Question

1A.) Which phases of cell cycle would be arrested by treatment of cells with ionizing radiation?

1B.) What upstream signaling is activating cyclin D complex?

1C.) What cell cycle transition is cyclin D regulating?

1D.) How is cyclinD1 and cdk4/6 activity required for cell cycle progression?

2A.) How are Caspases 3, 6 and 7 at the crossroads of intrinsic and extrinsic apoptosis?

2B.) How does DAG vs. IP3 result in different signaling options?  How do they both feed into Protein Kinase C activation?

2C.) How does Ca²⁺ function in the cell in response to ER release? What are two examples of targets that Ca²⁺can bind?

Answer 1

1A) When cells are exposed to ionizing radiation, a co,plex response is initiated with the arrest of cell cycle in G1 and G2. This event is preceded by DNA damage and apoptosis. DNA damage due to ionizing radiation initiates signal transduction by activating apoptosis. The intracellular signals for DNA damage results in cell cycle arrest as it activates the DNA-dependent protein kinase which initiate cell cycle arrest in G1 and G2 phase by activating the p53 tumor suppressor protein.

1B) The Cyclin D or Cdk4 complex is a multiprotein structure consisting of cyclin D and cyclin dependent kinase 4, or Cdk4. This complex regulates different cell cycle transitions by activating protein kinases. The cyclin D complex is activated by the upstream binding of Growth factors (GFs) that are either from the cells itself or adjoining cells. The activation of cyclin D complex stimulates the receptors of tyrosine kinase proteins to activate which in turm activates other proteins.

1C) The cyclin D or Cdk4 forms the integral checkpoint for the progression of cell cycle from Growth phase 1 to Synthetic phse (G1S) of the cell cycle.

1D) Cyclin D1 is a protein that regulates the cell cycle progression in G1 phase. It is a regulatory subunit of cyclin dependent kinases Cdk4 and Cdk6. The cyclin D1-Cdk4 complex helps in the progression through G1 phase by inhibiting retinoblastoma protein (pRb) by means of phosphorylation and allows transcription factors to transcribe genes which are essential for the entry in S-phase. This complex also activates the cyclin E-Cdk2 complex.

2A) Caspases are a family of endoproteases that are crucial in inflammation and programmed cell death. They are broadly classified according to their roles in apoptosis (caspase-3,6,7,8 and 9) and in inflammation (caspase-1,4,5,12). Caspases involved in apoptosis are subdivided according to the mode of mechanism into initiator (caspase-8) and executioner caspases (3,6 and 7). There are two distinct pathways of apoptosis namely, extrinsic and intrinsic pathway. The extrinsic pathway implies its activation by binding of a donor to an apoptic receptor, which in turn recruit, dimerize and activate caspase-8 with the help of adapter proteins. The activated caspase-8 then may either initiate apoptosis directly thereby activating executioner caspases 3,6 and7 or initiates intrinsic pathway. The intrinsic pathway is activated with the response of various cellular stresses that release cytochrome c from mitochondria. This is followed by the formation of apoptosome comprised of different subunits which in turn activates caspase-9. The active Caspase-9 then iniitiates apoptosis by cleaving and thereby turning on caspase-3,6 and 7. Hence, caspases 3, 6 and 7 are at the crossroads of intrinsic and extrinsic pathways.