P237
CD36 IS A NOVEL HIGH CAPACITY RECEPTOR FOR ALBUMIN IN PROXIMAL
TUBULAR CELLS
Baines, R1,2, Chana, R2, Febbraio, M3, Brunskill, N1,2
1John Walls Renal Unit, Leicester General Hospital, 2Department ofInfection,
Immunity and Inflammation, University of Leicester, United Kingdom. 3Department of
Cell Biology, Cleveland Clinic, Cleveland, Ohio, USA
Filtered proteins are efficiently retrieved from the proximal tubule by the megalin/cubulin complex.
However megalin deficient proximal tubular cells (PTC) may also demonstrate protein uptake. CD36
is an 88 kDa member of the class B scavenger receptor family that is widely expressed in different
cell types, including PTC. PTC expression of CD36 is upregulated in proteinuric conditions in vivo
and in vitro. We investigated whether CD36 functioned as a receptor for albumin.
Opossum kidney (OK) cells were stably transfected with CD36 and several over-expressing clones
were established (OKCD36). Wild type OK (OKWT) were used as controls. Albumin interaction with
cells was measured by incubation with FITC-Alb. To inhibit potential binding of albumin to CD36
anti-CD36 antibodies (ab) or the specific antagonist sulphosuccinimidyl-oleate (SSO) were used.
Protein:Creatinine ratios were measured in CD36 knockout mice and CD36 expression in human renal
biopsy specimens assessed by immunohistochemistry.
Immunoblotting of lysates from transfected cell lines confirmed CD36 to be overexpressed, confocal
imaging showed it to be predominantly associated with the apical cell membrane. Uptake and binding
of FITC-Alb to both cell lines was concentration dependent and saturable. OKCD36 cells showed
significantly greater albumin binding and uptake than OKWT in proportion to the degree of CD36
overexpression. Both cell lines demonstrated 2 binding sites, characterised by high affinity/low
capacity (HA) and low affinity/high capacity (LA). In OKCD36 the kinetics of the HA site were
changed indicating a different uptake mechanism. The affinity (Kd) of this receptor was 5 fold that of
control and capacity (Vmax) increased x8.24. Uptake by LA, and binding to HA and LA were
increased in OKCD36 which was the net result of an increased number of binding sites of a lower
affinity. Unlabelled albumin blocked FITC-alb binding to both cell lines. Co-localisation of CD36 and
FITC-Alb was observed at the cell membrane but CD36 did not accompany FITC-Alb through its
intracellular trafficking. Incubation with 250 M SSO and anti-CD36 ab reduced FITC-Alb uptake in
CD36OK to OKWT levels. Protein:Creatinine ratios were 1.45±0.34 mg/mg in CD36 knockout mice
(n=8) and 0.95±0.07 in wild type controls (n=4) (p≤0.05). Using human renal biopsy material CD36
expression was increased in a number of proteinuric conditions.
CD36 may function as a receptor for albumin in PTC where it facilitates endocytosis. The function of
CD36 in PTC warrants further investigation.