Indiana University-Purdue University Indianapolis (IUPUI)Insulin resistance starts years before type 2 diabetes (T2D) diagnosis, even before
recognition of prediabetes. Mice on a high fat diet have a similar early onset of insulin
resistance, yet the mechanism remains unknown. Several studies have demonstrated that
skeletal muscle insulin resistance resulting from obesity or high fat feeding does not stem
from defects in proximal insulin signaling. Our lab discovered that excess plasma
membrane cholesterol impairs insulin action. Excess cholesterol in the plasma membrane
causes a loss of cortical actin filaments that are essential for glucose transporter GLUT4
regulation by insulin. Our cell studies further revealed that increased hexosamine
biosynthesis pathway (HBP) activity increases O-linked N-acetylglucosamine
modification of the transcription factor Sp1, leading to transcription of HMG-CoA
reductase (HMGR), the rate-limiting enzyme in cholesterol biosynthesis. Our central
hypothesis is that cholesterol accumulation mediated by HBP activity is an early
reversible mechanism of high-fat diet-induced insulin resistance. We performed a series
of studies and found that early high-fat feeding-induced insulin resistance is associated
with a buildup of cholesterol in skeletal muscle membranes (SMM). Akin to the
antidiabetic effect of caloric restriction, we found that high-fat diet removal fully
mitigated SMM cholesterol accumulation and insulin resistance. Furthermore, using the
cholesterol-binding agent methyl-β-cyclodextrin (MβCD), studies established causality
between excess SMM cholesterol and insulin resistance. To begin to assess the role of the HBP/Sp1 in contributing to de novo cholesterol biosynthesis, SMM accumulation, and
insulin resistance we treated high-fat fed mice with an Sp1 inhibitor, mithramycin. We
found that mithramycin prevented SMM cholesterol accumulation and insulin resistance.
This series of studies provide evidence that HBP/Sp1-mediated cholesterol accumulation
in SMM is a causal, early and reversible mechanism of whole body insulin resistance
