As common nodes for metabolic control have been revealed, a plethora of mechanisms
have emerged to explain how the expression and activities of these enzymes are
controlled, and these mechanims are far more complex than the product inhibition
models previously taught in introductory biochemistry classes. A few examples
follow.
One of the newly appreciated steps by which mitogenic signals from oncogenic receptor
tyrosine kinases connect to the glycolytic machinery is via inhibition of the M2
isoform of pyruvate kinase (PKM2) a key regulator of glycolysis [
21,
22]. This inhibition is accomplished by a novel mechanism in which
binding of PKM2 to other tyrosine-phosphorylated proteins triggers release of the
allosteric activator, fructose-1, 6-bisphosphate (FBP) which inhibits PKM2.
Alternative splicing controls the swapping in and out of a single short exon that
encodes residues in PKM2 that allow binding to phosphotyrosine and the linkage
between phosphotyrosine binding and release of FBP. In contrast the PKM1
alternatively spliced form contains residues that maintain the enzyme in a highly
active state, independent of FBP. Interestingly, the PKM1 isoform is expressed in
most terminally differentiated, non-dividing cells, whereas all dividing cells that
have been investigated, both normal and cancerous, express the M2 isoform. This
splicing event is controlled, in part, by the expression of hnRNP A1 and A2 proteins
whose promoters are in turn regulated by c-Myc [
23,
24]. In addition to the
splicing to PKM2, the promoter is upregulated by HIF-1a in an mTORC1-dependent way,
thus giving tumors with elevated mTOR activity higher levels of PKM2 [
25]. Finally, the activity of PKM2 is also
decreased by acetylation under high glucose conditions [
26] consistent with suppression by pro-growth signals, such
as tyrosine phosphorylation. Another unique function of the M2, but not M1, isoform
of PKM was reported to be translocation to the nucleus and direct transactivation of
HIF-1a [
27]. In this capacity, PKM2 would
be reinforcing its own expression and that of other HIF-1a targets reprogramming
glucose metabolism.
Another example of multifaceted regulation is that of GLS1, which encodes a form of
glutaminase that resides in the mitochondria and converts glutamine to glutaminate,
for use in TCA cycle anaplerosis. As aforementioned, glutaminase mRNA is upregulated
by c-Myc through an indirect mechanism involving repression of mIR23a/b, which
targets GLS1. GLS1 mRNA has also been shown to be upregulated by NFkB, downstream of
other oncogenic signals [
28]. In
addition, GLS1 is alternatively spliced to form a shorter protein, GAC, whose
expression is elevated in certain cancers. Further links between call proliferation
and metabolism come from the observation that glutaminase protein levels are
targeted by the APC/C protease during the cell cycle, ensuring a careful temporal
control over glutaminase activity during the cell cycle [
29].
Another metabolic enzyme whose turnover is controlled by APC/C is the
“inducible” form of phosphofructokinase 2 (iPFK2/PFKFB3) [
29]. PFKFBP3 is a HIF-1a target gene,
upregulated in Ras-dependent tumor cells, and also has the unique distinction of
being directly phosphorylated by Akt and AMPK on different sites [
30]. The convergence of tyrosine and
serine/threonine kinases, acetylases, ubiquitin ligases, miRNA, splicing, and
transcriptional controls onto these 3 enzymes (PKM2, GLS1, and iPFK2/PFKFBP3) gives
a hint of the level of complexity that is likely to emerge for many of the proteins
at the nodes of metabolic control.