Dissertations, Theses, and Capstone Projects

Date of Degree

6-2022

Document Type

Dissertation

Degree Name

Ph.D.

Program

Biochemistry

Advisor

David A. Foster

Committee Members

Patricia Rockwell

Jill Bargonetti

Amy Ikui

Poulikos Poulikakos

Subject Categories

Biochemistry

Abstract

The mammalian target of rapamycin (mTOR) acts as the central regulator of multiple cellular processes including cell growth, proliferation, and survival by integrating signals via nutrients, growth factors, hormones, and energy sensing. In cancer cells, the mTOR pathway is highly dysregulated providing survival signals to the cells for their uncontrolled growth. Hence, mTOR has evolved to be a potential therapeutic target for cancer treatment for the past two decades. Application of micro-molar doses of Rapamycin in vitro has been found to successfully inhibit mTOR complex 1 (mTORC1) by blocking the phosphorylation of its downstream substrates- a) ribosomal protein p70 S6 kinase (S6K) and b) eukaryotic initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1). Previous studies have shown that Rapamycin acts as a cytostatic drug and causes G1 cell cycle arrest in the presence of transforming growth factor- β (TGF-β). However, in the absence of TGF-β signal, the drug induces cell-death. The rationale behind the cytotoxic effect of Rapamycin in the absence of TGF-β is that without TGF-β, the cells do not arrest in G1 and progress into S phase where they lack further survival signals from mTORC1 which has been inhibited by Rapamycin – therefore, the cells undergo apoptosis. Of significance, we have found that cancer cells with mutated RB and CDKN2A are not susceptible to cell-death upon Rapamycin treatment in the absence of TGF-β. The gene products of RB and CDKN2A (pRb and p14ARF respectively) suppress E2F family transcription factors that promote cell cycle progression from G1 into S. Restoration of wild type RB or inhibition of E2F activity in cancer cells led to Rapamycin sensitivity. These data provide evidence that the combination of mutant RB and mutant CDKN2A in cancer cells leads to Rapamycin resistance, which has implications for precision medicine approaches to anti-cancer therapies.

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