Protien kinases whose activation depends on a cyclin protien, called cyclin dependant kinases regulate the transitions between the various phases. The enzymatic activity of the kinase is dependant on its assosciation with a cyclin and its phosphorylation state. Degradation of cyclins is carried out by the ubiquitin-proteasome complexes(during Phase M).

• Phosphorylation of Histone
• Phosphorylation of nuclear lamins
• Phosphorylation of microtubule assosciated protiens

leading to

• chromosome condensation
• breakdown of nuclear lamins
• spindle formation

respectively.

Cyclin B/cdc2 complex is frequently refered to as the Mitosis promoting factor(MPF).

The retinoblastoma gene product example
The retinoblastoma gene product designated Rb, is present normally in a hypophophorylated state. In this state, the molecule serves to scavenge E2F (a transcription factor) molecules. Free E2F can heterodimerize with other transcription factors like DP1 and directly signal the basal transcription machinery to start DNA replication. But when bound to Rb, DP1 is unable to activate the replication machinery(the activity of the E2F/DP1 is reduced by the Rb bound to DP1). Removal of Rb from DP1 by multiple phosphorylation at several pocket binding sites on the Rb releases free DP1 and transcription can proceed

In other words, Rb promotes differentiation while 'inhibition ' of Rb brings about transcriptional activity. Mulitple phosphorylation of the Rb molecule is carried out by two cdks-

• Cyclin D/cdk4 or cdk6
• Cyclin E/cdk2

Growth factors promote cdk activity, wheras TGFb, contact inhibition, cAMP inhibit cdk acrivity through various mechanisms.

Activity of the cdks in also modulated by cdki-cdk inhibitors
Usually designated as p(some number), eg; p21, inhibitors are low molecular weight protiens that inhibit cdks. Genetic alterations in cdki esp. p16 and p15 ahve been found very frequently in various tumors.

p53, a cdki is known as the guardian of the genome at the G1 checkpoint. It is expressed in case of extensive DNA damage. It can either induce p21 production to halt cell cycle progression or set of f an apoptotic pathway if the DNA damage is too great. Mutations in p53 are the most common genetic alteration found in human cancers. Mutations always seem to involve the region between codon 120 and 290. p53 is the target of many tumor causing viruses like

• human papilloma virus(protien E6 of the virus)
• Adenovirus E1B protien
• SV40 large T antigen

All seem to interfere with p53 function.

Codon 249 is mutated by the chemical substance aflatoxin, leading to hepatomas.

1. Activation of CREBs through the Protien Kinase A pathways
2. Inactivation of certain inhibitory molecules leading to translocation of intranuclear transcription factors through the Protein Kinase C pathway.

1. N.B I couldn't figure out the mode of action of cAMP response elements and CRE binding proteins. The pathway, as I saw it, was 'self-terminating'. If you are able to figure it out or know it already, do tell me.

2. The more important pathway concerns the activation of protien kinase C.The basic framework of the regulatory pathway involving PKC starts with cAMP releasing intracellular calcium stores and bringing in extra cellular calcium. This leads to production of second messengers IP3 and DAG via the Phospholipase C pathway. DAG stimulates Protien Kinase C which activates a family of serine/threonine kinases which directly then act on factors regulating transcription. For eg.

DAG-mediated activation of Protien Kinase C
|
|
\/
Activation of the MAP Kinase pathway
|
|
\/
Phosphorylation of IkB(an inhibitor molecule, prevents an transcription acvivating heterodimer complex from reaching the nucleus)
|
|
\/
Release of the activating heterodimers(these belong to the rel family).
|
|
\/
Nuclear translocation of the activating heterdimers
|
|
\/
Transcription

• PDGF
• FGFs
• EGFs
• TGF(+)
• nerve growth factor
• heregulin
• insulin
• insulin-like growth factors
• Stem cell factors
• Colony stimulating factors

are all Receptor tyrosine kinase complexes.
A brief note on the structure of these receptors
Receptor tyrsoine kinase complexes are simple in structure as compared to the G-protien receptor complexes. But the activation of intracellular events by these receptors take as sinuous a path as is the case of the G-Protien transducer elements.

Ligand binding induces dimerization or oligomerization of the receptor subunits leading to activation of the tyrosine kinase activity on the intracellular domains and susequent autophosphorylation of certain segments on the intracellular chain. These phosphorylated segments then act as binding sites for other signal transduction molecules. Many enzymes (like the nonreceptor tryosine kinase src) bind to these phosphorylated binding sites and get activated. Some protiens serve as docking proteins exclusively in that , they lack enzymatic activity but they bind to the binding sites and subsequently activate other enzymes, serving as bridges. The docking protiens bind to the binding sites on the receptor using SH2, SH3 or PH domains and activate downstream events.
Tyrosine Kinase receptors have been shown to be involved in the activation of the
ras
fos/jun/AP-1 family of transcription factors
c-myc activation

Overexpression of tyrosine kinase receptors has been seen in many cancers,
-lung
-bladder
-head and neck
breast
ovarian cancers.

The receptors for
-growth hormone
-prolactin
erythropoeitin
-IL-2, IL-3, IL-4, IL-6, IL-7
-granulosyte CSF
interferon gamma
are all tryosine kinase assosciated receptors.

The kinases assosciate with the receptors by docking proteins. to mainly, three classes of kinases
the src family of kinases
The Janus family of kinases
The syk family of kinases
and lead to similar types of signalling cascades as mentioned above. For eg, the Janus family of kinases activate a class of molecules called STATs(signal transduction and activation).

These receptors are frequently overexpressed in tumours of hematopoietic origin.

These receptors recognise TGFb and bone morphogenetic factors. TGFb promotes cell differentiation by induction of cdki. Loss of expression of TGFb receptors in seen in colon carcinoma and lymphomas

CD45 on T-cells is the prototype here. T-cell activation involves removal of a phosphate residue on lck , a kinase of the src kinase family. This activation of lck leads to signal transduction through the T-cell antigen receptor. It seems that activation of lck depends on the activation of the CD45 receptor on the T-Cell surface. Role of CD45 in neoplasia in not known.