Bl. Riser et al., TGF-beta receptor expression and binding in rat mesangial cells: Modulation by glucose and cyclic mechanical strain, KIDNEY INT, 56(2), 1999, pp. 428-439
Background. Transforming growth factor-beta (TGF-beta) is a causal factor i
n experimental glomerulosclerosis, and it mediates the increased extracellu
lar matrix (ECM) accumulation that occurs in cultured mesangial cells (MCs)
exposed to high glucose concentrations and cyclic mechanical strain. This
change is associated with increased levels of TGF-beta, but may also involv
e alterations in receptor expression and binding.
Methods. Rat MCs cultured in media containing either 8 or 35 mM glucose wer
e seeded into culture plates with elastin-coated flexible bottoms. Thereaft
er, they were subjected to cyclic stretch or static conditions and then exa
mined for I-125-TGF-beta 1 binding and expression of TGF-beta receptors at
the gene and protein levels.
Results. Kinetic studies showed that MCs bound TGF-beta 1 in a time- and co
ncentration-dependent manner, expressing 6800 high-affinity receptors per c
ell, with an apparent dissociation constant (K-d) Of 15.4 PM, while cross-l
inking analysis identified three TGF-beta receptors (beta R) corresponding
to beta RI, beta RII, and beta RIII of 54, 73, and 200 kDa, respectively. I
mmunocytochemical studies of beta RI and beta RII protein revealed MC expre
ssion in a homogeneous, punctate distribution, whereas Northern analysis de
monstrated the presence of the corresponding mRNAs. Exposure to cyclic stre
tching significantly increased (10%) the overall number of TGF-beta recepto
rs, whereas ligands associated with beta Rs I, II, and III also increased (
25 to 50%). The finding of increased (30 to 40%) beta RI and beta RII trans
cript levels and immunoreactive protein (163 and 59%, respectively) in the
absence of significant changes in the apparent K-d indicated that stretch-i
nduced binding was the result of increased receptor synthesis and expressio
n and not due to a change in binding affinity. In a similar, but more drama
tic fashion, exposure to high glucose also elevated (50%) the receptor numb
er, as well as the amount of ligands associated with beta Rs I, II, and III
(100 to 250%). This same treatment also increased the levels of beta RI and
beta RII mRNA (30 to 40%) and the immunoreactive protein (82 and 82%, resp
ectively), without significantly altering the binding affinity of the recep
tor. A concerted or synergistic effect of both stimuli was not evidenced.
Conclusion. These results suggest that the modulation of TGF-beta receptors
may be an additional control point in mediating the glucose- and mechanica
l force-induced increase in ECM deposition by MCs.