The contribution of hormone sensitive lipase to adipose tissue lipolysis and its regulation by insulin in periparturient dairy cows

Abstract

Hormone sensitive lipase (HSL) activation is part of the metabolic adaptations to the negative energy balance common to the mammalian periparturient period. This study determined HSL contribution to adipose tissue (AT) lipolysis and how insulin regulates its activity in periparturient dairy cows. Subcutaneous AT (SCAT) samples were collected at 11 d prepartum (dry) and 11 (fresh) and 24 d (lactation) postpartum. Basal and stimulated lipolysis (ISO) responses were determined using explant cultures. HSL contribution to lipolysis was assessed using an HSL inhibitor (CAY). Basal lipolysis was higher in SCAT at dry compared with fresh. CAY inhibited basal lipolysis negligibly at dry, but at fresh and lactation it reduced basal lipolysis by 36.1 ± 4.51% and 43.1 ± 4.83%, respectively. Insulin inhibited lipolysis more pronouncedly in dry compared to fresh. Results demonstrate that HSL contribution to basal lipolysis is negligible prepartum. However, HSL is a major driver of SCAT lipolytic responses postpartum. Lower basal lipolysis postpartum suggests that reduced lipogenesis is an important contributor to fatty acid release from SCAT. Loss of adipocyte sensitivity to the antilipolytic action of insulin develops in the early lactation period and supports a state of insulin resistance in AT of cows during the first month postpartum.

Figure 1

Serum free fatty acid (FFA), β-hydroxybutyrate (BHB) and insulin concentrations. Least squares means of FFA concentration (mEq/L) (A), BHB concentration (mmol/L) (B) and insulin concentration (µg/L) (C) during the periparturient period. Error bars represent the SEM. Time-points with different letters differ significantly (abcd, P < 0.05). P-values for the effect of wk relative to calving on the different metabolites are derived from the linear mixed effect model.

Figure 2

Adipocyte volume and diameter and number of adipocytes per gram adipose tissue. Least squares means of adipocyte diameter (µm) (A), adipocyte volume (pL) (B) and number of adipocytes per gram AT (×10⁶ adipocytes per g AT) (C) in the SCAT samples at dry (−11 ± 1 d), fresh (+11 ± 0.2 d) and lactation (+24 ± 0.4 d relative to calving) from Holstein dairy cows. Error bars represent the SEM. Time-points with different letters differ significantly (abc, P < 0.05). P-values for the effect of period on the different variables are derived from the linear mixed effect model.

Figure 3

Glycerol release and the effect of adipocyte volume during inhibition of HSL activity. Least squares means of basal glycerol release (nmol/10⁶ adipocytes per 3 h) (A), CAY inhibited basal glycerol release (% decrease of basal glycerol release) (B), isoproterenol (ISO) stimulated glycerol release (% increase of basal glycerol release) (C), CAY inhibited ISO stimulated glycerol release (% decrease of ISO stimulated glycerol release) (D) and insulin (0.2 and 1 µg/L) inhibited ISO stimulated glycerol release (% decrease of ISO stimulated glycerol release) (E,F). Effect of adipocyte volume on basal glycerol release (G) and CAY inhibited ISO stimulated glycerol release (H) in SCAT explants from Holstein dairy cows at dry (−11 ± 1 d), fresh (+11 ± 0.2 d) and lactation (+24 ± 0.4 d relative to calving). Error bars represent the SEM. Time-points with different letters differ significantly (ab, P < 0.05). P-values for the effect of period and adipocyte size (volume in pL) on the different variables are derived from the linear mixed effect model.

Figure 4

Effect of adipocyte volume on the mRNA expression of DGAT2 (A) and GLUT4 (B) in SCAT samples. mRNA expression of the genes is presented as relative mRNA abundance after normalization with the reference genes (EIF3K, RPLPO, RPS9).

Figure 5

HSL and PKA activity in periparturient cows. Least squares means of the ratio of pHSL(Ser563):HSL (A) and pAKT:AKT (C) in explants cultured in basal incubation condition from SCAT samples taken during the dry (−11 ± 1 d), fresh (+11 ± 0.2 d) and lactation (+24 ± 0.4 d relative to calving) period from Holstein dairy cows. Change in ratio of pHSL(Ser563):HSL in explants incubated with CAY compared with basal incubation conditions (B) and change in ratio of pAKT:AKT in explants incubated with insulin (1 µg/L) compared with basal incubation conditions (D). Error bars represent the SEM. Time-points with different letters differ significantly (ab, P < 0.05). P-values for the effect of period on the ratio of protein phosphorylation and the change in ratio of protein phosphorylation are derived from the linear mixed effect model.