Biophysical assessment of AQP9 as principal facilitative pathway in mouse liver import of glucogenetic glycerol

GIUSEPPE CALAMITA; PATRICIA GENA; DOMENICO FERRI; ROSITO A; ROJEK A; NIELSEN S; MARINELLI RA; FRÜHBECK G; MARIA SVELTO

BIOLOGY OF THE CELL

PORTLAND PRESS LTD

Año: 2011

0248-4900

BACKGROUND INFORMATION: Lipolytic glycerol, released from adipocytes, flows through bloodstream to the liver where its utilization in supplying hepatocyte gluconeogenesis is rate-limited by the permeation step. An aquaglyceroporin expressed in hepatocytes, AQP9, has been often linked to liver uptake of glycerol. However, the truthfulness of this postulation as well as 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 in AQP9(+/+) wild type mice in different nutritional states and circulating insulin levels. The liver glycerol permeability 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/mole) and strong inhibition by phloretin, an AQP9 blocker, that characterized the transport. While fasting markedly increased hepatic AQP9 a straight parallelism was seen both in quantitative and time-space terms between glycerol permeability and AQP9 protein in AQP9(+/+) mice kept in fed or fasted/refed states. In line with these findings, the highest glycerol permeability (P(gly) ≈ 14.0 x 10(-6) cm/s at 20 C), at 18-hours fasting, coincided with the highest percent of phloretin inhibition (63%). Besides being markedly lower than in AQP9(+/+) mice the liver P(gly) of the AQP9 null mice did not increase during fasting. RT-PCR analysis showed lack of compensation by AQP3 and AQP7, the other known murine glycerol facilitators, in AQP9 null mice. CONCLUSIONS: Overall, these results prove experimentally major functional significance for AQP9 in maximizing 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.