Biophysical assessment of aquaporin-9 as principal facilitative pathway in mouse liver import of glucogenetic glycerol

Background information: Lipolytic glycerol, released from adipocytes, flows through the bloodstream to the liver, where its utilisation in supplying hepatocyte gluconeogenesis is rate-limited by the permeation step. An aquaglyceroporin expressed in hepatocytes, aquaporin-9 (AQP9), has been often linked to liver uptake of glycerol. However, the truthfulness of this postulation and the potential existence of additional pathways of glycerol import by hepatocytes have never been assessed directly. Here, we define the identity and extent of liver glycerol transport and evaluate the correlation between hepatic AQP9 expression and glycerol permeability (P(gly) ) in AQP9(+/+) wild-type mice in different nutritional states and circulating insulin levels. The liver P(gly) of AQP9 null mice is also assessed.

Results: By stopped-flow light scattering, facilitated diffusion of glycerol into hepatocytes was indicated by the low Arrhenius activation energy (3.5 kcal/mol) and strong inhibition by phloretin, an AQP9 blocker, that characterised the transport. Although fasting markedly increased hepatic AQP9, a straight parallelism was seen both in quantitative and time-space terms between P(gly) and AQP9 protein in AQP9(+/+) mice kept in fed or fasted/refed states. In line with these findings, the highest P(gly) (P(gly) ≈ 14.0 × 10(-6) cm/s at 20°C) at 18-h fasting coincided with the highest percent of phloretin inhibition (63%). Besides being markedly lower than that in AQP9(+/+) mice, the liver P(gly) of the AQP9 null mice did not increase during fasting. Reverse-transcription PCR analysis showed lack of compensation by AQP3 and AQP7, the other known murine glycerol facilitators, in AQP9 null mice.

Conclusions: Overall, these results experimentally prove major functional significance for AQP9 in maximising liver glycerol import during states requiring increased glucose production. If any, alternative facilitated pathways would be of minor importance in transporting glucogenetic glycerol into hepatocytes during starvation. Refining the understanding of liver AQP9 in metabolic and energy homeostasis may reveal helpful for therapeutic purposes.