Research
Interests:
When a coronary
artery is obstructed by either cholesterol
plaque or thrombus, the region served by that
artery loses its blood supply and, therefore,
its supply of oxygen and metabolite. Heart
muscle cells begin to die within 20 or 30
minutes. To restore coronary flow in these
patients thrombolytic drugs can be given to
dissolve the blood clot or catheter-based
techniques can be used to remove or compress
the plaque and reestablish luminal patency.
Unfortunately there is always some delay in
restoration of flow, and there is always
unavoidable death of heart muscle. Because
heart muscle cells cannot be regenerated, the
loss of contractile mass leaves the patient
with a permanently weakened heart which often
leads to heart failure, a major cause of
morbidity and death in these patients. The
goal of my research is discovery of an
intervention which will delay the rate of
cell death in such a patient so that more
muscle would survive the heart attack.
In 1986, it was shown that heart cells
could be made very resistant to death
following loss of blood flow if they were
first exposed to a brief period of blood flow
deprivation followed by reperfusion. Within
minutes of this cycle of ischemia/reperfusion
the heart actually adapted itself to much
better tolerate a subsequent, more prolonged
cessation of blood flow. If understood, this
process, called preconditioning, should
provide a key to designing a therapy which
could spare ischemic myocardium. My colleague
James M. Downey found that
the protection is primarily triggered by
adenosine which is released from cardiac
tissue soon after blood flow ceases.
Adenosine populates receptors on the heart
muscle cells which act to stimulate protein
kinase C (PKC). PKC modulates the function of
the cell's proteins by phosphorylating them.
Dr. Downey and I have spent several years
investigating possible signal transduction
pathways in the myocardial cell which might
be involved in this powerful preconditioning
phenomenon. We now know that any receptor in
the heart which couples to PKC can trigger
preconditioning. This includes receptors for
adrenaline, angiotensin II, bradykinin,
endothelin and opioids. While all of these
substances are released by the heart when
blood flow is interrupted, bradykinin,
adenosine and opioids appear to be the major
mediators of this protection. Free radicals
also contribute to PKC's activation.
Currently we believe that following ligand
binding to its surface receptor on the
cardiomyocyte (acetylcholine or ACh in the
diagram), a G protein is activated which in
turn stimulates a metalloproteinase in the
membrane. The latter enzyme causes
release of growth factors which activate
their specific receptors through a process
called transactivation. Receptor
tyrosine kinases are autophosphorylated, src
tyrosine kinase and phosphatidylinositol 3-kinase
are attracted to the signaling module,
membrane phospholipids are metabolized so
that 3’-phosphoinositide-dependent
kinases are activated which in turn
phosphorylate Akt. The latter activates
nitric oxide synthase resulting in production
of nitric oxide which activates guanylyl
cyclase eventually leading to phosphorylation
of PKA. Somehow this kinase opens the
mitochondrial KATP channel leading
to the release of reactive oxygen species
which stimulate downstream PKC and other
tyrosine kinases, e.g., p38MAP kinase in a
cascade. The ultimate end-effector is
not yet known.
In collaboration
with Dr. Downey I study the preconditioning
phenomenon in rabbit hearts and employ models
of preconditioning which range from isolated
heart cells in culture to the intact heart in
the awake rabbit instrumented with a
pneumatic occluder on its coronary artery.
While the agents listed above can
pharmacologically protect the heart of
experimental animals, they have not proven to
be clinically practical because of
unfavorable side effects (e.g., hypotension,
carcinogensis), a need for pretreatment which
is seldom possible in these patients, and the
development of tolerance. Current work is
focusing on 1) developing a practical drug; 2)
understanding what actually protects the
cell; 3) working out the signal transduction
pathways distal to PKC; and 4) developing a
cardioprotective strategy which can be
applied at reperfusion, and, therefore, one
which has great clinical potential.
Recent
Publications:
1. Oldenburg, O.,
Cohen, M.V., Yellon, D.M., and Downey, J.M.:
Mitochondrial KATP Channels: Role
in Cardioprotection. Cardiovascular Research 55:
429-437, 2002.
2. Oldenburg, O.,
Qin, Q., Sharma, A.R., Cohen, M.V., Downey, J.M.,
and Benoit, J.N.: Acetylcholine Leads to Free
Radical Production Dependent on KATP
Channels, Gi Proteins,
Phosphatidylinositol 3-Kinase and Tyrosine
Kinase. Cardiovascular Research 55:
544-552, 2002.
3. Krieg, T.,
Qin, Q., McIntosh, E.C., Cohen, M.V., and
Downey, J.M.: ACh and Adenosine Activate PI3-Kinase
in Rabbit Hearts Through Transactivation of
Receptor Tyrosine Kinases. American Journal
of Physiology 283: H2322-H2330, 2002.
4. Qin, Q.,
Downey, J.M., and Cohen, M.V.: Acetylcholine
But Not Adenosine Triggers Preconditioning
Through PI3-Kinase and a Tyrosine Kinase.
American Journal of Physiology 284: H727-H734,
2003.
5. Krieg, T.,
Landsberger, M., Alexeyev, M.F., Felix, S.B.,
Cohen, M.V., and Downey, J.M.: Activation of
Akt Is Essential for Acetylcholine to Trigger
Generation of Oxygen Free Radicals.
Cardiovascular Research 58: 196-202,
2003.
6. Oldenburg, O.,
Critz, S.D., Cohen, M.V., and Downey, J.M.:
Acetylcholine-induced Production of Reactive
Oxygen Species in Adult Rabbit Ventricular
Myocytes Is Dependent on Phosphatidylinositol
3- and Src Kinase Activation and
Mitochondrial KATP Channel Opening.
Journal of Molecular and Cellular Cardiology 35:
653-660, 2003.
7. Krieg, T.,
Cohen, M.V., and Downey, J.M.: Mitochondria
and Their Role in Preconditioning’s
Trigger Phase. Basic Research in Cardiology 98:
228-234, 2003.
