Insulin action in adipose tissue is crucial for whole-body
glucose homeostasis, with
insulin resistance being a major risk factor for
metabolic diseases such as
type 2 diabetes. Recent studies have proposed mitochondrial
oxidants as a unifying driver of adipose
insulin resistance, serving as a signal of nutrient excess. However, neither the substrates for nor sites of
oxidant production are known. Because
insulin stimulates
glucose utilization, we hypothesized that
glucose oxidation would fuel respiration, in turn generating mitochondrial
oxidants. This would impair
insulin action, limiting further
glucose uptake in a negative feedback loop of "
glucose-dependent"
insulin resistance. Using primary rat adipocytes and cultured 3T3-L1 adipocytes, we observed that
insulin increased respiration, but notably this occurred independently of
glucose supply. In contrast,
glucose was required for
insulin to increase mitochondrial
oxidants. Despite rising to similar levels as when treated with other agents that cause
insulin resistance,
glucose-dependent mitochondrial
oxidants failed to cause
insulin resistance. Subsequent studies revealed a temporal relationship whereby mitochondrial
oxidants needed to increase before the
insulin stimulus to induce
insulin resistance. Together, these data reveal that (a) adipocyte respiration is principally fueled from nonglucose sources; (b) there is a disconnect between respiration and oxidative stress, whereby mitochondrial
oxidant levels do not rise with increased respiration unless
glucose is present; and (c) mitochondrial oxidative stress must precede the
insulin stimulus to cause
insulin resistance, explaining why short-term,
insulin-dependent
glucose utilization does not promote
insulin resistance. These data provide additional clues to mechanistically link nutrient excess to adipose
insulin resistance.