Kerith Sharkey

Mammalian target of rapamycin (mTOR) is a key regulator of cell growth acting via two independent targets, ribosomal protein S6 kinase 1 (S6K1) and 4EBP1. While each is known to regulate translational efficiency, the mechanism by which they control cell growth remains unclear. In addition to increased initiation of translation, the accelerated synthesis and accumulation of ribosomes are fundamental for efficient cell growth and proliferation. Using the mTOR inhibitor rapamycin, we show that mTOR is required for the rapid and sustained serum-induced activation of 45S ribosomal gene transcription (rDNA transcription), a major rate-limiting step in ribosome biogenesis and cellular growth. Expression of a constitutively active, rapamycin-insensitive mutant of S6K1 stimulated rDNA transcription in the absence of serum and rescued rapamycin repression of rDNA transcription. Moreover, overexpression of a dominant-negative S6K1 mutant repressed transcription in exponentially growing NIH 3T3 cells. Rapamycin treatment led to a rapid dephosphorylation of the carboxy-terminal activation domain of the rDNA transcription factor, UBF, which significantly reduced its ability to associate with the basal rDNA transcription factor SL-1. Rapamycin-mediated repression of rDNA transcription was rescued by purified recombinant phosphorylated UBF and endogenous UBF from exponentially growing NIH 3T3 cells but not by hypophosphorylated UBF from cells treated with rapamycin or dephosphorylated recombinant UBF. Thus, mTOR plays a critical role in the regulation of ribosome biogenesis via a mechanism that requires S6K1 activation and phosphorylation of UBF.

In mammals, the mechanisms regulating the number of active copies of the approximately 200 ribosomal RNA (rRNA) genes transcribed by RNA polymerase I are unclear. We demonstrate that depletion of the transcription factor upstream binding factor (UBF) leads to the stable and reversible methylation-independent silencing of rRNA genes by promoting histone H1-induced assembly of transcriptionally inactive chromatin. Chromatin remodeling is abrogated by the mutation of an extracellular signal-regulated kinase site within the high mobility group box 1 domain of UBF1, which is required for its ability to bend and loop DNA in vitro. Surprisingly, rRNA gene silencing does not reduce net rRNA synthesis as transcription from remaining active genes is increased. We also show that the active rRNA gene pool is not static but decreases during differentiation, correlating with diminished UBF expression. Thus, UBF1 levels regulate active rRNA gene chromatin during growth and differentiation.

The distribution of scarce healthcare resources is an increasingly important issue due to factors such as expensive 'high tech' medicine, longer life expectancies and the rising prevalence of chronic illness. Furthermore, in the current healthcare context lifestyle-related factors such as high blood pressure, tobacco use and obesity are believed to contribute significantly to the global burden of disease. As such, this paper focuses on an ongoing debate in the academic literature regarding the role of responsibility for illness in healthcare resource allocation: should patients with self-caused illness receive lower priority in access to healthcare resources? This paper critically describes the lower priority debate's 12 key arguments and maps out their relationships. This analysis reveals that most arguments have been refuted and that the debate has stalled and remains unresolved. In conclusion, we suggest progression could be achieved by inviting multidisciplinary input from a range of stakeholders for the development of evidence-based critical evaluations of existing arguments and the development of novel arguments, including the outstanding rebuttals.

Clinician gate-keeping is the process whereby healthcare providers prevent access to eligible patients for research recruitment. This paper contends that clinician gate-keeping violates three principles that underpin international ethical guidelines: respect for persons or autonomy; beneficence or a favourable balance of risks and potential benefits; and justice or a fair distribution of the benefits and burdens of research. In order to stimulate further research and debate, three possible strategies are also presented to eliminate gate-keeping: partnership with professional researchers; collaborative research design and clinician education.