4.5 PHA Monomers as Drugs
Sodium salts of d-3-hydroxybutyrate (d-3HB), dl-3-hydroxybutyrate (dl-3HB), and
3HBME are derivatives of 3HB, a body ketone that is produced in vivo in animals,
including human. d-3HB is the most common degradation product of microbial
PHA that have been investigated for tissue engineering applications. 3HB and its
derivatives (collectively called 3HB derivatives) were reported to have an effect on
cell apoptosis and the cytosolic Ca2+ concentration of mouse glial cells (Xiao et al.
2007). The percentage of cells undergoing apoptosis decreased significantly in the
presence of 3HB and its derivatives, as evidenced by flow cytometry. The in vitro
study on the cytosolic Ca2+ concentration demonstrated that 3HB derivatives elevated
dramatically the cytosolic Ca2+ concentration. Both the extracellular and the
intracellular Ca2+ contributed as sources of such Ca2+ concentration elevation. The effect
of 3HB derivatives on cytosolic Ca2+ concentration could be reduced by nitredipine,
an L-type voltage-dependent calcium channel antagonist. In comparison, 3HBME
worked more efficiently than d-3HB and dl-3HB did as 3HBME is most efficient
in permeation into the cells. All the results indicated that 3HB derivatives had an
inhibitory effect on cell apoptosis which is mediated by signaling pathways related
to the elevation of cytosolic Ca2+ concentration. This positive effect helps explain
the biocompatibility observed for PHA; it also points to the possibility of 3HB
derivatives regardless of chirality becoming effective neural protective agents.
Learning and memory require energy-demanding cellular processes and can be
enhanced when the brain is supplemented with metabolic substrates. It was found that
neuroglial cell metabolic activity was significantly elevated when neuroglial cells
were cultured in the presence of the PHB degradation product 3HB and derivatives.
We demonstrated that the receptor for 3HB, namely, protein upregulated in macrophages
by interferon-g (PUMA-G), was expressed in brain and upregulated in mice
treated with 3HBME. We also affirmed increased expression of connexin 36
protein and phosphorylated extracellular-signal-regulated kinase 2 (ERK2) in brain
tissues following 3HBME treatment, although these differences were not statistically
significant. Mice treated with 3HBME performed significantly (p