Personal account of my career
My career goal is to conduct medical research
and teaching in Canada. This has been set before I graduated from
McGill University. During my PhD studies, I have learnt basic molecular
and biochemical techniques and studied regulation of a hormone (somatostatin)
gene expression. The late Dr. Yogesh C. Patel introduced me into
molecular endocrinology. I took a challenge with Dr. Derek LeRoith
in order to further my training at the National Institutes of Health
(NIH, Bethesda, MD), by performing Cre/loxP-induced conditional
gene targeting on IGF-I. We were highly successful in clarifying
the role of liver-derived IGF-I in growth and metabolism. Several
years ago when I designed my independent research projects I decided
to march towards diabetes research, based on the facts that GH/IGF
system is highly relevant to insulin production and action, my transgenic
mice are very unique tools to solve the questions, and that diabetes
incidence is increasing rapidly in recent years due to obesity and
lack of physical activities.
MOST SIGNIFICANT CONTRIBUTIONS:
2005 The essential role of growth
hormone signaling in maintaining normal pancreatic islet size, insulin
production, insulin sensitivity and glucose homeostasis. Decades
of research on human, rodents and in cell cultures support a key
role of growth hormone signals in pancreatic islet cell growth and
regulating insulin responsiveness. Using the growth hormone receptor
gene deficient (GHR-/-) mice, I demonstrate that lack of growth
hormone signals caused decreased islet β-cell mass and insulin
production. At the same time, these mice are hyper sensitive to
insulin and exhibit hypoglycemia. At least part of these effects
is independent of the general growth retardation, due to their early
onset and the extent of islet cell mass reduction. Interestingly,
GHR-/- mice exhibit lower serum insulin level, elevated insulin
sensitivity, and a lean phenotype, in contrast to human Laron syndrome.
Very recently, pancreatic islet-specific expression of an IGF-I
transgene compensates islet cell growth in GHR-/- mice.
2004 Tissue-specific effects of IGF-I
on pancreatic islet growth and insulin production. Using Cre/loxP-mediated
conditional gene targeting, I have studied the effects of IGF-I
deficiency in the liver (LID) and pancreas (PID) on pancreatic islet
growth and insulin production. (1) LID mice exhibit insulin resistance
that results in islet cell hyperplasia and hyperinsulinemia. Insulin
resistance occurs mostly in skeletal muscles and can be relieved
by inhibiting growth hormone secretion. Islet hyperplasia is a secondary
response thus provides no protection against streptozotocin-induced
islet cell damage and diabetes. Therefore, liver-produced IGF-I
enhances insulin sensitivity at least in part by inhibiting growth
hormone release. (2) PID mice exhibit islet enlargement and hypoglycemia;
are significantly resistant to streptozotocin-induced type 1 diabetes
by preventing islet ?-cell apoptosis; and are resistant to high-fat-diet-induced
type 2 diabetes by stimulating islet cell growth. Our results suggest
that locally produced IGF-I within the pancreas inhibits islet cell
growth, its deficiency provides a protective environment to the
β-cells and a potential in combating diabetes.
2002 I have organized a virtual conference
“Conditional gene targeting in mice”, invited by the
journal Endocrine. Six groups of experts from the US, Sweden and
Switzerland summarized achievement in 7 years using the Cre/loxP-mediated
conditional gene targeting.
1999 Revised the Somatomedin Hypothesis.
Using the Cre/loxP-induced conditional gene targeting, we demonstrated
that lack of IGF-I production from the liver, which accounts for
75% of the circulating level, has no effect on normal growth and
development. Therefore, we proposed that growth hormone acts directly
on target tissues (rather than through liver IGF-I, as previously
proposed) and promotes local (paracrine) production of IGF-I, thereby
promoting tissue growth. It represents a breakthrough in 40 years
of somatomedin research. The concept is revolutionary and has been
frequently cited in over 250 articles in only 5 years.
1997 Creation of mice with loxP-tagged
IGF-I gene via homologous recombination (IGF-I/loxP mice). Exon
4 of the IGF-I gene was tagged by two loxP repeats that serve as
recognition sites for Cre recombinase. This is the first peptide
hormone gene to be tagged, and it enables Cre-mediated conditional
knockout of the IGF-I gene. As IGF-I is expressed extensively in
various tissues and throughout development, international collaborations
have been established to specifically knock out IGF-I in the following
targets: bone, mammary gland, ovary, prostate gland, heart, brain,
and pancreatic islets. We have achieved liver- and pancreatic specific
and interferon-inducible IGF-I knockouts (LID & PID). More recent
successes include in colon cancer (Can Res 2002; 62: 1030-1035)
and the spleen (GH IGF Res 2003; 13: 254-263).
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