LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

CHAPTER 4
DISCUSSION
different from gamma irradiation, signaling component and intensity of activation will be
different by both types of irradiation. These kinases play an important role in repair, cell cycle
arrest and apoptosis. However, only 11% survival after high LET radiation suggests that most of
the damage could not be repaired and that the persistent activation of these kinases may be a
signature for activation of apoptotic responses. Further studies in this direction will reveal the
importance of such response.
4.4 Radiation induced bystander signaling in mammalian cells.
The signaling events seen in totality in the foregoing work may have contribution from the
surrounding microenvironment that is of necessity exposed to radiation. The cells nearby which
are not directly exposed to the radiation but are juxtaposed to the irradiated cells are also affected
because of signals transduced from the ‘hit’ cell. The classical paradigm of radiation biology
asserts that all radiation effects on cells, tissues and organisms are due to the direct action of
radiation. However, there has been a recent growth of interest in the indirect actions of radiation
including the radiation-induced adaptive response, the bystander effect, low-dose
hypersensitivity, and genomic instability, which are specific modes of stress exhibited in
response to low-dose/low-dose rate radiation.
Radiation-induced bystander signals (whether observed through use of medium harvested from
irradiated cells, or through observation of non-hit cells in contact with irradiated cells), appear to
coordinate tissue or population responses so that cells not directly exposed or traversed by
radiation show responses, which may be part of higher order homeostatic control. Bystander
effects appear to be the result of a generalized stress response in tissues or cells. The signals may
be produced by all exposed cells but the response may require a quorum in order to be expressed.
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