In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Metfromin markedly lowered isoproterenol - induced elevation in the levels of TLR4 mRNA , myeloid differentiation protein 88 ( MyD88 ) , tumor necrosis factor - alpha ( TNF - a ) , and interleukin 6 ( IL - 6 ) in the heart tissues .
### Output Text:
Metfromin markedly lowered <span class="chemical">isoproterenol</span> - induced elevation in the levels of TLR4 mRNA , myeloid differentiation protein 88 ( MyD88 ) , tumor necrosis factor - alpha ( TNF - a ) , and interleukin 6 ( IL - 6 ) in the heart tissues . 