DNA damage checkpoints are biochemical pathways that delay or halt cell cycle progression in response to DNA damage. Cell cycle proceeds in four phases in all somatic eukaryotic cells, G1, S, G2, and M, and one outside the cycle per se, G0 [1].

The G1/S cell cycle checkpoint controls the passage of q cell from the first 'gap' phase (G1) into the DNA synthesis phase (S). Many different stimuli exert G1/S checkpoint control, including TGF-beta, DNA damage, contact inhibition, replicative senescence, and growth factor withdrawal.

Transforming growth factor-beta 1 (TGF-beta 1) [2] and transforming growth factor-beta 2 (TGF-beta 2) [3] participate in regulation of the G1/S checkpoint. TGF-beta signals are transmitted by contacting two distantly related transmembrane serine/threonine kinases, named receptors I (TGF receptor I) and II (TGF receptor II). TGF-beta 1 and/or TGF-beta 2 bind directly to TGF receptor II, which is a constitutively active kinase. The bound TGF-beta factors are then recognized by TGF receptor I, which is recruited into the complex and becomes phosphorylated by TGF receptor II. Phosphorylation allows TGF receptor I to propagate the signal to downstream substrates [4].

TGF-beta signaling leads to an excitation of at least two pathways.

TGF-beta factors induce an association of its receptor with regulatory subunit of protein phosphatase-2A (PP2A). Concomitantly, three PP2A subunits (regulatory, structural and catalytic) lead to dephosphorylation and inactivation of ribosomal protein S6 kinase, 70kD, polypeptide 1 (p70 kinase 1) [5]. Inactivated p70 kinase 1 fails to oppress activity of glycogen synthase kinase 3 beta (GSK3 beta). In this case, GSK3 beta phosphorylates cyclin D, making it accessible to the subsequent ubiquitination and proteosomal degradation [6]. Stimulation of GSK3 beta activity is observed also in the case of growth factor withdrawal [7].

In addition, TGF receptor I activates by phosphorylation SMAD family member 2 and 3 (Smad2 and Smad3), which form complexes with SMAD family member 4 (Smad4) that accumulate in the nucleus and regulate transcription of target genes [8]. Smads stimulate transcription of proteins from INK4 (p16INK and p15) and Kip/Cip (p21 and p27KIP1) families of cell cycle kinase inhibitors directly or indirectly (for example, via b-Jun and SP1 transfactors). These inhibitors interfere with interactions between cyclin-dependent kinases (CDK) and cyclins. Contact inhibition [9], replicative senescence [10], [11] and DNA damage [1] also may stimulate the G1/S checkpoint arrest through p15, p21 or p27KIP1.

Moreover, DNA damage leads to phosphorylation of checkpoint homologues (Chk). Phosphorylated Chk, in turn, inactivates by phosphorylation cell division cycle 25A phosphatase (Cdc25A). Lack of active Cdc25A results in the accumulation of the phosphorylated (inactive) form of Cdk2 and Cdk4, which are incapable to participate in initiation of replication [1].

Phosphorylated CDC25A may be exposed to ubiquitination by Anaphase-promoting complex (APC) and/or SCF E3 ubiquitin ligase complex in Smad3-dependent manner (with following proteosomal degradation) [12].

Polo-like kinase 3 (PLK3) may stimulate G1/S transition, e.g., via activation of CDC25A [13], [14].

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