CDKN1B

The p27Kip1 gene has a DNA sequence similar to other members of the "Cip/Kip" family which include the p21Cip1/Waf1 and p57Kip2 genes. In addition to this structural similarity the "Cip/Kip" proteins share the functional characteristic of being able to bind several different classes of Cyclin and Cdk molecules. For example, p27Kip1 binds to cyclin D either alone, or when complexed to its catalytic subunit CDK4. In doing so p27Kip1 inhibits the catalytic activity of Cdk4, which means that it prevents Cdk4 from adding phosphate residues to its principal substrate, the retinoblastoma (pRb) protein. Increased levels of the p27Kip1 protein typically cause cells to arrest in the G1 phase of the cell cycle. Likewise, p27Kip1 is able to bind other Cdk proteins when complexed to cyclin subunits such as Cyclin E/Cdk2 and Cyclin A/Cdk2. == Regulation ==

In general, extracellular growth factors which promote cell division reduce transcription and translation of p27Kip1. Also, increased synthesis of CDk4,6/cyclin D causes binding of p27 to this complex, sequestering it from binding to the CDk2/cyclin E complex. Furthermore, an active CDK2/cyclin E complex will phosphorylate p27 and tag p27 for ubiquitination. A mutation of this gene may lead to loss of control over the cell cycle leading to uncontrolled cellular proliferation. Loss of p27 expression has been observed in metastatic canine mammary carcinomas. Decreased TGF-beta signalling has been suggested to cause loss of p27 expression in this tumor type. A structured cis-regulatory element has been found in the 5' UTR of the P27 mRNA where it is thought to regulate translation relative to cell cycle progression. P27 regulation is accomplished by two different mechanisms. In the first its concentration is changed by the individual rates of transcription, translation, and proteolysis. P27 can also be regulated by changing its subcellular location Both mechanisms act to reduce levels of p27, allowing for the activation of Cdk1 and Cdk2, and for the cell to begin progressing through the cell cycle. Transcription Transcription of the CDKN1B gene is activated by Forkhead box class O family (FoxO) proteins which also acts downstream to promote p27 nuclear localization and decrease levels of COP9 subunit 5(COPS5) which helps in the degradation of p27. Transcription for p27 is activated by FoxO in response to cytokines, promyelocytic leukaemia proteins, and nuclear Akt signaling. However it is rarely expressed in early G1 where p27 levels first begin to decrease. During early G1 proteolysis of p27 is regulated by KIP1 Ubiquitylation Promoting Complex (KPC) which binds to its CDK inhibitory domain. P27 also has three Cdk-inhibited tyrosines at residues 74, 88, and 89. Once p27 is excluded from the nucleus it cannot inhibit the cell's growth. In the cytoplasm it may be degraded entirely or retained. This step occurs very early when the cell is exiting the quiescent phase and thus is independent of Skp2 degradation of p27. MicroRNA regulation Because p27 levels can be moderated at the translational level, it has been proposed that p27 may be regulated by miRNAs. Recent research has suggested that both miR-221 and miR-222 control p27 levels although the pathways are not well understood. == Role in cancer ==

Proliferation p27 is considered a tumor suppressor because of its function as a regulator of the cell cycle. Src was also shown to reduce the half life of p27 meaning it is degraded faster. In breast cancer cytoplasmic p27 reduced RHOA activity which increased a cell's propensity for motility. This role for p27 may indicate why cancer cells rarely fully inactivate or delete p27. By retaining p27 in some capacity it can be exported to the cytoplasm during tumorigenesis and manipulated to aid in metastasis. 70% of metastatic melanomas were shown to exhibit cytoplasmic p27, while in benign melanomas p27 remained localized to the nucleus. P27 is misplaced to the cytoplasm by the MAP2K, Ras, and Akt pathways although the mechanisms are not entirely understood. Additionally, phosphorylation of p27 at T198 by RSK1 has been shown to mislocalize p27 to the cytoplasm as well as inhibit the RhoA pathway. Because inhibition of RhoA results in a decrease in both stress fibers and focal adhesion, cell motility is increased. P27 can also be exported to the cytoplasm by oncogenic activation of the P13K pathway. In these cells, p27 bound to stathmin which prevents stathmin from binding to tubulin and thus polymerization of microtubules increased and cell motility decreased. MicroRNA regulation Studies of various cell lines including glioblastoma cell lines, three prostate cancer cell lines, and a breast tumor cell line showed that suppressing miR-221 and miR-22 expression resulted in p27-dependent G1 growth arrest Then when p27 was knocked down, cell growth resumed indicating a strong role for miRNA regulated p27. Studies in patients have demonstrated an inverse correlation between miR-221&22 and p27 protein levels. Additionally nearby healthy tissue showed high expression of the p27 protein while miR-221&22 concentrations were low. == Regulation in specific cancers ==

In most cancers reduced levels of nuclear p27 are correlated with increased tumor size, increased tumor grade, and a higher propensity for metastasis. However the mechanisms by which levels of p27 are regulated vary between cancers. Breast In breast cancer, Src activation has been shown to correlate with low levels of p27 Prostate A mutation in the CDKN1B gene has been linked to an increased risk for hereditary prostate cancer in humans. Multiple Endocrine Neoplasia Mutations in the CDKN1B gene has been reported in families affected by the development of primary hyperparathyroidism and pituitary adenomas, and has been classified MEN4 (multiple endocrine neoplasia, type 4). Testing for CDKN1B mutations has been recommended in patients with suspected MEN, in whom previous testing for, the more common MEN1/RET mutation, is negative. == Clinical significance ==

Prognostic value Several studies have demonstrated that reduced p27 levels indicate a poorer patient prognosis. Similar studies have correlated low levels of p27 with a worse prognosis in breast cancer. Colorectal carcinomas that lacked p27 were shown to have increased p27-specific proteolysis and a median survival of only 69 months compared to 151 months for patients with high or normal levels of p27. The authors proposed clinicians could use patient specific levels of p27 to determine who would benefit from adjuvant therapy. those with colon, So far studies have only evaluated the prognostic value of p27 retrospectively and a standardized scoring system has not been established. Similarly low levels of p27 correlated with poor results from adjuvant chemotherapy in breast cancer patients. Value as a therapeutic target P27 has been explored as a potential target for cancer therapy because its levels are highly correlated to patient prognosis. This is true for a wide spectrum of cancers including colon, breast, prostate, lung, liver, stomach, and bladder. Use of microRNAs for therapy Because of the role miRNAs play in p27 regulation, research is underway to determine if antagomiRs that block the activity of the miR221&222 and allow for p27 cell grow inhibition to take place could act as therapeutic cancer drugs. ==Role in Regeneration==

Knockdown of CDKN1B stimulates regeneration of cochlear hair cells in mice. Since CDKN1B prevents cells from entering the cell cycle, inhibition of the protein could cause re-entry and subsequent division. In mammals where regeneration of cochlear hair cells normally does not occur, this inhibition could help regrow damaged cells who are otherwise incapable of proliferation. In fact, when the CDKN1B gene is disrupted in adult mice, hair cells of the organ of Corti proliferate, while those in control mice do not. Lack of CDKN1B expression appears to release the hair cells from natural cell-cycle arrest. Because hair cell death in the human cochlea is a major cause of hearing loss, the CDKN1B protein could be an important factor in the clinical treatment of deafness. == Interactions ==

CDKN1B has been shown to interact with: • AKT1, • Cyclin E1, • Cyclin-dependent kinase 2, • Cyclin-dependent kinase 4, • Grb2, • NUP50 • SKP2, • SPDYA, and • XPO1. . == See also ==