Regulation of mammary stearoyl-CoA desaturase and the effects on milk fat composition in lactating mice : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Human Nutrition at Massey University, Palmerston North, New Zealand
The research described in this thesis tested, in the lactating mammary gland of female Swiss mice, a model for the control of lipogenesis developed for the liver of male mice. In male mice feeding a fat free diet, or a diet containing 0.5% w/w clofibrate, induces hepatic stearoyl CoA desaturase (SCD) mRNA transcription, which increases SCD activity and the amount of oleate incorporated into membrane phospholipids and the triacylglycerols of liver lipoprotein. In a preliminary trial, SCD mRNA in liver and mammary gland and fatty acids (FA) in the liver, mammary gland and milk fat were measured in three groups (n=3) of lactating mice fed either a control diet or the control diet with added clofibrate (0.05% w/w) or a fat free diet. Concentrations of SCD mRNA in liver and mammary gland and proportions of individual FA in liver, mammary gland and milk were not significantly different between the control and clofibrate groups. There were, however, positive linear correlations between liver SCD mRNA and hepatic 16:1/16:0 FA ratio (r =0.495, P<0.05), 18:1/18:0 FA ratio (r =0.520, P<0.05) and milk 16:1/16:0 FA ratio (r =0.552, P<0.05). In a second trial, four groups (n=6) of lactating Swiss mice were used to compare the effect of clofibrate ingestion (control diet v. diet containing clofibrate (0.05% w/w)) and clofibrate injection (olive oil vehicle subcutaneously v. 15 mg clofibrate/100g LW in olive oil subcutaneously) for 7 days. Mammary SCD mRNA, but not liver SCD mRNA, was induced by ingested and injected clofibrate (P<0.05), compared to their control treatments. FA composition of liver, mammary gland and milk was not affected by either treatment. Correlations between mammary SCD mRNA and mammary tissue 16:1/16:0 FA ratio (r =0.660, P<0.05), and 18:1/18:0 FA ratio (r =0.59, P<0.05) were significant in the group ingesting clofibrate. Liver SCD mRNA for both treatments and mammary SCD mRNA for the injected group were not significantly correlated with FA composition. It was concluded that female mice that are lactating may be less sensitive to the effects of clofibrate than male mice. In the preliminary trial, SCD mRNA transcription was induced (P<0.05) 2.1 fold in the mammary gland and 5.3 fold in the liver (P<0.05) of the mice fed the fat free diet over the control treatment. Induction of transcription was not transmitted to an effect on the FA composition of the liver, mammary gland or milk. However, there was a trend (P<0.10) for milk 16:1/16:0 FA ratio to be increased in the fat free treatment over the control treatment. Liver SCD mRNA was correlated (r =0.552, P<0.05) with milk 16:1/16:0 FA ratio, liver 18:1/18:0 FA ratio (r =0.520, P<0.05) and liver 16:1/16:0 FA ratio (r =0.61, P<0.05). In a third trial, lactating Swiss mice were allocated to three groups (6 mice/group) which were either fed a fat free diet, a safflower oil diet (25% w/w) or an olive oil diet (25% w/w) over a 7 day period. The safflower oil diet was included because polyunsaturated FA inhibit SCD activity in the liver while a fat free diet stimulates its activity. The olive oil treatment was included as a reference point with which to compare the responses to the other treatments. In the event, the intake of polyunsaturated FA by the mice on this diet may have been sufficient to inhibit the induction of SCD mRNA so that only relative responses between the various diets could be considered. Mammary SCD and liver mRNA transcription levels were greater in the fat free treatment (P<0.05), compared with the olive and safflower oil treatments. Mammary SCD enzyme activity was not significantly affected by treatment., The fat free treatment increased liver 16:1/16:0 FA ratio and 18:1/18:0 FA ratio (P=0.05) and the mammary 16:1/16:0 FA ratio (P<0.05) but not the 18:1/18:0 FA ratio compared with the other two diets. The olive oil treatment increased palmitoleate and oleate concentration in the liver, mammary gland and milk (P<0.05). The increase in the concentration of oleate reflected the composition of the olive oil in the diet. Similarly, dietary intake influenced the significantly greater proportion of linoleate in the milk of the safflower treatment (P<0.05). The oleate concentration, 16:1/16:0 and 18:1/18:0 FA ratios were greater (P<0.05) in the milk of the group fed the fat free diet than those in the milk of the group on the safflower oil diet. An accumulation of stearate (P<0.10), indicating SCD inhibition, was present in the milk of the safflower oil treatment compared to the fat free treatment. The proportion of saturated fatty acids from octanoate to palmitate was greater in the milk from the mice on the fat free diet compared with those on the safflower oil treatment. The proportions of long chain fatty acids of molecular weight greater than linoleate were higher in the milk from the mice fed the diets containing the oils.