4. protein kinase C-beta–66 kDa isoform of mammalian ShcA gene mitochondrial axis, mitophagy, and
adipose dysfunction
Nutrient excess is known to lead to mitochondrial dysfunction with
consequential effects on lipid and glucose metabolism. Mitochondrial
dysfunction in adipose tissue has been reported in patientswith obesity
and/or type 2 diabetes [57,58]. Mitochondrial DNA copy number, and
mitochondrial mass and activity are decreased in the White adipose tissue of mouse
models of obesity [59,60]. In humans, there is a correlation between
downregulation of the expression and activity of the components of
oxidative phosphorylation in White adipose tissue and obesity [61,62]. Dysfunctional
mitochondriamay lead to oxidative stress through excessive production
of reactive oxygen species (ROS). Indeed, oxidative stress is associated
with adiposity and lipid peroxide levels, and hydrogen peroxide generation
is elevated in adipose tissue, but not in skeletalmuscle or the aorta
[63]. Considering that mitochondria are the most important source of
intracellular ROS, it is possible that the adipose tissue in obese individuals
represents a major source of ROS. In human studies, the extent of
fat accumulation has been correlated with various markers of systemic
oxidative stress [64].
A significant conceptual advance in our understanding of the molecular
mechanism by which protein kinase C-beta signaling is involved in obesity came
from an earlier study linking oxidative stress, protein kinase C-beta activation, phosphorylation
of the redox enzyme 66 kDa isoform of mammalian ShcA gene; and its translocation into the
mitochondrial intermembrane space, and mitochondrial lifespan [65].
Activation of protein kinase C-beta by oxidative stress was shown to be required for
phosphorylation of Ser36 on 66 kDa isoform of mammalian ShcA gene; and its subsequent translocation to
mitochondria. Consistent with this signaling pathway, like protein kinase C-beta −/−
mice, 66 kDa isoform of mammalian ShcA gene; −/− mice are also characterized by reduced triglyceride
accumulation in adipocytes, increased metabolism, resistance to dietinduced
obesity, and reduced insulin resistance [66–68]. Based on
these findings, the following pathway has emerged: during oxidative
stress,protein kinase C-beta is activated and induces p66shc phosphorylation, thus
allowing 66 kDa isoform of mammalian ShcA gene; to be recognized by Pin1 and isomerized for import
into mitochondria after dephosphorylation by protein phosphatase-
2A. The p66shc protein exhibits oxidoreductase activity, generating