Mutations that activate oncogenes in cancer cells stimulate uncontrolled cell proliferation. Tumor-suppressor genes oppose the actions of oncogenes, and their inactivation predisposes to cancer. The realization that oncogenes and tumor-suppressor genes govern diverse biological processes, such as gene expression, cell differentiation, tissue development, and responses to environmental stress, provided key mechanistic insights into tumor formation.
A Hallmark of Cancer: Inactivation of the Rb and p53 Network
Transformation of normal cells to cancer cells commonly involves the disruption of signaling networks regulated by two canonical tumor-suppressor genes encoding the retinoblastoma protein (Rb) and the p53 transcription factor. Although the functions of Rb and p53 are not normally required for cell division, both proteins govern protective "checkpoint" responses that are triggered by DNA damage or by oncogene stimulation. Activation of Rb and p53 induces global changes in gene expression, which either arrest cell division or induce the death of damaged cells. Disruption of Rb and p53 compromises these safeguards, allowing incipient cancer cells to continue to divide, to consolidate mutational damage, and, ultimately, to form tumors.
D-type Cyclins and their Cyclin-Dependent kinases (CDKs)
Extracellular mitogens that drive cellular proliferation trigger the synthesis of D-type cyclins and their assembly with CDK4 and CDK6. In turn, the latter kinases phosphorylate Rb, releasing Rb-bound transcription factors (E2Fs) that, once freed from Rb constraint, activate a suite of genes necessary for chromosomal DNA replication (Figure 2). In this manner, the D-type cyclins act as growth factor sensors to integrate extracellular cues with the core cell cycle machinery. Oncogenic mutations can target components of cell receptor-mediated signaling pathways to render them constitutively active and independent of the extracellular mitogens that normally govern their functions. Such mutations enforce cyclin D-dependent CDK4/6 activities, Rb inactivation, and chromosomal DNA replication. Indeed, subsets of cancer-specific mutations directly impinge on the D-type cyclin and CDK4/6 genes to amplify their activities and enforce aberrant cell proliferation. Remarkably, the tumor suppressive p16Ink4a and p15Ink4b proteins encoded by the Ink4-Arf locus are induced by inappropriate hyperproliferative signals and serve to protect cells from mutated oncogenic drivers. Cancer cells result both from oncogene activation and a failure of tumor suppressor functions, so that mutations that inactivate the Ink4-Arf locus and Rb, or that increase the expression of D-type cyclins, CDK4, and CDK6 are among the most frequently detected in tumors.
Dating from the early discoveries more than two decades ago of the D-type cyclins, CDK4, and CDK6, there have been longstanding attempts to develop chemical inhibitors of CDK4 and CDK6 that might mimic the anti-proliferative effects of Ink4 proteins. Improvements in chemistry and drug screening have finally led to the generation of highly potent, orally available drugs that specifically inhibit CDK4/6 activities without significant off-target effects or clinically unmanageable dose-limiting toxicities in patients. These CDK4/6 inhibitors synergize with other newly developed drugs that target the signal transduction pathways that induce cyclin D synthesis, enabling the application of new combinatorial targeted therapies to treat various cancers. To date, one such CDK4/6 inhibitor has achieved FDA approval, and its use in combination with inhibitors of estrogen signaling, has led to significant increases in progression-free survival of patients with hormone-responsive breast cancer. Additional CDK4/6 inhibitors are in late clinical trials for a variety of other cancer indications.
Disparate Physiological Functions of Arf Suggest Novel Roles during Tissue Development
Arf is not generally expressed in normal tissues but is induced in response to activated oncogenes, thereby triggering p53-dependent elimination of incipient cancer cells (Figure 2). Paradoxically, we detected Arf expression in three tissues during early mouse development: (1) in the hyaloid vasculature of the eye, (2) in male germ cells (spermatogonia) within seminiferous tubules, and (3) in the fetal yolk sac, a tissue which arises from extraembryonic endoderm. Inactivation of Arf in these three tissues results in focal developmental defects that respectively lead to blindness, impaired spermatogenesis, and delayed formation of extraembryonic endoderm in the earliest stages of embryogenesis. Studies in which Arf deletions were specifically targeted to affected tissues, combined with the use of engineered "indicator strains," in which engagement of the Arf promoter was monitored throughout mouse development, revealed that Arf inactivation provokes these disparate defects in a manner that is independent of p53. By defining the segments of the Arf protein responsible for its unusual developmental activities in the eye, testis, and extraembryonic tissues, we hope to gain further insights into Arf’s p53-independent tumor suppressive functions.
As of March 3, 2016