Deoxycholic acid (DCA) is a secondary bile acid postulated to be involved in the etiology and the progression of colorectal cancer, but its specific mechanisms are not fully understood. DCA has been shown to induce apoptosis allowing selection for apoptosis-resistant cells, which highlights the importance of understanding the mechanisms of action of DCA. Previously, it has been demonstrated that DCA perturbs the plasma membrane, leading to the activation of receptor tyrosine kinases. Because the insulin-like growth factor-1 receptor (IGF-IR), a receptor tyrosine kinase, is demonstrated to play a significant role in protecting colorectal cancer cells from apoptosis, we hypothesized that DCA modulates IGF-IR functions in colorectal cancer cells. We demonstrated that DCA induced the dynamin-dependent endocytosis of IGF-IR through both clathrin-mediated and caveolin-1-dependent mechanisms. Endocytosis of IGF-IR sensitized cells to DCA-induced apoptosis, which demonstrated that IGF-IR played a role in protecting cells against DCA-induced apoptosis. Since DCA-induced endocytosis of IGF-IR was determined to be a caveolin-1 dependent process, caveolin-1 knockdown in HCT116 (HCT116-Cav1-AS) prevented the DCA-mediated endocytosis of IGF-IR. However, we observed an increased sensitivity of DCA-induced apoptosis in the Cav1-AS cells. This suggested that caveolin-1 knockdown altered the plasma membrane dynamics such that although IGF-IR was maintained at the plasma membrane, it facilitated a pro-apoptotic signal. We demonstrated that DCA induced the activation of the pro-apoptotic p38 signaling pathway in HCT116-Cav1-AS, but not in HCT116-Mock, via IGF-IR. Inhibition of both the IGF-IR and p38 independently in HCT116-Cav1-AS significantly decreased their sensitivity to DCA-induced apoptosis. These observations demonstrated that, in a caveolin-1 dependent manner, IGF-IR played a dynamic role in the DCA-mediated apoptosis. Finally, we provided preliminary evidence demonstrating that autophagy played a central role in protecting DCA-resistant cells from DCA-induced apoptosis.Since resistance to DCA also confers apoptosis-resistance, understanding the mechanisms that lead to or prevent DCA-induced cell death is significant, since they can lead to the development of novel therapeutic strategies to sensitize apoptosis-resistant colorectal cancer cells to undergo cell death.

Insulin-like growth factor-1 (IGF-1) is essential for oligodendrocyte (OL) development, promoting their survival, proliferation and differentiation. Furthermore, IGF-1 null mutant mice have a decrease in CNS myelination and in the number of OL progenitors (OLPs). IGF-1 interacts with the Type I IGF receptor to activate two main downstream signalling pathways, the PI3K/Akt and the Ras-Raf-MEK/ERK cascades, which mediate survival or proliferation of OLPs. The objective of this study is to elucidate the transduction pathways involved in IGF-I-stimulated protein synthesis, important for growth and differentiation of OLs. In other cellular systems, the PI3K/Akt pathway is involved in protein translation. mTOR and the p70 S6 kinase are downstream effectors that phosphorylate translation initiation factors (e.g. eIF-4E) and their regulators (e.g. 4E-BP1). OLPs were obtained from primary cultures and were treated with IGF-1 with or without inhibitors LY294002 or wortmannin (PI3K), rapamycin (mTOR), Akt III or IV, an adenovirus with a dominant negative form of Akt or PD98059 (ERK). Protein synthesis was assessed by metabolic labeling with [35S]-methionine, and protein phosphorylation by Western blotting. Results from the former showed that IGF-1 stimulates protein synthesis in a dose-dependent manner. Moreover, IGF-1 increases protein synthesis in OLPs through PI3K, mTOR, Akt and ERK activation. Concordantly, Western blot analysis reveals that IGF-1 stimulates phosphorylation of Akt, mTOR, ERK, S6 and 4E-BP 1. Activation of S6 and inactivation of 4E-BP1 occur through phosphorylation and are required for protein synthesis to take place. These events are dependent on the upstream activation of PI3K, Akt and mTOR.

The study of diabetes mellitus is vital in this day and age because its incidence is increasing at an alarming rate. Diabetes results in the loss of function of beta-cells within the pancreas. Insulin resistance contributes to diabetes but the human body can compensate in various ways such as increasing the islet cell mass, glucose disposal and insulin secretion, in order to prevent the onset of diabetes. Growth hormone (GH) and insulin-like growth factor-I (IGF-I) are two integral hormones important in both glucose homeostasis and islet cell growth. Early studies using cultured islet cells have demonstrated positive regulation of beta-cell growth by both GH and IGF-I. To evaluate their relevance on normal beta-cell growth, compensatory growth, as well as in insulin responsiveness, we have used two mouse models that represent opposite manipulations of the GH/IGF-I axis. Specifically, the growth hormone receptor gene deficient (GHR-/-) and the IGF-I overexpression (MT-IGF) mice, to help understand the role of glucose homeostasis and islet cell growth in the GH/IGF-I axis. GH is essential for somatic growth and development as well as maintaining metabolic homeostasis. It is known that GH stimulates normal islet cell growth. Moreover, GH may also participate in islet cell overgrowth and compensate for insulin resistance induced by obesity. To determine whether the islet cell overgrowth is dependent on GH signaling, we studied the response of GHR-/- mice to high-fat diet (HFD)-induced obesity. We also studied the insulin responsiveness in GHR-/- mice. On the other hand, IGF-I promotes embryonic development, postnatal growth and the maturation of various organ systems. The notion that IGF-I stimulates islet cell growth has been challenged in recent years by results from IGF-I and receptor gene targeted models. We have characterized MT-IGF mice which overexpress the IGF-I gene. / The results of our studies indicate that (1) GH is essential for normal islet cell growth, but not required for compensatory overgrowth of the islets in response to obesity, (2) GHR gene deficiency caused delayed insulin responsiveness in skeletal muscle; in contrast to elevated insulin sensitivity in the liver; (3) although overexpression does not stimulate islet cell growth, a chronic IGF-I elevation caused significant hypoglycemia, hypoinsulinemia, and improved glucose tolerance, (4) finally IGF-I overexpression mice are resistant to experimental diabetes.

Bibliography :leaves 250-302. / xix, 302, [19] leaves, [4] leaves of plates : ill. (chiefly col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Results suggest that IGF-I peptides significantly influence gastrointestinal growth in normal adult and suckling rats and indicate they may have therapeutic implications both in conditions of impaired gut function in the adult gastrointestinal tract and in the treatment of gut disease in the immature intestine. / Thesis (Ph.D.)--University of Adelaide, Dept. of Obstetrics & Gynaecology, 1995?

Thesis (Ph.D.) -- University of Adelaide, Department of Animal Science, 1992.

Thesis (Ph.D.)--University of Adelaide, Department of Animal Science, 1994.

Thesis (Ph.D.) -- University of Adelaide, Dept. of Biochemistry, 1994.

Diss. (sammanfattning) Stockholm : Karol. inst., 2005. / Härtill 6 uppsatser.

Thesis (M.S.)--West Virginia University, 2004. / Title from document title page. Document formatted into pages; contains iv, 124 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 82-124).

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 199-225).