fibroblasts (similar to the behavior of romidepsin, a depsipeptide HDAC inhibitor
approved for the treatment of cutaneous T-cell lymphoma). Histone hyperacetylation
and mRNA expression presented similar patterns, supporting the hypothesis that
expression is induced at the transcription level by inhibition of HDAC activity.
Transgene expression was also induced in mice (especially in the liver) and in
cultured epidermal sheets [82, 83]. Cancer growth suppression has also been evaluated for peptide T1.2. Preliminary GI 50 values of 13 nM and 14 nM against human
esophageal cancer cell lines (T.Tn and TE2, respectively) and T.Tn cancer growth
suppression measured in mice encouraged further studies. The in vitro mechanism of
action involved induction of apoptosis, which was further investigated and shown to
occur through the intrinsic pathway, i.e., upregulation of the Bax/Bcl-2 ratio in a
p53-independent manner [84]. Furthermore, esophageal squamous cell carcinoma
(ESCC) patients who were treated with compound T1.2 presented upregulation of
miR-375. The tumor-suppressive microRNA produced was identified to target
LDHB, which, upon knockdown, showed tumor suppression. This gene, miR-375,
is present in an area on chromosome 2q35, accessible by nucleosome disruption,
therefore supporting HDAC-mediated effect of macrocycle T1.2 in this context [85].
Chlamydocin derivative 6.9 (Fig. 6) was reported as cytotoxic against mouse
myeloma cell lines HS-72 and P3U1, but not against healthy spleen B and T cells.
This effect occurred through a mitochondrial intrinsic apoptotic pathway, mediated
by caspase-3 and caspase-9, which was completely inhibited by transfecting HS-72
cells with a bcl-2 expression plasmid. Other effects of exposure to compound 6.9
were accumulation of hyperacetylated histone 3 and downregulation of the expression of HDAC1 and HDAC2 [86].
Compound 7.1 was tested in two different disease models in vitro: first, as a
molecular chaperone for the recovery of the function of ΔF508-cystic fibrosis
transmembrane conductance regulator (CFTR), where HDAC inhibitors with
Zn
2+ -binding groups other than the hydroxamic acid were more promising, most
likely due to toxicity associated with the high potency of hydroxamic acidcontaining chemotypes [87], and, second, as activator of the survival of motor
neuron 2 (SMN2) gene, which can overcome the loss of SMN1 in spinal muscular
atrophy, for which this compound proved more promising. However, the effects of
various macrocyclic inhibitors in the last-mentioned study did not correlate with
their in vitro HDAC inhibitory profiles, and further experiments would be needed in
order to define such activity [88].
5 Chemical Tools for the Study of HDAC Structure
and Function
Linear hydroxamic acid-containing peptides have not attracted as much attention for
the design of HDAC inhibitors as their cyclic counterparts. Such molecules present
lower membrane permeability than macrocycles due to N- and C-terminal electrical
44
C. Moreno-Yruela and C. A. Olsen
approved for the treatment of cutaneous T-cell lymphoma). Histone hyperacetylation
and mRNA expression presented similar patterns, supporting the hypothesis that
expression is induced at the transcription level by inhibition of HDAC activity.
Transgene expression was also induced in mice (especially in the liver) and in
cultured epidermal sheets [82, 83]. Cancer growth suppression has also been evaluated for peptide T1.2. Preliminary GI 50 values of 13 nM and 14 nM against human
esophageal cancer cell lines (T.Tn and TE2, respectively) and T.Tn cancer growth
suppression measured in mice encouraged further studies. The in vitro mechanism of
action involved induction of apoptosis, which was further investigated and shown to
occur through the intrinsic pathway, i.e., upregulation of the Bax/Bcl-2 ratio in a
p53-independent manner [84]. Furthermore, esophageal squamous cell carcinoma
(ESCC) patients who were treated with compound T1.2 presented upregulation of
miR-375. The tumor-suppressive microRNA produced was identified to target
LDHB, which, upon knockdown, showed tumor suppression. This gene, miR-375,
is present in an area on chromosome 2q35, accessible by nucleosome disruption,
therefore supporting HDAC-mediated effect of macrocycle T1.2 in this context [85].
Chlamydocin derivative 6.9 (Fig. 6) was reported as cytotoxic against mouse
myeloma cell lines HS-72 and P3U1, but not against healthy spleen B and T cells.
This effect occurred through a mitochondrial intrinsic apoptotic pathway, mediated
by caspase-3 and caspase-9, which was completely inhibited by transfecting HS-72
cells with a bcl-2 expression plasmid. Other effects of exposure to compound 6.9
were accumulation of hyperacetylated histone 3 and downregulation of the expression of HDAC1 and HDAC2 [86].
Compound 7.1 was tested in two different disease models in vitro: first, as a
molecular chaperone for the recovery of the function of ΔF508-cystic fibrosis
transmembrane conductance regulator (CFTR), where HDAC inhibitors with
Zn
2+ -binding groups other than the hydroxamic acid were more promising, most
likely due to toxicity associated with the high potency of hydroxamic acidcontaining chemotypes [87], and, second, as activator of the survival of motor
neuron 2 (SMN2) gene, which can overcome the loss of SMN1 in spinal muscular
atrophy, for which this compound proved more promising. However, the effects of
various macrocyclic inhibitors in the last-mentioned study did not correlate with
their in vitro HDAC inhibitory profiles, and further experiments would be needed in
order to define such activity [88].
5 Chemical Tools for the Study of HDAC Structure
and Function
Linear hydroxamic acid-containing peptides have not attracted as much attention for
the design of HDAC inhibitors as their cyclic counterparts. Such molecules present
lower membrane permeability than macrocycles due to N- and C-terminal electrical
44
C. Moreno-Yruela and C. A. Olsen