Regulatory mechanisms of tumor suppressor P16(INK4A) and their relevance to cancer

Abstract

P16(INK4A) (also known as P16 and MTS1), a protein consisting exclusively of four ankyrin repeats, is recognized as a tumor suppressor mainly because of the prevalence of genetic inactivation of the p16(INK4A) (or CDKN2A) gene in virtually all types of human cancers. However, it has also been shown that an elevated level of expression (upregulation) of P16 is involved in cellular senescence, aging, and cancer progression, indicating that the regulation of P16 is critical for its function. Here, we discuss the regulatory mechanisms of P16 function at the DNA level, the transcription level, and the posttranscriptional level, as well as their implications for the structure-function relationship of P16 and for human cancers.

Figure 1. The P16-CDK4/6-pRb Pathway

Arrows and minus signs represent positive and negative regulatory effects, respectively. P, phosphorylated. This figure was modified from Reference .

Figure 2. Primary, secondary, and tertiary structures of P16

A, Sequence and secondary structure of P16. Italic residues at the N-terminal represent missing residues in the cDNA gene first reported. Red and dashed lines represent helical and loop regions, respectively. Residues with * marks are those with mutations in human cancers. AR, ankyrin repeat. B, Tertiary structure and domains of P16. The solution structure of P16 (PDB code: 2A5E) is presented here (30), in which D84, the residue critical for CDK4 inhibition, is highlighted. JNK, c-Jun N-terminal kinases.

Figure 3. Schematic structure of the INK4b/ARF/INK4a locus

Rectangles represent DNA and mRNA, and cylinders represent proteins. RD: the regulatory sequence of the INK4/ARF locus; e1, e2: exons 1 and 2 of p15^INK4B; E1β, E1α, E2, E2γ, E3: exons 1β, 1α, 2, 2γ, and 3 of p16^INK4A; In1: intron 1 of p16^INK4A; kD, kilo Daltons. Sizes of coding regions and proteins are not in proportion strictly. This figure was modified from Reference .

Figure 4. The structure of the p16^INK4A promoter and regulators of p16^INK4A transcription

Empty arrows represent the transcription directions, and plus and minus marks indicate positive and negative effects on the transcription of p16^INK4A, respectively. RD: the regulatory sequence of the INK4b/ARF/INK4a locus; ITSE, the INK4a transcription silencing element; PPRE, the peroxisome proliferator response element. Sizes of the DNA elements are not in proportion strictly.

Figure 5. Coordination of P16 and other proteins in modulating CDK4/6-mediated phosphorylation of pRb

Regulators to be investigated in our proposed studies are in green shadow. Arrows and bars represent positive and negative regulation, respectively. Dotted lines, genomic DNA; P, phosphorylated.

Figure 6. Multifactorial effects of aberrant gankyrin over-expression on cell growth and carcinogenic progression

In both A and B, arrows and crosses represent positive and negative regulatory effects, respectively. In B, red crosses indicate that over-expressed gankyrin preludes the inhibition of P16 and P21 on CDK4/6, while red arrows represent the enhancing effects on phosphorylation of pRb, ubiquitination of P53, and proteasome-mediated degradation of both pRb and P53. P, phosphorylated. This figure was modified from Reference .