of adipocyte precursors [
102]. There is further compelling evidence that adipogenesis depends on mTORC1 signaling [
103]. As miR-21 reduces the expression of various tumor suppressor genes of the mTORC1 signaling pathway, milk-derived exosomal miR-21 may promote adipogenesis. In fact, genetic deletion of S6K1 in mice (S6K1
−/− mice), the downstream target of mTORC1, inhibited the transformation of mesenchymal stem cells into adipocytes and reduced the total number and size of adipocytes [
104]. Ectopic expression of miR-103 in preadipocytes accelerated adipogenesis as measured by both the up-regulation of many adipogenesis markers and by an increase in triglyceride accumulation at an early stage of adipogenesis [
105]. miR-103 is a component of cow´s milk [
51] (Table
1).
Table 1
Selected milk-derived miRs and their potential impact on metabolism
miR-21 | 50 | miR-21: Inhibition of various tumor suppressor gene mRNAs (PTEN, Spouty1, Sprouty2, PDCD4) | |
Increased adipogenesis | |
miR-29a | 54 | miR-29a: Down-regulation of Insig-1 with increased lipogenesis and adipocyte differentiation | |
|
miR-29b | 51 | miR-29b: Reduction of BCKD and reduced BCAA catabolism | |
|
miR-103 | 51 | miR-103: Increased adipogenesis | |
miR-155 | 51 | miR-155: Reduction of brown adipose tissue and thermogenesis | |
Let-7a, b, c, f | 51 | Let-7: overexpression results in insulin resistance and disturbed glucose homeostasis | |
Insulin-induced gene 1 (insig-1) mRNA is a validated target of miR-29a [
106], a further miR detected in cow´s milk [
54]. Insig-1 binds sterol regulatory element-binding protein (SREBP) cleavage-activating protein in the endoplasmic reticulum, thereby blocking proteolytic processing required for SREBP activation [
107]. Insig-1 restricts lipogenesis in mature adipocytes and inhibits differentiation in preadipocytes [
108]. Inactivation of insig-1 mRNA by milk-derived miR-29a may thus be another potential mechanisms by which milk consumption my promote adipogensis and BMI increase as observed in children, adolescents and adults [
81‐
83,
91‐
93].
Remarkably, cow´s milk contains substantial amounts of miR-155 [
51]. The target of miR-155 is the adipogenic transcription factor
CCAAT/
enhancer-
binding protein β (C/EBPβ). Overexpression of miR-155 in mice has been shown to reduce brown adipose tissue (BAT) mass [
109]. Thus, milk miR-155 intake may attenuate thermogenesis of BAT, an unfavorable condition promoting lipid and energy storage in WAT promoting obesity. However, attenuation of BAT thermogenesis may be a developmentally appropriate postnatal event.
Elevated plasma BCAAs have been associated with reduced cellular BCAA utilization and/or incomplete BCAA oxidation [
110]. The mitochondrial BCAA oxidation checkpoint, which commits BCAAs to degradation is the branched-chain α-ketoacid dehydrogenase (BCKD) complex. BCKD activity is reduced in WAT of obese individuals [
110]. miR-29b is targeted to the mRNA for the dihydrolipoamide branched chain acyltransferase component of BCKD and prevents translation when bound [
111]. Thus, miR-29b inhibits the pathway of BCAA catabolism. Intriguingly, miR-29b is a major miR of bovine milk [
51]. Milk´s miR-29b may thus function to maintain high plasma BCAAs levels important for mTORC1-dependent growth and appropriate amino acid uptake for protein de novo biosynthesis of functional and structural proteins required for postnatal growth. High plasma levels of milk-derived BCAAs together with milk-miR-29b-mediated inhibition of BCAA catabolism may thus over-activate mTORC1-S6K1 signaling. Insulin resistance during mTORC1-driven phases of growth may represent a negative feedback mechanism, induced by up-regulated S6K1, which via IRS-1 phosphorylation inhibits downstream insulin signaling [
112]. These metabolic events may explain the link between elevated BCAA plasma levels and insulin resistance [
113]. In fact, Hoppe
et al. demonstrated that milk consumption but not meat (which misses miR-29b) induced insulin resistance in Danish prepubertal boys [
17]. Thus, milk, the starter kit of mammalian evolution, appears to execute a highly sophisticated metabolic program, which orchestrates BCAA-, miR-21- and miR-29b-driven mTORC1-mediated protein biosynthesis. In synergy with activated protein biosynthesis, the mTORC1-driver milk should promote lipid synthesis too. Remarkably, it has recently been recognized that activated mTORC1 induces the expression of key transcription factors of lipogenesis, sterol response element binding protein-1 (SREBP-1) [
4,
114‐
116], and peroxisome proliferator-activated receptor-γ (PPARγ) [
117,
118]. Furthermore, activated mTORC1 has been shown to suppress lipolysis, stimulates lipogenesis and promotes fat storage [
119]. Thus, milk appears to provide an endocrine signaling environment for increased mTORC1-driven lipogenesis and fat storage as well as miR-155-induced suppression of BAT differentiation resulting in BMI elevations and fat deposition in WAT [
81‐
83,
91‐
93].
Milk contains substantial amounts of let7a, let7b, let7c and let7f [
50]. There is accumulating evidence that the Lin28/let-7 axis regulates glucose metabolism [
120]. Muscle specific loss of Lin28a and overexpression of let-7 resulted in insulin resistance and impaired glucose tolerance in mice [
120]. Intriguingly, let-7 targets are enriched for genes that contain SNPs associated with type 2 diabetes and fasting glucose in human genome-wide association studies [
120]. Lin28, a developmentally regulated RNA-binding protein, selectively blocks the processing of pri-let-7 miRs [
121]. Notably, the restoration of the Lin28 protein blocked let-7 expression and restored glucose metabolism in adipose-derived stem cells derived from obese tissues [
122]. The most interesting miRs in inflammatory microvesicles in association with metabolic and cardiovascular diseases recently reported are the let-7 family, miR17/92 family, miR-21, miR-29, miR-126, miR-133, miR-146 and miR-155 [
123]. Notably, there is a substantial overlap with miRs derived from milk exosomes (Table
1).