Chapter 080. Cancer Cell Biology and Angiogenesis (Part 2)

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Cancer Cell Biology The treatment of most human cancers with conventional cytoreductive agents has been unsuccessful due to the Gompertzian-like growth kinetics of solid tumors (i.e., tumor growth is exponential in small tumors, with increasing doubling times as tumors expand; since conventional chemotherapeutic agents target proliferating cells, noncycling cells in large tumors are relatively resistant). Genetic instability is inherent in most cancer cells and predisposes to the development of intrinsic and acquired drug resistance. Thus, although tumors arise from a single cell (i.e., they are clonal), large tumors become very heterogeneous with multiple related subclones, some of which will...

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Chapter 080. Cancer Cell Biology and Angiogenesis (Part 2) Cancer Cell Biology The treatment of most human cancers with conventional cytoreductive agents has been unsuccessful due to the Gompertzian-like growth kinetics of solid tumors (i.e., tumor growth is exponential in small tumors, with increasing doubling times as tumors expand; since conventional chemotherapeutic agents target proliferating cells, noncycling cells in large tumors are relatively resistant). Genetic instability is inherent in most cancer cells and predisposes to the development of intrinsic and acquired drug resistance. Thus, although tumors arise from a single cell (i.e., they are clonal), large tumors become very heterogeneous with multiple related subclones, some of which will be resistant to specific therapies, leading to the selection of progressively more resistant tumors as
treatment progresses. Since a 1-cm tumor often contains 109 cells, and patients typically present to their physicians with 1010–1011 tumor cells, the obstacle to curative treatment becomes more understandable. Rationally designed, target- based therapeutic agents, directed against the specific molecular derangements that distinguish malignant from nonmalignant cells, have become possible with advances in the understanding of oncogene and tumor-suppressor pathways. This chapter describes the convergence of scientific, pharmacologic, and medical knowledge that has led to the targeted therapy of cancer. Therapeutic Approaches to Cell Cycle Abnormalities in Cancer The mechanism of cell division is substantially the same in all dividing cells and has been conserved throughout evolution. The process assures that the cell accurately duplicates its contents, especially its chromosomes. The cell cycle is divided into four phases. During M-phase, the replicated chromosomes are separated and packaged into two new nuclei by mitosis and the cytoplasm is divided between the two daughter cells by cytokinesis. The other three phases of the cell cycle are called interphase: G1 (gap 1), during which the cell determines its readiness to commit to DNA synthesis; S (DNA synthesis), during which the genetic material is replicated and no re-replication is permitted; and G2 (gap 2), during which the fidelity of DNA replication is assessed and errors are corrected.
Deregulation of the molecular mechanisms controlling cell cycle progression is a hallmark of cancer. Progression from one phase of the cell cycle to the next is controlled by the orderly activation of cyclin-dependent kinases (CDKs) that are regulated by signaling events that couple a cell's physiologic response to its extracellular milieu. In normal cells, specific molecular signals, called checkpoints, prevent progression into the next phase of the cell cycle until all requisite physiologic processes are complete. Cancer cells often have defective cell cycle checkpoints. The transition through G1 into S-phase is a critical regulator of cell proliferation, and the phosphorylation state of the retinoblastoma tumor-suppressor protein (pRB) at the restriction point in late G1 determines whether a cell will enter S-phase. The complex of CDK4 or CDK6 with D type cyclins forms a G1-specific kinase whose activity is regulated by growth factors, nutrients, and cell-cell and cell-matrix interactions. Subsequent formation of an active CDK2/cyclin E complex results in full phosphorylation of pRB, relieving its inhibitory effects on the S-phase-regulating transcription factor E2F/DP1, and permitting the activation of genes required for S-phase (such as dihydrofolate reductase, thymidine kinase, ribonucleotide reductase, and DNA polymerase). The activity of CDK/cyclin complexes can be blocked by CDK-inhibitors including p21Cip1/Waf1, p16Ink4a, and p27Kip1, which block S-phase progression by preventing the phosphorylation of pRB.
Genetic lesions that render the retinoblastoma pathway nonfunctional are thought to occur in all human cancers. Loss of function of pRB as guardian of the G1 restriction point enables cancer cells to enter a mitotic cycle without the normal input from external signals. Current therapeutic efforts to reverse the derangements of the retinoblastoma pathway have taken two main approaches. All kinases require the binding of ATP (and substrate) to the enzyme active site, followed by transfer of the γ-phosphate to serine, threonine, or tyrosine residues of the substrate. Flavopiridol was the first relatively selective CDK inhibitor identified, with Ki or IC50s in the 40- to 400-nM range. Although flavopiridol was initially thought to prevent tumor cell proliferation by inhibition of cell cycle CDKs, it is now clear that regulation of cellular transcription elongation by the CDK7/cyclin H and CDK9/cyclin T1 complexes may be the critical target of flavopiridol. Phase II clinical trials of flavopiridol are in progress; responses have been reported in chronic lymphocytic leukemia after a dosing schedule was defined to optimize the pharmacokinetics of the drug. Laboratory efforts are focused on the development of novel classes of CDK inhibitors capable of specifically targeting individual CDK/cyclin complexes. A second therapeutic endeavor to regain control of pRB function involves reversing the epigenetic silencing of p16Ink4a gene and is discussed below. p53, the "guardian of the genome," is a sequence-specific transcription factor whose activity is regulated through tight control of p53 protein levels.
Normally, levels of p53 are kept low by its association with the mdm2 oncogene product, which binds p53 and shuttles it out of the nucleus for proteolytic degradation. p53 levels are regulated by two checkpoint pathways that are activated in response to DNA damage or oncogene-induced cell proliferation (Fig. 80-1). The loss of p53 function abrogates these checkpoints and enables tumor cells to escape cell cycle arrest, senescence, or apoptosis despite accumulation of mutations and aberrant passage through the cell cycle.