Diurnal rhythmicity in glucose uptake is mediated by temporal periodicity in the expression of the sodium-glucose cotransporter (SGLT1)

Abstract

Background: Intestinal transport exhibits distinct diurnal rhythmicity. Understanding the mechanisms behind this may reveal new therapeutic strategies to modulate intestinal function in disease states such as diabetes and obesity, as well as short bowel syndrome. Although diurnal rhythms have been amply documented for several intestinal transporters, the complexity of transepithelial transport has precluded definitive attribution of rhythmicity in glucose uptake to a single transporter. To address this gap, we assessed temporal changes in glucose transport mediated by the Na(+)/glucose cotransporter SGLT1.

Methods: SGLT1 expression was assessed at 4 times during the day: ZT3, ZT9, ZT15, and ZT21 (ZT, Zeitgeber time; lights on at ZT0; n = 8/ time). SGLT1 activity, which is defined as glucose uptake sensitive to the specific SGLT1 inhibitor phloridzin, was measured in everted intestinal sleeves. Changes in Sglt1 expression were assessed by real-time polymerase chain reaction (PCR) and immunoblotting.

Results: Glucose uptake was significantly higher at ZT15 in jejunum (P < 0.05 vs ZT3). Phloridzin significantly reduced glucose uptake and completely abolished its rhythmicity. Sglt1 mRNA levels were significantly greater at ZT9 and ZT15 in jejunum and ileum, respectively (P < 0.05 vs ZT3), whereas SGLT1 protein levels were significantly greater at ZT15 in jejunum (P < 0.05 vs ZT3).

Conclusions: Our results definitively link diurnal changes in intestinal glucose uptake capacity to changes in both SGLT1 mRNA and protein. These findings suggest that modulation of transporter expression would enhance intestinal function and provide an impetus to elucidate the mechanisms that underlie diurnal rhythmicity in transcription. Modulation of intestinal function would benefit the management of malnutrition as well as diabetes and obesity.

Fig. 1. Diurnal variation in radiolabeled glucose uptake by the everted sleeve method

¹⁴C-labelled glucose uptake was significantly higher at ZT15 compared with ZT3 (*p < 0.05 uptake in the absence of phloridzin at ZT15 vs. ZT3, n=8). The addition of phloridzin, a specific inhibitor of SGLT1, abolished this difference in uptake and significantly reduced ¹⁴C-labelled glucose uptake across all timepoints compared to incubation without phloridzin (^†p<0.005, n=8). This shows that the observed rhythmicity in sugar absorption was caused by alterations in the activity of SGLT1. There is no significant difference between the 4 time points for post-phloridzin glucose uptake.

Fig. 2. Diurnal rhythmicity of Sglt1 mRNA expression in jejunal and ileal mucosa

mRNA levels of Sglt1 were significantly higher at ZT9 in jejunum compared to ZT3 and ZT21. mRNA levels in ileum demonstrated a blunted but more sustained elevation at ZT9 and ZT15, both significantly higher than ZT3 and ZT21( *p < 0.01 compared with ZT3 and ZT21, n=6).

Fig. 3

Fig. 3a: Diurnal variation in SGLT1 protein expression as shown by Western blotting. Intensity of each band as measured on densitometry was indexed to the first band on each gel and normalized to the housekeeping protein actin as an internal control. A representative blot is shown below. Fig. 3b: Diurnal variation in jejunal SGLT1 protein expression. SGLT1 levels were significantly higher at ZT15 compared to ZT3 (*p < 0.05 compared with ZT3, n=6).

Fig. 3

Fig. 3a: Diurnal variation in SGLT1 protein expression as shown by Western blotting. Intensity of each band as measured on densitometry was indexed to the first band on each gel and normalized to the housekeeping protein actin as an internal control. A representative blot is shown below. Fig. 3b: Diurnal variation in jejunal SGLT1 protein expression. SGLT1 levels were significantly higher at ZT15 compared to ZT3 (*p < 0.05 compared with ZT3, n=6).