BHLHE41

BHLHE41 is expressed in the suprachiasmatic nucleus with levels peaking during subjective day.[14] The gene encodes for a transcription factor that belongs to the Hairy/Enhancer of Split (Hes) subfamily of basic helix-loop-helix factor genes which encode transcriptional repressors that function as downstream targets to regulate cell fate during tissue development.[26] BHLHE41 acts as a transcriptional repressor and as a regulator of the Circadian clock.[8] In the clock, the transcriptional factors Clock and Bmal form a heterodimer. This heterodimer binds to the E-Box promoter element, thereby promoting transcription of downstream genes such as Per and BHLHe41.[27] After transcription and translation, the protein product of BHLHE41 (DEC2) reenters the nucleus and competes with Clock-Bmal1 heterodimer for E-Box element binding (through competitive inhibition); this acts as a suppressor for per gene transcription.[14]

BHLHE41 has also been implicated in multiple other pathways. Deregulation of BHLHE41 transcription levels has been characterized as a marker in the progression of several cancers. Low levels of BHLHE41 transcript has been associated with tumor growth suggesting that BHLHE41 suppresses tumor proliferation; however, no definite mechanism of action has been discovered.[28] Dec2 has also been hypothesized to be involved in the regulation of immune responses.[10] Further research on characterizing these pathways and BHLHE41's specific role is still being conducted.

In mice lacking SHARP1/BHLHE41 and SHARP2, IGF-2 is elevated and leads to enhanced memory consolidation.[29]

A point mutation substituting C to G in DEC2/BHLHE41 DNA sequence results in the substitution of proline at position 385 with arginine. The proline at position 385 (384 according to the talk page) of BHLHE41 is located close to the C-terminal histone deacetylase-interacting region of BHLHE41, which is a highly conserved region within the proline-rich domain.[24] This mutation mitigates BHLHe41's transcriptional inhibitory function.[30] Mice with this mutation sleep less. They get less of both REM and non-REM sleep and recover more easily from sleep deprivation.[10] These effects are not seen in BHLHE41 knockout mice. As such, scholars believe the Dec2-P385R mutation is a dominant negative mutation.[31]

Ying-Hui Fu's lab found this mutation in familial natural short sleepers (FNSS).[8][32][24] FNSS naturally sleep less, about 6-6.5 hours a night.[10] It is not clear how this works. It is possible BHLHE41 makes sleep shorter through pathways independent of those which regulate the molecular core clock, such as the pathway involving the PER2 gene.[10] Both BHLHE41 and PER2 also affect immune function, maybe because tuning the immune system could be one of the reasons we sleep at all.[10]

BHLHE41 knockout mice, also known as BHLHE41 -/- or BHLHE41 null, showed no change in their free-running period with respect to activity. After being exposed to an in vivo model of allergic asthma, BHLHE41 knockout mice show decreased TH2 cytokine production, defective TH2 responses after being repeatedly stimulated with OVA peptide, and reduced alveolar infiltrate.[10] BHLHE41 knockout mice had increased post-natal regeneration of muscle after injury. However, these mice showed no deficits in embryonic muscle repair.[33]

BHLHE41 has been shown to be regulator of T-cell activation. BHLHE41 upregulates CD25 expression through a Stat6-dependent mechanism, which enhances the IL-2 receptor-mediated signal pathway, which promotes TH2 differentiation. Gata3 enhances T helper cell 2 (Th2) differentiation signals by regulating BHLHE41 expression through an autoregulatory loop.[25]

Hypoxia stimulates hypoxia-inducible factor-1 alpha (HIF-1α) to be produced, which initiates the hypoxic response. HIF-1α induces the transcription of BHLHE41 and BHLHE40. This is believed to repress cell proliferation, which is not conducive to a hypoxic environment.[33] BHLHE41 can also block a hypoxic response by presenting HIF-1α to a proteasome complex, which induces HIF-1α's degradation.[25]

BHLHE41 has been shown to represses myogenic differentiation by inhibiting MyoD activity through multiple mechanisms. When BHLHE41 dimerizes with MyoD and E47, it prevents the formation of MyoD-E47 heterodimers, which are functional. When BHLHE41 is sumoylated at K240 and K255, it recruits the histone methyltransferase G9a. G9a then catalyzes repressive histone 3 lysine 9 dimethylation (H3K9me2) at promoter sites of target genes of MyoD. G9a also methylates MyoD, which inhibits MyoD's transcriptional activity.[25]

BHLHE41 and BHLHE40 are transcriptional targets of SREBP-1 (also known as ADD-1) isoforms SREBP-1a and SREBP-1c. After being induced by SREBP-1, BHLHE41 and BHLHE40 have been shown to repress myogenesis by blocking MYOD1 transcription. BHLHE40 and BHLHE41 are also known to alter the expression of several contractile proteins and mitochondrial proteins in skeletal muscle. BHLHE41 and BHLHE40 also repress SREBP-1. This forms a negative feedback loop between SREBP-1, BHLHE40, and BHLHE41 in muscles that runs on a 24-hour circadian cycle, which has a 12-hour offset between SREBP-1 and BHLHE40/BHLHE41.[33] In addition, BHLHE41 is known to inhibit inflammation and adipogenic differentiation in muscles.[34]

BHLHE41 has been shown to suppress the expression of vascular endothelial growth factor (VEGF) in sarcoma cells and oral cancer cells. BHLHE41 also suppresses cytochrome P450 2D6 (CYP2D6) in hepatocellular carcinoma cells. While BHLHE40 induces apoptosis, senescence, and epithelial-mesenchymal transition (EMT) in tumor cells, BHLHE41 shows a circadian expression and inhibits EMT, apoptosis, and metastasis in sarcoma cells and hepatocellular carcinoma cells.[34] It has been shown that the normal tissue adjacent to colon carcinomas show high levels of BHLHE41 expression.[35] Research is currently examining whether BHLHE40 and BHLHE41 can be used as target genes for chemotherapy.[34]

BHLHE41 is thought to be a critical regulator of the metastasis of triple-negative-breast cancer (TNBC).[36] Regulated by the p63 metastasis suppressor, BHLHE41 inhibits TNBC through the inhibition of HIF-1α and hypoxia-inducible factor 2α (HIF-2α).[36] Studies have shown that BHLHE41 is both required and sufficient to limit the expression of HIF-target genes, by mechanistically binding to HIFs and promoting proteasomal degradation.[36] Breast cancer tumors that show high expression of BHLHE41 and CyclinG2 are believed to have a lower metastatic risk.[37][38]