William G. Kaelin Jr., Sir Peter J. Ratcliffe, and Gregg L. Semenza identified the molecular machinery that regulates how genes respond to varying oxygen levels. This machinery is the hypoxia-inducible factor (HIF) transcription factor that binds to DNA under low oxygen conditions to activate genes like erythropoietin that help cells adapt. Under normal oxygen levels, HIF is targeted for destruction by the proteasome after being hydroxylated in an oxygen-dependent way, but under low oxygen, HIF binds DNA to turn on adaptive response genes.

1. BILAL AHMAD MIR
RESEARCH SCHOLAR
ZOOLOGY-MANUU HYDERABAD

2.  Fundamental importance of O2 has been understood for
centuries, but how cells adapt to change in level of O2
has long been unknown.

3.  William G. Kaelin, Jr.
 Sir Peter J. Ratcliffe
 Gregg L. Semenza
 They identified molecular machinery that regulate the
activity of genes responsible to varying levels of O2.

4.  O2 is essential for animal life
 Required for oxidation reaction
 Used by the mitochondria in order to convert
food into useful energy. This conversion is
purely an enzymatic process (Warburg 1931,
Noble prize winner)

5.  Hypoxia is a state in which oxygen is not
available in sufficient amounts at tissue level
to maintain adequate homeostasis.
 this can result from inadequate oxygen
delivery to the tissues either due to low blood
supply or low oxygen content in blood
(hypoxemia).

7.  In addition to the carotid-controlled rapid
adaptation to hypoxia, rise in EPO level is also
a physiologic response to hypoxia
 special cells of carotid body senses the O2
level of blood (Corneille Hayman, noble prize
winner 1938),

9.  EPO gene was studied using transgenic mice
and some specific DNA segments located
next to EPO gene were found to mediate the
response to hypoxia.(semenza and Ractcliffe)

10.  O2 sensing mechanism was found to be
present in virtually all the tissues not only
kidney cells were EPO is present.

11.  Cellular component mediating this process
was found to be HIF (hypoxia inducible factor)
that binds to the identified DNA segments
(HRE) in O2 dependent manner.

12. HIF
(a transcription factor)
HIF-1ALFA ARNT
(O2 regulated) (O2 Non
regulated)

13.  hypoxia causes increases in the
transcriptional expression of the
erythropoietin hormone (EPO) in kidney
(Bondurant and Koury,1986)
 This finding in turn had its own roots:
Exposure to high altitude increased red
blood cell counts (Bert, 1882)

14.  Von Hippel-Lindau syndrome (VHL) is a
hereditary condition where tumor suppressor
gene VHL are associated with tumors arising
in multiple organs.
 Normal Von Hippel-Lindau tumor suppressor
gene when introduced in VHL mutant cell
lines suppress the tumor growth.(Kaelin
1995)

15.  Kaelin also showed that cancer cells lacking a
functional VHL gene express abnormally high
levels of hypoxia-regulated genes. This was
an important clue showing that VHL was
somehow involved in controlling responses to
hypoxia
 In 1999, Ratcliffe demonstrated the
association between VHL and HIF-1Alfa ( VHL
regulated HIF-1alfa posttranslational O2
dependent degradation).

16.  High O2 level very little HIF-1alfa
content
 Low O2 level HIF-1Alfa increases
In the later case, HIF-1Alfa binds to and thus
regulates EPO gene.

17. When O2 levels are normal
ubiquitin is added to HIF-1Alfa protein
Proteosomal degradation of HIF-1Alfa protein

18.  VHL physically interact with HIF-1Alfa and is
required for its degradation at normal O2
level
 O2 level regulate the interaction between VHL
and HIF-1Alfa protein.

20.  Under Normal O2 level, Hydroxyl groups are
added at two specific positions of HIF-
1Alfa(hydroxylation).which allows VHL to
recognize and bind to HIF-1Alfa.

21. In hypoxia, the HIF complex binds to HRE elements in the genome, activating gene expression of gene products important for
adaptation to low oxygen.
Under high oxygen (normoxic) conditions, HIF1a is targeted for destruction by the proteasome after being hydroxylated in an
oxygen-dependent manner.

22. Thank You